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Body temperature distributions of active diurnal lizards in three deserts: Skewed up or skewed down?
Abstract
The performance of ectotherms integrated over time depends in part on the position and shape of the distribution of body temperatures ( T b ) experienced during activity. For several complementary reasons, physiological ecologists have long expected that T b distributions during activity should have a long left tail (left‐skewed), but only infrequently have they quantified the magnitude and direction of T b skewness in nature.
To evaluate whether left‐skewed T b distributions are general for diurnal desert lizards, we compiled and analysed T b (∑ = 9,023 temperatures) from our own prior studies of active desert lizards in three continents (25 species in Western Australia, 10 in the Kalahari Desert of Africa and 10 species in western North America). We gathered these data over several decades, using standardized techniques.
Many species showed significantly left‐skewed T b distributions, even when records were restricted to summer months. However, magnitudes of skewness were always small, such that mean T b were never more than 1°C lower than median T b . The significance of T b skewness was sensitive to sample size, and power tests reinforced this sensitivity.
The magnitude of skewness was not obviously related to phylogeny, desert, body size or median body temperature. Moreover, a formal phylogenetic analysis is inappropriate because geography and phylogeny are confounded (i.e. are highly collinear).
Skewness might be limited if lizards pre‐warm inside retreats before emerging in the morning, emerge only when operative temperatures are high enough to speed warming to activity T b , or if cold lizards are especially wary and difficult to spot or catch. Telemetry studies may help evaluate these possibilities.
A plain language summary is available for this article.
... Beyond the statistical relationship between body and environmental temperatures, examining the full temperature data, rather than just the means, can provide insights into the frequency distribution of a species' body temperatures. Huey and Pianka (2018) found that body temperature distributions of diurnal desert lizards were left-skewed, with a mode near the warmer end of the T b range with a long 'tail' across a wide range of lower temperatures. Leftskewed body temperature distributions are deemed better for squamate activity due to the similar asymmetry of performance curves, such that exceeding the optimum temperature leads to a sharper drop in performance or fitness than a shift of a similar magnitude towards lower temperatures (Martin & Huey, 2008). ...
... Leftskewed body temperature distributions are deemed better for squamate activity due to the similar asymmetry of performance curves, such that exceeding the optimum temperature leads to a sharper drop in performance or fitness than a shift of a similar magnitude towards lower temperatures (Martin & Huey, 2008). This, however, is probably only true for squamates active at high temperatures, where the optimum is nearer the maximum, such as the diurnal lizards studied by Huey and Pianka (2018), and has not been studied in other groups. ...
... Most body temperature distributions were left-skewed (Figure 7) as found for desert lizards by Huey and Pianka (2018). Thus, T b during activity is more frequently closer to the maximum than to the minimum temperature limit (i.e. ...
... Beyond the statistical relationship between body and environmental temperatures, examining the full temperature data, rather than just the means, can provide insights into the frequency distribution of a species' body temperatures. Huey and Pianka (2018) found that body temperature distributions of diurnal desert lizards were left-skewed, with a mode near the warmer end of the T b range with a long 'tail' across a wide range of lower temperatures. Leftskewed body temperature distributions are deemed better for squamate activity due to the similar asymmetry of performance curves, such that exceeding the optimum temperature leads to a sharper drop in performance or fitness than a shift of a similar magnitude towards lower temperatures (Martin & Huey, 2008). ...
... Leftskewed body temperature distributions are deemed better for squamate activity due to the similar asymmetry of performance curves, such that exceeding the optimum temperature leads to a sharper drop in performance or fitness than a shift of a similar magnitude towards lower temperatures (Martin & Huey, 2008). This, however, is probably only true for squamates active at high temperatures, where the optimum is nearer the maximum, such as the diurnal lizards studied by Huey and Pianka (2018), and has not been studied in other groups. ...
... Most body temperature distributions were left-skewed (Figure 7) as found for desert lizards by Huey and Pianka (2018). Thus, T b during activity is more frequently closer to the maximum than to the minimum temperature limit (i.e. ...
Aim: Squamate fitness is affected by body temperature, which in turn is influenced by environmental temperatures and, in many species, by exposure to solar radiation. The biophysical drivers of body temperature have been widely studied, but we lack an integrative synthesis of actual body temperatures experienced in the field, and their relationships to environmental temperatures, across phylogeny, behaviour, and climate.
Location: Global (25 countries on six continents).
Taxa: Squamates (210 species, representing 25 families).
Methods: We measured the body temperatures of 20,231 individuals of squamates in the field while they were active. We examined how body temperatures vary with substrate and air temperatures across taxa, climates, and behaviours (basking and diel activity).
Results: Heliothermic lizards had the highest body temperatures. Their body temperatures were the most weakly correlated with substrate and air temperatures. Body temperatures of non-heliothermic diurnal lizards were similar to heliotherms in relation to air temperature, but similar to nocturnal species in relation to substrate temperatures. The correlation of body temperature with air and substrate temperatures was stronger in diurnal snakes and non-heliothermic lizards than in heliotherms. Body-substrate and body-air temperature correlations varied with mean annual temperatures in all diurnal squamates, especially in heliotherms. Thermal relations vary with behaviour (heliothermy, nocturnality) in cold climates but converge toward the same relation in warm climates. Non-heliotherms and nocturnal species body temperatures are better explained by substrate temperature than by air temperature. Body temperature distributions become left-skewed in warmer-bodied species, especially in colder climates.
Main conclusions: Squamate body temperatures, their frequency distributions, and their relation to environmental temperature, are globally influenced by behavioural and climatic factors. For all temperatures and climates, heliothermic species’ body temperatures are consistently higher and more stable than in other species, but in regions with warmer climate these differences become less pronounced. A comparable variation was found in non-heliotherms, but in not nocturnal species whose body temperatures were similar to air and substrate irrespective of the macroclimatic context.
... We used published data for the observed body temperature range of 23.8-40.2°C for active P. platyrhinos (Huey & Pianka, 2018), using the lower value for two behavioural thresholds, both temperature of emergence (T emerg ) and basking temperature (T bask ). Preferred temperature (T pref ) is the midpoint value of the range of selected temperatures recorded in a laboratory gradient (34.2°C; ...
... Lastly, we compare the body temperature distribution from the model output with published data by Huey and Pianka (2018) for P. platyrhinos (see paper for details and supplemental code used to calculate skewness coefficient). We focus on a qualitative comparison of mean body temperature, median body temperature, mean absolute deviation and D'Agostino coefficient of skewness as their measurements took place between 1962 and 1969 and our model output is based on climate from 2017. ...
... We compare the basking and foraging activity state body temperatures of the model output with the data reported in Huey and Pianka (2018). For field collected P. platyrhinos measurements, the skewness was less than −1 for both northern and southern regions ...
Aim
Temperature plays an important role in determining distributions of ectothermic species, yet many ectotherms have wide distributions across diverse environmental conditions. Our goal was to model regional differences in annual and seasonal activity patterns among populations of Phrynosoma platyrhinos across its climatically diverse distribution. We hypothesize that microhabitat availability and behavioural thermoregulation will not mitigate large‐scale climatic differences across the species' distribution, resulting in alteration of activity patterns of populations.
Location
Western deserts of North America.
Taxon
Desert Horned Lizard (Phrynosoma platyrhinos).
Methods
We used biophysical models to (a) model microhabitat conditions at 31 localities across the species' range, (b) model body temperature profiles of a theoretical lizard using microhabitat conditions, physiological and thermal parameters and the ability of lizards to thermoregulate behaviourally and (c) predict year‐round activity patterns of P. platyrhinos across its range, quantified as the percentage of time permissible for basking or foraging.
Results
The models identified that different populations of P. platyrhinos experience differences in predicted activity patterns, both daily and seasonally, with up to 67% more time suitable for foraging in the southern than northern deserts. Southern populations have more potential for year‐round activity, but predicted activity is reduced during hot summer months. The northern populations have less time for activity overall because of low temperatures in the spring and fall. However, summer‐predicted activity is greater than in the southern regions.
Main conclusions
We detected pronounced regional differences in the times available to P. platyrhinos for activity. We conclude that behaviour and microclimate conditions are not able to fully mitigate the effects of large‐scale thermal variation, and that lizards compensate for climatic variation by shifting their daily and seasonal predicted activity patterns. These differences can be tied to inter‐population variation in life history traits.
... Moreover, individual decisions are subject to some stochasticity either when there is no preference for one microhabitat or when unexpected events change animal behavior (Woods et al. 2015;Vickers and Schwarzkopf 2016;Malishev et al. 2017). Altogether, those thermoregulatory and nonthermoregulatory decisions determine the daily distribution of body temperatures experienced by an individual or a population of ectotherms (Huey et al. 1977;Bauwens et al. 1996;Huey and Pianka 2017). Daily T b distributions have long been used by field studies with thermoregulating terrestrial ectotherms to infer characteristics of their thermal biology (Huey et al. 1977;Hertz 1992;Hertz et al. 1993;Bauwens et al. 1996;Huey and Pianka 2017), the quality of thermal habitats, or the effectiveness of behavioral thermoregulation (Hertz et al. 1993;Row and Blouin-Demers 2006;Zamora-Camacho et al. 2014). ...
... Altogether, those thermoregulatory and nonthermoregulatory decisions determine the daily distribution of body temperatures experienced by an individual or a population of ectotherms (Huey et al. 1977;Bauwens et al. 1996;Huey and Pianka 2017). Daily T b distributions have long been used by field studies with thermoregulating terrestrial ectotherms to infer characteristics of their thermal biology (Huey et al. 1977;Hertz 1992;Hertz et al. 1993;Bauwens et al. 1996;Huey and Pianka 2017), the quality of thermal habitats, or the effectiveness of behavioral thermoregulation (Hertz et al. 1993;Row and Blouin-Demers 2006;Zamora-Camacho et al. 2014). Understanding the processes that modulate animal activity routines, microhabitat selection, and T b distributions is therefore crucial to improve the predictive capacity of biophysical models. ...
... With this information, the model derives the T b distribution of the population at each instant of time. We validated model predictions using empirical T b data of desert lizards published by Huey and Pianka (2017). Desert lizards are among the best-studied examples of thermoregulating ectotherms because they rely on behavior to avoid ambient temperatures beyond their critical thermal limits (Huey et al. 1977;Pianka 1986;Sunday et al. 2014). ...
Realistic projections of the biological impacts of climate change require predicting fitness responses to variations in environmental conditions. For ectotherms, this challenge requires methods to scale-up microclimatic information into actual body temperatures, T b , while dealing with uncertainty regarding individual behaviors and physiological constraints. Here, we propose an information-theoretical model to derive microhabitat selection and T b distributions of ectotherm populations from microclimatic data. The model infers the most probable allocation of individuals among the available microenvironments and the associated population-level T b distribution. Using empirical T b data of 41 species of desert lizards from three independently evolved systems— Western North America, Kalahari Desert, and Western Australia—we show that the model accurately predicts empirical T b distributions across the three systems. Moreover, the framework naturally provides a way to quantify the importance of thermoregulation in a thermal environment and thereby a measurement for the constraint imposed by the climatic conditions. By predicting T b distributions of ectotherm populations even without exhaustive information on the underpinning mechanisms, our approach forms a solid theoretical basis for upscaling microclimatic and physiological information into a population-level fitness trait. This scaling process is a first step to reliably project the biological impacts of climate change to broad temporal and spatial scales.
... Many biochemical processes and other physical performance proxies (e.g. sprint speed) reach peak performance within a narrow window of temperatures, and many reptiles (especially those in temperate zones) will invest substantial amounts of energy into regulating their body temperature within this optimal range (Brown & Weatherhead 2000;Herczeg et al. 2006;Cadena & Tattersall 2009;Caldwell et al. 2017;Huey & Pianka 2017). ...
... The thermal preference (Tsel) of many reptile species lies very close to their upper temperature threshold (CTmax) and this is also true of temperate-dwelling species that may occupy environments where optimal environmental temperatures are rarely available (Huey & Slatkin 1976;Martin & Huey 2008;Cadena & Tattersall 2009;Huey & Pianka 2017). However, since it is dangerous to maintain body temperatures so close to lethal temperatures, in the wild some reptiles may maintain their body temperatures at the lower end of their thermal preference range and sacrifice maximum performance to avoid overheating (Martin & Huey 2008;Cadena & Tattersall 2009;Huey & Pianka 2017). ...
... The thermal preference (Tsel) of many reptile species lies very close to their upper temperature threshold (CTmax) and this is also true of temperate-dwelling species that may occupy environments where optimal environmental temperatures are rarely available (Huey & Slatkin 1976;Martin & Huey 2008;Cadena & Tattersall 2009;Huey & Pianka 2017). However, since it is dangerous to maintain body temperatures so close to lethal temperatures, in the wild some reptiles may maintain their body temperatures at the lower end of their thermal preference range and sacrifice maximum performance to avoid overheating (Martin & Huey 2008;Cadena & Tattersall 2009;Huey & Pianka 2017). This maintenance of a slightly lower Tb has been reported for several lizard species and is referred to as negative skew (Cadena & Tattersall 2009;Huey & Pianka 2017). ...
Garter snakes are the most widespread reptile in North America. Despite occupying vastly different biogeoclimatic zones across their range, evidence suggests that the thermal preference (Tsel) of garter snakes has not significantly diverged among populations or different Thamnophis species. The reason for garter snake’s wide thermal tolerance could lie in their flexible thermoregulatory behaviours and habitat use. I aimed to investigate this relationship by first identifying the Tsel of a common garter snake species (Thamnophis sirtalis sirtalis) via a thermal gradient in the lab. I used this Tsel parameter (27.82 – 32.15 ºC) as a benchmark for calculating the thermal quality of open, mixed, and forested habitats all used by the species. I measured the thermal profiles of these habitats by installing a series of operative temperature models that mimicked the thermal properties of living garter snakes and recorded environmental temperatures as living snakes experience them. Lastly, I used coverboards to survey the habitat usage of T. s. sirtalis. Of the three habitats, I found that the open habitat offered the highest thermal quality throughout the snake’s active season. In contrast, I recorded the greatest number of snakes using the mixed habitat which had considerably lower thermal quality. Although the open habitat offered the greatest thermal quality on average, environmental temperatures regularly exceeded the upper limits of the animals’ thermal tolerance, greatly restricting the activity window for Eastern garter snakes. Therefore, the open habitat may be less thermally attractive to T. s. sirtalis. My data show that not all habitat types used by Eastern garter snakes share the same quality and the relationship is not as simple as warm, open habitats are better. Rather, these animals may preferentially seek out areas that offer a mix of both open and closed-canopy spaces to suit their thermoregulatory needs. In conclusion, T. s. sirtalis may select more thermally stable habitats over those that present them with thermal extremes and greater constraints to their surface activity.
... Our model can predict mean growth rate during some time interval or life stage by integrating over the distribution of body temperatures experienced during the interval or stage (p(T b ); Huey and Slatkin 1976;Deutsch et al. 2008;Vasseur et al. 2014). Body temperature distributions of active ectotherms are typically left skewed and unimodal and can be broad or narrow (Huey and Pianka 2017). We arbitrarily selected activity T b of two desert lizards (Pedioplanus lineoocellata, from the Kalahari Desert; Phrynosoma playrhinos, from northern US deserts) as exemplars of T b distributions (Huey and Pianka 2017) of ectotherms with a relatively narrow versus broad distribution, respectively. ...
... 2B, 2D). Third, our model assumes that the distribution of T b is unimodal and left skewed: this is valid for ectotherms such as diurnal desert lizards when active (Huey and Pianka 2017). However, growth is sensitive to T b over 24 h, not just during activity. ...
Climate warming may lower environmental resource levels, growth, and fitness of many ectotherms. In a classic experiment, Brett and colleagues documented that growth rates of salmon depended strikingly on both temperature and food levels. Here we develop a simple bioenergetic model that explores how fixed temperatures and food jointly alter the thermal sensitivity of net energy gain. The model incorporates differing thermal sensitivities of energy intake and metabolism. In qualitative agreement with Brett's results, it predicts that decreased food intake reduces growth rates, lowers optimal temperatures for growth, and lowers the highest temperatures sustaining growth (upper thermal limit). Consequently, ectotherms facing reduced food intake in warm environments should restrict activity to times when low body temperatures are biophysically feasible, but-in a warming world-that will force ectotherms to shorten activity times and thus further reduce food intake. This "metabolic meltdown" is a consequence of declining energy intake coupled with accelerating metabolic costs at high temperatures and with warming-imposed restrictions on activity. Next, we extend the model to explore how increasing mean environmental temperatures alter the thermal sensitivity of growth: when food intake is reduced, optimal temperatures and upper thermal limits for growth are lowered. We discuss our model's key assumptions and caveats as well as its relationship to a recent model for phytoplankton. Both models illustrate that the deleterious impacts of climate warming on ectotherms will be amplified if food intake is also reduced, either because warming reduces standing food resources or because it restricts foraging time.
... To emphasize this, we compared the simulated temperatures with real microclimates in Sweden, Spain and extremely thermally buffered Ecuadorian forests, revealing that natural temperatures are much more variable (and cooler on average) than those in the simulations (Figure 1a). This holds true also for comparisons with operative body temperatures of, for instance, lizards (Huey & Pianka, 2018), insects (Pincebourde et al., 2021) and non-hibernating mammals (Levesque et al., 2018)-even as temperature and its variance increase during the simulation. Note that our focus is on temporal variation; we only use microclimates to represent a broad range of thermal regimes. ...
In their simulation study, Garcia‐Costoya et al. (2023) conclude that evolutionary constraints might aid populations facing climate change. However, we are concerned that this conclusion is largely a consequence of the simulated temperature variation being too small, and, most importantly, that uneven limitations to standing variation disadvantage unconstrained populations. image
... In both locations, the species showed similar distribution. This situation also supports the species' cryptic habit, using rocky outcrops for protection (Garda et al., 2013;Huey & Pianka, 2017). ...
Ecological aspects are essential for testing hypotheses about the characteristics, evolutionary processes and behaviour patterns of different taxa. However, for many neotropical lizards, like Gymnodactylus darwinii, such information is scant in the literature. This lizard is endemic to the Atlantic rainforest, occurring from Rio Grande do Norte to São Paulo. Herein, the aspects of trophic ecology, microenvironmental specificity and sexual dimorphism were analysed from 65 collected individuals and 15 sighted individuals in two conservation units: Camucim and Tapacurá, located in São Lourenço da Mata, north‐eastern region of Brazil. The data found classify G. darwinii as a saxicolous lizard, with a preference for rocky outcrops, without sexual dimorphism. According to its trophic ecology, the species is considered a generalist and sit–wait forager. Arthropods, especially Orthoptera and Araneae, dominate the lizard's diet. Neither of these aspects differed between seasons. The present study also shows the importance of the domain conservation, with the aim to provide resources for many vertebrates, especially endemic species.
... We then evaluated the thermoregulatory behaviours and parameters from the experimental data. We used a two-fold approach to assess whether the distribution of T sel was negatively skewed (Huey and Pianka 2018). First, we checked if the skewness of the T sel distributions was significantly different from a normal distribution with a t-test using the "t.test" function (stats package; R Core Team 2021). ...
Thermal biology research compares field with laboratory data to elucidate the evolution of temperature-sensitive traits in ectotherms. The hidden challenge of many of these studies is discerning whether animals actively thermoregulate, since motivation is not typically assessed. By studying the behaviours involved in thermoregulation, we can better understand the mechanisms underlying body temperature control. Using an integrative approach, we assessed the thermoregulatory and thermotactic behaviours of two sympatric snake species with contrasting life histories: the generalist Eastern Garter Snake (Thamnophis sirtalis sirtalis ( Linnaeus, 1758 )) and the semi-fossorial Northern Red-bellied Snake (Storeria occipitomaculata occipitomaculata ( Storer, 1839 )). We expected that thermoregulatory behaviours would be optimized based on life history, in that T. s. sirtalis would show higher evidence for thermally oriented behaviours than S. o. occipitomaculata due to its active nature. Thamnophis sirtalis sirtalis actively thermoregulated, had higher thermal preferences (29.4 ± 2.5 vs. 25.3 ± 3.6 °C), and was more active than S. o. occipitomaculata, which showed relatively low evidence for thermotaxis. Our results build on the notion that evaluating movement patterns and rostral orientation towards a heat source can help ascertain whether animals make thermally motivated choices. Our data provide insight into the thermoregulatory strategies used by snakes with different life histories and maximize the information provided by behavioural thermoregulation experiments.
... The relatively stable state of T b compared to the variation in T ex between months indicates that the role of behavior in maintenance of optimal body temperature might be important, as the larger difference between T b and T a in warmer periods of year might indicate a greater frequency of avoidance of overheating . Given that both of these two species are diurnal, with the most active times being in spring and summer, efficient thermoregulatory behavior is crucial for heat avoidance while maintaining an optimal body temperature for foraging and metabolism (Huey and Pianka, 2018;Sepúlveda et al., 2014). It is also striking that while smaller P. parva has a higher T ex than L. diplochondrodes in July when ground temperature is at its highest, this trend is reversed in September when temperatures are lower. ...
Sympatric lizard species present convenient models for studying differentiation in thermal behavior and the role of morphological differences in their thermal biology. Here we studied the thermal biology of two sympatric lizard species which occur sympatrically in the Phrygian Valley of Western Anatolia. These two species differ in body size, with Lacerta diplochondrodes being larger than Parvilacerta parva. The surface body temperatures of the individuals belonging to both species were recorded when active in the field. Additionally, several environmental parameters including solar radiation, substrate temperature, air temperature and wind speed were monitored to investigate the relative effects of these abiotic parameters on the thermal biology of the two species. The surface body temperature and temperature excess (difference between body and substrate temperature) of the two species, while being relatively close to each other, showed seasonal differences. Solar radiation, substrate temperature and air temperature were the main factors influencing their thermal biology. Additionally, although body size did not have a direct effect on body temperature or temperature excess, the interaction between body size and solar radiation on temperature excess was significant. In conclusion, our study partially supports the conservation of body temperature of related lizard species.
... Therefore, accounting for its strict endemism, as well as the WT and TSM values in shade, we can point out the importance of microhabitats for the persistence of this species and consequently the importance of implementing urgent measures to protect the current suitable habitats that support populations of L. cuyumhue. In conclusion, similar to other studies, we show that the quality of the microhabitats is essential for the persistence of desert lizards (Flesch et al. 2017;Huey and Pianka 2018). Future research activities should be focused on studying basic aspects of the biology such as feeding, reproduction, and use of time space for the implementation of an appropriate management plan for this species. ...
The body temperature of lizards is strongly influenced by the thermal quality of microhabitats, exploiting the favourable environmental temperatures, and avoiding exposure to extreme thermal conditions. For these reasons, reptile populations are considered to be especially vulnerable to changes in environmental temperatures produced by climate change. Here, we study the thermal physiology of the critically endangered Añelo Sand Dunes Lizard (Liolaemus cuyumhue Avila, Morando, Perez and Sites, 2009). We hypothesise that (i) there is a thermal coadaptation between optimal temperature for locomotor performance of L. cuyumhue and its thermal preference; (ii) L. cuyumhue lives in an environment with low thermal quality; and (iii) a rise in environmental temperatures due to global warming will impose a decrement in locomotor speed represented by lower warming tolerance and narrower thermal safety margins, increasing their already high vulnerability. We recorded field body temperatures (Tb), preferred body temperatures (Tpref), the operative temperature (Te), and the thermal sensitivity of locomotion at different body temperatures. Our results indicate that this lizard is not currently under environmental stress or exceeding its thermal limits, but that it is thermoregulating below Tpref to avoid overheating, and that an increase in environmental temperature higher than 3.5 °C will strongly affect the use of microhabitats with direct sun exposure.
... temperature variation, humidity, or precipitation) influence the fundamental TPC. For example, TPCs are often considered to be left-skewed for physiological reasons (e.g. the response of thermoregulation proceeds faster at higher temperatures) (Woodin et al., 2013;Huey and Pianka, 2018), but our results and those of many other studies have shown that TPCs can be diverse in their shape (Dell et al., 2011;Monaco et al., 2017). Although we show that variation in the shape of TPCs can be due to interspecific competition, other types of biotic interactions, including mutualistic and host-parasites interactions (Cohen et al., 2017), should also be carefully considered and compared when quantifying fundamental TPCs. ...
Understanding how climate-mediated biotic interactions shape thermal niche width is critical in an era of global change. Yet, most previous work on thermal niches has ignored detailed mechanistic information about the relationship between temperature and organismal performance, which can be described by a thermal performance curve. Here, we develop a model that predicts the width of thermal performance curves will be narrower in the presence of interspecific competitors, causing a species’ optimal breeding temperature to diverge from that of its competitor. We test this prediction in the Asian burying beetle Nicrophorus nepalensis, confirming that the divergence in actual and optimal breeding temperatures is the result of competition with their primary competitor, blowflies. However, we further show that intraspecific cooperation enables beetles to outcompete blowflies by recovering their optimal breeding temperature. Ultimately, linking abiotic factors and biotic interactions on niche width will be critical for understanding species-specific responses to climate change.
... temperature variation, humidity, or precipitation) influence the fundamental TPC. For example, TPCs are often considered to be left-skewed for physiological reasons (e.g. the response of thermoregulation proceeds faster at higher temperatures) (Woodin et al., 2013;Huey and Pianka, 2018), but our results and those of many other studies have shown that TPCs can be diverse in their shape (Dell et al., 2011;Monaco et al., 2017). Although we show that variation in the shape of TPCs can be due to interspecific competition, other types of biotic interactions, including mutualistic and host-parasites interactions (Cohen et al., 2017), should also be carefully considered and compared when quantifying fundamental TPCs. ...
Understanding how climate-mediated biotic interactions shape thermal niche width is critical in an era of global change. Yet, most previous work on thermal niches has ignored detailed mechanistic information about the relationship between temperature and organismal performance, which can be described by a thermal performance curve. Here, we develop a model that predicts the width of thermal performance curves will be narrower in the presence of interspecific competitors, causing a species’ optimal breeding temperature to diverge from that of its competitor. We test this prediction in the Asian burying beetle Nicrophorus nepalensis, confirming that the divergence in actual and optimal breeding temperatures is the result of competition with their primary competitor, blowflies. However, we further show that intraspecific cooperation enables beetles to outcompete blowflies by recovering their optimal breeding temperature. Ultimately, linking abiotic factors and biotic interactions on niche width will be critical for understanding species-specific responses to climate change.
... temperature variation, humidity, or precipitation) influence the fundamental TPC. For example, TPCs are often considered to be left-skewed for physiological reasons (e.g. the response of thermoregulation proceeds faster at higher temperatures) (Woodin et al., 2013;Huey and Pianka, 2018), but our results and those of many other studies have shown that TPCs can be diverse in their shape (Dell et al., 2011;Monaco et al., 2017). Although we show that variation in the shape of TPCs can be due to interspecific competition, other types of biotic interactions, including mutualistic and host-parasites interactions (Cohen et al., 2017), should also be carefully considered and compared when quantifying fundamental TPCs. ...
Understanding how climate-mediated biotic interactions shape thermal niche width is critical in an era of global change. Yet, most previous work on thermal niches has ignored detailed mechanistic information about the relationship between temperature and organismal performance, which can be described by a thermal performance curve. Here, we develop a model that predicts the width of thermal performance curves will be narrower in the presence of interspecific competitors, causing a species' optimal breeding temperature to diverge from that of its competitor. We test this prediction in the Asian burying beetle Nicrophorus nepalensis , confirming that the divergence in actual and optimal breeding temperatures is the result of competition with their primary competitor, blowflies. However, we further show that intraspecific cooperation enables beetles to outcompete blowflies by recovering their optimal breeding temperature. Ultimately, linking abiotic factors and biotic interactions on niche width will be critical for understanding species-specific responses to climate change.
... Before modelling field body temperature and water loss rates, we checked the deviation from normality of these dependent variables. Field body temperature was slightly left-skewed (À0.82), as it is usually the case in lizards (Huey & Pianka, 2018). Raw TEWL data was right skewed (0.62). ...
... Before modelling field body temperature and water loss rates, we checked the deviation from normality of these dependent variables. Field body temperature was slightly left-skewed (À0.82), as it is usually the case in lizards (Huey & Pianka, 2018). Raw TEWL data was right skewed (0.62). ...
The maintenance of optimal body temperatures has profound consequences on all aspects of ectotherms life history, like fitness and performance, and has been the subject of research for decades. In contrast, for reptiles, comparatively less is known on the potential effects of water balance on physiology, ecology and behaviour. In recent years, several recent studies are pointing out the importance of studying thermoregulation and water balance within the same framework. Here, we used a Mediterranean lizard, Psammodromus algirus, to investigate how field body temperature and water loss rates may vary among populations, between sexes and along an altitudinal gradient. We found little variation in field body temperatures among populations, while within each population, field body temperature may change daily, seasonally and differ between sexes. On the contrary, water loss rates decreased with elevation and showed no seasonal trend or difference between sexes. Microclimatic data indicated that highland lizards experienced more fluctuating conditions, a factor that may explain their lower water loss rates. Other factors, like inter‐population differences in ectoparasite intensity, might also contribute in explaining the observed patterns. We present here the first data for a lacertid lizard on the inter‐population variability and sexual difference in water loss rates and point out that water balance may play a fundamental role in regulating lizard activity during the hottest and driest period of the year.
... More broadly, skewness is an intrinsic feature of physiological processes, since they increase exponentially with temperature [54]. Recently, an extensive review of skewness in several desert lizard species concluded that left-skewness is common across geography and phylogeny, and does not correlate with body size or median body temperature [55]. Our results reinforce such a view, and the four species we tested reacted in a similar way to water deprivation (Fig 2). ...
Climate change is negatively affecting many species. The increase in mean air temperature is often associated with shifts in distribution, changes in phenology, and local extinctions. Other factors that only partially correlate with air temperature, like water shortage, may also
contribute to the negative consequences of climate change. Although the effect of temperature on lizards’ ecophysiology is highly studied, many lizards are also at risks of increased water loss and dehydration, which are predicted to increase under climate change. Here we
aimed for the first time to explore if lacertid lizards exposed to dehydration thermoregulate less precisely than hydrated lizards and if dehydrated lizards are less active, change the daily pattern of thermoregulation and balance water balance against thermoregulation. We exposed four lizard species with differences in the thermal preference to thermal gradients with or without a source of water. We measured preferred body temperatures, daily pattern
of thermoregulation, and the use of space. Dehydration negatively affected thermoregulation in all investigated species. Dehydrated lizards reduced their preferred body temperature and showed a species-specific pattern of hourly change in thermal preference. Furthermore, they more frequently used the colder parts of the gradients and spent more time hidden. Lizards experiencing dehydration may suffer a reduction in survival and fitness because of
poor thermoregulation. Similarly, they may spend more time hidden, waiting for more favourable weather conditions. Such inactivity may carry ecological costs especially in those regions that undergo either short or prolonged periods of droughts.
... To predict lizard abundances at survey sites, we used a 30-m raster land use land cover layer, which we aggregated as a binary raster (1 or 0) to represent either urban, agricultural, transportation, or mining-related development or none of these categories from the most recent year available (CalFIRE, 2015). Finally, lizard activity is strongly affected by temperature (Huey & Pianka, 2018;McCain, 2010). We used the average of the two ambient temperature measurements made during each survey to represent average temperature as a surveylevel covariate predicting lizard activity. ...
Multispecies wildlife monitoring across large geographical regions is important for effective conservation planning in response to expected impacts from climate change and land use. Unlike many species of birds, mammals, and amphibians which can be efficiently sampled using automated sensors including cameras and sound recorders, reptiles are often much more challenging to detect, in part because of their typically cryptic behavior and generally small body sizes. Although many lizard species are more active during the day which makes them easier to detect using visual encounter surveys, they may be unavailable for sampling during certain periods of the day or year due to their sensitivity to temperature. In recognition of these sampling challenges, we demonstrate application of a recent innovation in distance sampling that adjusts for temporary emigration between repeat survey visits. We used transect surveys to survey lizards at 229 sites throughout the Mojave Desert in California, USA, 2016. We estimated a total population size of 82 million (90% CI: 65–99 million) for the three most common species of lizards across this 66,830 km² ecoregion. We mapped how density at the 1‐km² scale was predicted to vary with vegetation cover and human development. We validated these results against independent surveys from the southern portion of our study area. Our methods and results demonstrate how multispecies monitoring programs spanning arid ecoregions can better incorporate information about reptiles.
... Although latitude affects the thermal biology of other lizards (Huey et al. 2009), this was not included herein because the latitudinal variation in the present dataset is low (Fig. 1), which, alongside a limited sample size would probable add more noise to the results. In addition, phylogeny was also not taken into account, as this study focused only on 1 family, and it would probably also add noise to the results (see also Huey & Pianka 2018). Nonetheless, the importance of testing the effects of geography and phylogeny on lizard (1) step 1 is the assessment of heterogeneity with the homogeneity test and the value of I 2 , (2) step 2 is the selection of the best moderator, according to the Logworth value with p-value correction, and (3) step 3 is the integration of effect sizes in final subsets (Ortega et al. 2016a). 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 thermoregulation is noteworthy (Felsenstein 1985;Garland et al. 2005). ...
The thermal ecology of ectotherms has been studied for almost two centuries, but additional attention is currently being paid to the issue in order to understand how organisms deal with the environment in a climate change context. However, a consensus is still far away, due to the large number of factors involved and their complex interactions. In this context, three analyses in lacertid lizards were carried out: (1) a meta‐analysis, to test for differences between body and air temperatures from 71 populations, (2) a meta‐analysis concerning correlations between body and air temperatures from 60 populations, and (3) a multimodel inference using thermoregulation effectiveness indices from 45 populations. The importance of different factors, including body size, habitat, insuraliry, altitude, climate, and season, was evaluated in all analyses in order to model the response variables. A strong seasonality effect was observed, with a consistent pattern of less effective thermoregulation in summer compared to other seasons. Altitude was the second most important factor, with a consistent higher thermoregulation effort in populations occurring at high elevations (> 1000 m asl). Other factors, such as insularity or body size, can also be important, but did not exhibit a clear pattern. Finally, thermoregulation was less affected by climate and habitat type.
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The comparative method has long been a fundamental exploratory tool in evolutionary biology, but this venerable approach was revolutionized in 1985, when Felsenstein published “Phylogenies and the Comparative Method” in The American Naturalist. This article forced comparative biologists to start thinking phylogenetically when conducting statistical analyses of correlated trait evolution rather than simply applying conventional statistical methods that ignore evolutionary relationships. It did so by introducing a novel analytical method (phylogenetically “independent contrasts”) that required a phylogenetic topology with branch lengths and that assumed a Brownian motion model of trait evolution. Independent contrasts enabled comparative biologists to avoid the statistical dilemma of nonindependence of species values, arising from shared ancestry, but came at the cost of needing a detailed phylogeny and of accepting a specific model of character change. Nevertheless, this article not only revitalized comparative biology but even encouraged studies aimed at estimating phylogenies. Felsenstein’s characteristically lucid and concise statement of the problem (il-lustrated with powerful graphics), coupled with an oncoming flood of new molecular data and techniques for estimating phylogenies, led Felsenstein’s 1985 article to become the second most cited article in the history of this journal. Here we present a personal review of comparative biology before, during, and after Joe’s article. For historical context, we append a perspective written by Joe himself that describes how his article evolved, unedited transcripts of reviews of his submitted manuscript, and a guide to some nontrivial calculations. These additional materials help emphasize that the process of science does not always occur gradually or predictably. © 2019 by The University of Chicago. 0003-0147/2019/19306-58868$15.00. All rights reserved.
Thermal performance curves (TPCs), which quantify how an ectotherm's body temperature (Tb) affects its performance or fitness, are often used in an attempt to predict organismal responses to climate change. Here, we examine the key ? but often biologically unreasonable ? assumptions underlying this approach; for example, that physiology and thermal regimes are invariant over ontogeny, space and time, and also that TPCs are independent of previously experienced Tb. We show how a critical consideration of these assumptions can lead to biologically useful hypotheses and experimental designs. For example, rather than assuming that TPCs are fixed during ontogeny, one can measure TPCs for each major life stage and incorporate these into stage-specific ecological models to reveal the life stage most likely to be vulnerable to climate change. Our overall goal is to explicitly examine the assumptions underlying the integration of TPCs with Tb, to develop a framework within which empiricists can place their work within these limitations, and to facilitate the application of thermal physiology to understanding the biological implications of climate change.
The impact of anthropogenic climate change on terrestrial organisms is often predicted to increase with latitude, in parallel with the rate of warming. Yet the biological impact of rising temperatures also depends on the physiological sensitivity of organisms to temperature change. We integrate empirical fitness curves describing the thermal tolerance of terrestrial insects from around the world with the projected geographic distribution of climate change for the next century to estimate the direct impact of warming on insect fitness across latitude. The results show that warming in the tropics, although relatively small in magnitude, is likely to have the most deleterious consequences because tropical insects are relatively sensitive to temperature change and are currently living very close to their optimal temperature. In contrast, species at higher latitudes have broader thermal tolerance and are living in climates that are currently cooler than their physiological optima, so that warming may even enhance their fitness. Available thermal tolerance data for several vertebrate taxa exhibit similar patterns, suggesting that these results are general for terrestrial ectotherms. Our analyses imply that, in the absence of ameliorating factors such as migration and adaptation, the greatest extinction risks from global warming may be in the tropics, where biological diversity is also greatest.
• biodiversity
• fitness
• global warming
• physiology
• tropical
Rectal body temperatures (BTs) of tuataras (Sphenodon punctatus) and of endemic, ovoviviparous gekkonid lizards—mainly Hoplodactylus maculatus (=H. pacificus) and Heteropholis manukanus—were taken together with ambient temperatures during early summer 1970 in areas of central New Zealand. The results, combined with earlier data, enable a number of conclusions to be drawn. (a) The preferred body temperature of heliotherm reptiles is best deduced from the mode of rectal BTS taken in the field, but that of non-heliotherms, when unimodal, from the median or mean. (b) Among Gekkonoidea, specific thermal relations are highly variable in several ways. (c) Sphenodon foraged on cool nights at BTs of 10.5–12.5°c, yet basked in the forest by day at BTs up to 24°c; in pasture it apparently basks within the burrow entrance. (d) Similarly, H. maculatus foraged at night at BTs of 10–13°c, but by day thermoregulated at BTs up to 33°c by ‘indirect basking’ (under thin cover) or ‘protected basking’ (in crevices penetrated by solar radiation). The average BT of females was 2°c higher than that of males, presumably because many females were gravid. (e) H. manukanus is (tertiarily) diurnal, and thermoregulated by basking up to a BT of 31 °c. Towards evening it apparently cooled down voluntarily. (f) Whereas a high daytime BT probably assists digestion in nocturnal foragers, a voluntary low night-time BT in diurnal reptiles may help to conserve energy.
Increases in the frequency, severity and duration of temperature extremes are anticipated in the near future. Although recent work suggests that changes in temperature variation will have disproportionately greater effects on species than changes to the mean, much of climate change research in ecology has focused on the impacts of mean temperature change. Here, we couple fine-grained climate projections (2050-2059) to thermal performance data from 38 ectothermic invertebrate species and contrast projections with those of a simple model. We show that projections based on mean temperature change alone differ substantially from those incorporating changes to the variation, and to the mean and variation in concert. Although most species show increases in performance at greater mean temperatures, the effect of mean and variance change together yields a range of responses, with temperate species at greatest risk of performance declines. Our work highlights the importance of using fine-grained temporal data to incorporate the full extent of temperature variation when assessing and projecting performance.
Aim
Temperature influences most components of animal ecology and life history – but what kind of temperature? Physiologists usually examine the influence of body temperatures, while biogeographers and macroecologists tend to focus on environmental temperatures. We aim to examine the relationship between these two measures, to determine the factors that affect lizard body temperatures and to test the effect of both temperature measures on lizard life history.
Location
World‐wide.
Methods
We used a large (861 species) global dataset of lizard body temperatures, and the mean annual temperatures across their geographic ranges to examine the relationships between body and mean annual temperatures. We then examined factors influencing body temperatures, and tested for the influence of both on ecological and life‐history traits while accounting for the influence of shared ancestry.
Results
Body temperatures and mean annual temperatures are uncorrelated. However, accounting for activity time (nocturnal species have low body temperatures), use of space (fossorial and semi‐aquatic species are ‘colder’), insularity (mainland species are ‘hotter’) and phylogeny, the two temperatures are positively correlated. High body temperatures are only associated with larger hatchlings and increased rates of biomass production. Annual temperatures are positively correlated with clutch frequency and annual longevity, and negatively correlated with clutch size, age at first reproduction and longevity.
Main conclusions
Lizards with low body temperatures do not seem to have ‘slower’ life‐history attributes than species with high body temperatures. The longer seasons prevalent in warm regions, and physiological processes that operate while lizards are inactive (but warm enough), make environmental temperatures better predictors of lizard life‐history variation than body temperatures. This surprisingly greater effect of environmental temperatures on lizard life histories hints that global warming may have a profound influence on lizard ecology and evolution.
Animals and plants often exhibit narrow ranges of thermal preference in variable environments; fitness-enhancing activities such as reproduction and growth tend to be concentrated during times in which body temperature lies within that narrow range. The relation between fitness-associated performance and temperature is modeled by a performance curve defined by two traits: performance breadth (T-br), a measure of thermal specialization, and the critical maximum temperature for performance (T-max). Optimality models are used to define the fitness landscape for these two traits under several different patterns of within- and among-generation variation in temperature. In constant environments and environments in which there is significant within-generation variation, specialists with narrow preference ranges are the favored phenotype. In environments in which there is considerable among-generation but little within-generation variation, generalists with broad preference ranges are favored. Specialists in a constant environment have a mean fitness an order of magnitude higher than any phenotype in more variable environments, demonstrating that homeostatic mechanisms can confer a large fitness advantage. In contrast to previous models of environmental tolerance, these performance models suggest that increasing temporal environmental variation can favor the evolution of thermal specialization.
At the end of their breeding season, male side-blotched lizards, Uta stansburiana from western Colorado decreased their home range to a size not different from that of females. Both males and females showed a high degree of overlap in home ranges, not found in populations previously studied in Texas. Uta's sprint speed was dependent on body temperature, with maximum sprint speed occurring at body temperatures between 35°C and 38°C, with lower speeds at higher and lower temperatures. An energy budget model was used to predict the range of body temperatures (and thus sprint speeds) available to lizards in four microhabitats within each animal's home range. Predicted body temperatures were converted to a space-time index. The distribution of the space-time index in each microhabitat was used to predict the spatial and temporal distributions of lizards. Predicted distributions accurately reflected the measured distributions of lizards in the morning and late afternoon, but did not reflect the measured distributions during midday. These inconsistencies are thought to be the result of lizard responses to other temperature-dependent processes, such as evaporite water loss.
Behavioural regulation of body temperature in thermally heterogeneous habitats requires different amounts of time that could otherwise be dedicated to foraging and social activities. In this study I examined how four populations of the lizard Zootoca vivipara along an altitudinal gradient (250-1450 m) adjust their thermal-physiology traits and thermoregulatory behaviour to compensate for increasing time costs of thermoregulation. I focused on variation in several physiological (set-point temperature, heating rate) and behavioural traits (microhabitat selection, basking frequency, extent of thermoregulation). To estimate potential time spent basking and foraging by lizards that were not employing any behavioural compensatory mechanism, I used a simple biophysical model of thermoregulation, including information about operative temperatures at the study sites, selected temperature range, and heating/cooling rates. Time costs of thermoregulation for each population were calculated as potential time spent basking relative to time spent foraging. Operative temperatures varied among study sites, resulting in different time costs of thermoregulation. Lizards at 1450 m should spend about 50% more time basking than those at 250 m. I found that the only mechanism which potentially compensated for the higher time costs incurred at high altitudes was a shift in the choice of basking sites. Lizards thermoregulated with similar accuracy and effectiveness over the 1200-m altitudinal range, indicating that there were no adjustments in the extent of thermoregulation. The observed basking frequencies of lizards were highly correlated with potential time spent basking without behavioural adjustments, suggesting a minor compensatory effect of thermoregulatory behaviour. Lizards responded to higher time costs of thermoregulation primarily by allocating different amounts of time to basking. These results suggest that Z. vivipara regulated body temperature at the expense of time that could be devoted to other activities.
Phrynosoma douglassi brevirostre has a median body temperature (Tb) of 32.9 °C in southeastern Alberta, with a sharply peaked and negatively skewed Tb frequency. The range of the Tb frequency distribution in daylight hours during the active season (activity temperature range) is wide (20.0 °C). The realized thermal niche is approximately 7 °C wide, centred loosely on the median Tb. The populations examined are active over a wide range of ambient temperatures. With regard to substrate temperature and air temperature at 10 cm, the populations examined are intermediate between thermoconformity and thermal independence, but with regard to air temperature at 1 m they display more thermal independence. Habitat use is most varied on sunny days. Tb is not significantly different between situations, indicating that shuttling is an important thermoregulatory strategy. There is a significant difference in median Tb between small lizards and large lizards, but not between either of these size groups and medium-sized lizards. Small lizards utilize significantly warmer substrates than the other two size groups. Large lizards are more closely coupled to substrate temperature, and less closely coupled to air temperature, than small lizards. Medium-sized lizards have a wider realized thermal niche than the other two size groups and are intermediate in their thermal relations relative to the other two size groups.
Control of body temperature in lizards is largely dependent upon their appropriate exploitation of their thermal environment. Desert Iguanas achieve behavioral thermoregulation by changes in location and posture within dynamic thermal environmental mosaics, achieving a body temperature distribution whose median is 38.5 C. This distribution is negatively skewed with the central 95, 68, and 50% of the distribution, ranging between 37.0-39.5, 36.1-40.1, and 33.2-41.8 C, respectively. Such skewness is what might be expected if the mechanism for thermoregulation depended upon detection of deviations from a set point of some rate process that in turn has a direct exponential relation to body temperature. Since physiological activity bears a direct exponential relation to temperature, corresponding body-temperature distributions would necessarily be negatively skewed. Such a system of regulation might also equally apply to other animals whose distribution of body temperatures is negatively skewed. Desert iguanas may exhibit differences between head and trunk temperatures of several degrees, and may hold head temperature at or below 44 degrees by panting and postural means, while the trunk temperature is allowed to go 3 degrees higher. Under extremely hot desert conditions that otherwise would confine them to their burrows, desert iguanas will permit body temperatures to rise above the preferred level to 42 C and higher. Body temperatures are also permitted to rise well above the preferred level during territorial fighting behavior and in the presence of predators (46.5 C is the highest recorded) but generally not during feeding and courtship.
It is predicted that climate change will cause species extinctions and distributional shifts in coming decades, but data to
validate these predictions are relatively scarce. Here, we compare recent and historical surveys for 48 Mexican lizard species
at 200 sites. Since 1975, 12% of local populations have gone extinct. We verified physiological models of extinction risk
with observed local extinctions and extended projections worldwide. Since 1975, we estimate that 4% of local populations have
gone extinct worldwide, but by 2080 local extinctions are projected to reach 39% worldwide, and species extinctions may reach
20%. Global extinction projections were validated with local extinctions observed from 1975 to 2009 for regional biotas on
four other continents, suggesting that lizards have already crossed a threshold for extinctions caused by climate change.
The field thermal biology of sympatric Anolis cooki and A. cristatellus were evaluated in January and in August in desert scrub forest at Playa de Tamarindo near Guanica, Puerto Rico. Data on randomly positioned copper models of lizards, each equipped with a built-in thermocouple, established null hypotheses about basking frequency and operative temperatures (T
e) against which the behavior and body temperatures (T
b) of live lizards were evaluated. Both species exhibited non-random hourly basking rates (more marked in cristatellus than in cooki), and cristatellus was virtually inactive during the warm mid-day hours. The relationship between lizards' T
b and randomly sampled T
e differed between the species: cristatellus's mean T
b was 2 to 3 C lower than randomly sampled mean T
e in both months, whereas cooki's mean T
b was slightly higher than mean T
e in January and slightly lower in August. Although cooki's mean T
b was higher than that of cristatellus in both months, the T
b's of the two species overlapped substantially over an annual cycle. Given the similarities in their field active T
b and the low thermal heterogeneity among microsites at Playa de Tamarindo, these species appear not to partition the thermal environment there in a coarse-grained way. Instead, the relatively small differences in their field active T
b probably result from small differences in their use of similar microhabitats within their mutually exclusive territories. Thermal resource partitioning by territorial animals is unlikely unless thermal heterogeneity is coarse-grained in relation to territory size.
This study was aimed to determine whether thermoregulation by juvenile garter snakes is affected by developmental temperature imposed by their mothers during pregnancy. Maternal temperature treatments were created by maintaining six groups of females at six different constant temperatures (24–32°C) during pregnancy. A seventh group of females was allowed to thermoregulate during pregnancy. The body temperatures of juveniles (N = 324) on thermal gradients were monitored in the morning and afternoon for 5 consecutive days, beginning 5 days after birth. The body temperatures of juveniles were consistent, apparently heritable and remarkably stereotyped. Juvenile body temperature was virtually unaffected by maternal temperature treatment.
Contemporary theory for thermal adaptation of ectothermic metazoans focuses on the maximization of energy gain and performance (locomotion and foraging). Little consideration is given to the selection for mechanisms that minimize resting energy loss in organisms whose energy gain is severely constrained. We tested a hypothetical framework for thermal performance of locomotor activity (a proxy for energy gain) and resting metabolism (a proxy for energy loss) in energetically compromised snails in the littoral fringe zone, comparing this with existing theory. In contrast to theory, the thermal ranges and optima for locomotor performance and metabolic performance of Echinolittorina malaccana are mismatched, and energy gain is only possible at relatively cool temperatures. To overcome thermal and temporal constraints on energy gain while experiencing high body temperatures (23-50°C), these snails depress resting metabolism between 35 and 46°C (thermally insensitive zone). The resulting bimodal relationship for metabolism against temperature contrasts with the unimodal or exponential relationships of most ectotherms. Elevation of metabolism above the breakpoint temperature for thermal insensitivity (46°C) coincides with the induction of a heat shock response, and has implications for energy expenditure and natural selection. Time-dependent mortality is initiated at this breakpoint temperature, suggesting a threshold above which the rate of energy demand exceeds the capacity for cellular energy generation (rate of ATP turnover). Mortality in a thermal range that elevates rather than limits aerobic metabolism contrasts with the hypothesis that cellular oxygen deficiency underlies temperature-related mortality. The findings of this study point to the need to incorporate aspects of resting metabolism and energy conservation into theories of thermal adaptation.
To understand the effects of temperature on biological systems, we compile, organize, and analyze a database of 1,072 thermal responses for microbes, plants, and animals. The unprecedented diversity of traits (n = 112), species (n = 309), body sizes (15 orders of magnitude), and habitats (all major biomes) in our database allows us to quantify novel features of the temperature response of biological traits. In particular, analysis of the rising component of within-species (intraspecific) responses reveals that 87% are fit well by the Boltzmann-Arrhenius model. The mean activation energy for these rises is 0.66 ± 0.05 eV, similar to the reported across-species (interspecific) value of 0.65 eV. However, systematic variation in the distribution of rise activation energies is evident, including previously unrecognized right skewness around a median of 0.55 eV. This skewness exists across levels of organization, taxa, trophic groups, and habitats, and it is partially explained by prey having increased trait performance at lower temperatures relative to predators, suggesting a thermal version of the life-dinner principle-stronger selection on running for your life than running for your dinner. For unimodal responses, habitat (marine, freshwater, and terrestrial) largely explains the mean temperature at which trait values are optimal but not variation around the mean. The distribution of activation energies for trait falls has a mean of 1.15 ± 0.39 eV (significantly higher than rises) and is also right-skewed. Our results highlight generalities and deviations in the thermal response of biological traits and help to provide a basis to predict better how biological systems, from cells to communities, respond to temperature change.
The past 30 years have seen a revolution in comparative biology. Before that time, systematics was not at the forefront of the biological sciences, and few scientists considered phylogenetic relationships when investigating evolutionary questions. By contrast, systematic biology is now one of the most vigorous disciplines in biology, and the use of phylogenies not only is requisite in macroevolutionary studies but also has been applied to a wide range of topics and fields that no one could possibly have envisioned 30 years ago. My message is simple: phylogenies are fundamental to comparative biology, but they are not the be-all and end-all. Phylogenies are powerful tools for understanding the past, but like any tool, they have their limitations. In addition, phylogenies are much more informative about pattern than they are about process. The best way to fully understand the past-both pattern and process-is to integrate phylogenies with other types of historical data as well as with direct studies of evolutionary process.
Thermoregulatory processes were compared among three-size groups of free-ranging Komodo dragons (Varanus komodoensis) comprising small (5–20 kg), medium (20–40 gm) and large (40–70 kg) lizards. While all size groups maintained a similar preferred body temperature of ≈35∘C, they achieved this end point differently. Small dragons appeared to engage in sun shuttling behavior more vigorously than large dragons as represented by their greater frequency of daily ambient temperature and light intensity changes as well as a greater activity and overall exposure to the sun. Large dragons were more sedentary and sun shuttled less. Further, they appear to rely to a greater extent on microhabitat selection and employed mouth gaping evaporative cooling to maintain their preferred operational temperature and prevent overheating. A potential ecological consequence of size-specific thermoregulatory habits for dragons is separation of foraging areas. In part, differences in thermoregulation could contribute to inducing shifts in predatory strategies from active foraging in small dragons to more sedentary sit-and-wait ambush predators in adults.
Benjamini and Hochberg suggest that the false discovery rate may be the appropriate error rate to control in many applied multiple testing problems. A simple procedure was given there as an FDR controlling procedure for independent test statistics and was shown to be much more powerful than comparable procedures which control the traditional familywise error rate. We prove that this same procedure also controls the false discovery rate when the test statistics have positive regression dependency on each of the test statistics corresponding to the true null hypotheses. This condition for positive dependency is general enough to cover many problems of practical interest, including the comparisons of many treatments with a single control, multivariate normal test statistics with positive correlation matrix and multivariate $t$. Furthermore, the test statistics may be discrete, and the tested hypotheses composite without posing special difficulties. For all other forms of dependency, a simple conservative modification of the procedure controls the false discovery rate. Thus the range of problems for which a procedure with proven FDR control can be offered is greatly increased.
Desert lizards are typically either widely foraging or sit-and-wait predators, and these foraging modes are correlated with major differences in ecology. Foraging mode is related to the type of prey eaten by lizards. Widely foraging lizards in the Kalahari desert, the Western Australian desert, and the North American desert generally eat more prey that are sedentary, unpredictably distributed, and clumped (e.g., termites) or that are large and inaccessible (inactive scorpions) than do sit-and-wait lizards. In contrast, sit-and-wait lizards eat more prey that are active. Foraging mode also appears to influence the types of predators that in turn eat the lizards. For example, a sit-and-wait snake eats predominately widely foraging lizards. Crossovers in foraging mode thus exist between trophic levels. Widely foraging lizards may also encounter predators more frequently, as suggested by analyses of relative tail lengths; but tail break frequencies are ambiguous. Daily maintenance energetic expenditures of widely foraging lizards appear to be about 1.3-1.5 times greater than those of sit-and-wait lizards in the same habitats, but gross food gains are about 1.3-2.1 times greater. Widely foraging species also have lower relative clutch volumes, apparently in response to enhanced risks of predation. Foraging mode within one species varies with changes in food availability. Physiology, morphology, and risk of predation might generally restrict the flexibility of foraging mode. Because foraging mode constrains numerous important aspects of ecology, any general model of foraging velocity must be complex.
synopsis. An understanding of interactions between the thermal physiology and ecology
of ectotherms remains elusive, partly because information on the relative performance of
whole-animal physiological systems at ecologically relevant body temperatures is limited.
After discussing physiological systems that have direct links to ecology (e.g., growth,
locomotor ability), we review analytical methods of describing and comparing certain as?
pects of performance (including optimal temperature range, thermal performance
breadth), apply these techniques in an example on the thermal sensitivity of locomotion in
frogs, and evaluate potential applications.
Body temperature distributions of ectotherms which are free to select any environmental temperature throughout a wide range usually are negatively skewed, that is, a wider range of selected temperature falls below the median than above it. This negative skewness can be understood as the consequence of the regulation of a temperaturedependent rate process whose rate, as an exponential function of temperature, is maintained within a normal curve of error. Analysis of frequency distributions of body temperature show that they can be characterized by the median and two additional parameters. One of these can be used to approximate the Q 10 of the hypothetical rate process. Mathematical and graphical techniques for the description of body temperature distributions are developed, and are proposed as useful tools for comparative and experimental studies of body temperature regulation.
Variation in traits across species or populations is the outcome of both environmental and historical factors. Trait variation is therefore a function of both the phylogenetic and spatial context of species. Here we introduce a method that, within a single framework, estimates the relative roles of spatial and phylogenetic variations in comparative data. The approach requires traits measured across phylogenetic units, e.g. species, the spatial occurrences of those units and a phylogeny connecting them. The method modifies the expected variance of phylogenetically independent contrasts to include both spatial and phylogenetic effects. We illustrate this approach by analysing cross-species variation in body mass, geographical range size and species-typical environmental temperature in three orders of mammals (carnivores, artiodactyls and primates). These species attributes contain highly disparate levels of phylogenetic and spatial signals, with the strongest phylogenetic autocorrelation in body size and spatial dependence in environmental temperatures and geographical range size showing mixed effects. The proposed method successfully captures these differences and in its simplest form estimates a single parameter that quantifies the relative effects of space and phylogeny. We discuss how the method may be extended to explore a range of models of evolution and spatial dependence.
Lizards thermoregulate by behavioral and physiological adjustments. The resultant control over metabolic processes is generally assumed to be beneficial. However, these thermoregulatory adjustments have associated costs which, if extensive, make thermoregulation impractical. We extend this idea into an abstract mathematical, cost-benefit model of thermoregulation in lizards. Investigation of the model leads to a set of predictions which includes: (1) the physiologically optimal temperature is not always the ecologically optimal temperature; (2) thermoregulation is beneficial only when associated costs are low; (3) thermal specialists will normally thermoregulate more carefully than thermal generalists unless costs are high; and (4) lizards will thermoregulate more carefully if productivity of the habitat is increased or if exploitation competition is reduced. Data on lizards, where available, generally agree with these predicitions.
The view that behavior and physiological performance are tightly coadapted is a central principle of physiological ecology. Here, we test this principle using a comparative study of evolutionary patterns in thermal preferences and the thermal dependence of sprinting in some Australian skinks (Lygosominae). Thermal preferences (Tp ) differ strikingly among genera (range 24° to 35°C), but critical thermal maxima (CTMax) (range 38° to 45°C) and optimal temperatures for sprinting (To , 32° to 35°C) vary less. Diurnal genera have relatively high Tp , To , and CTMax. In contrast, nocturnal genera have low Tp but have moderate to high To and CTMax. Both nonphylogenetic and phylogenetic (minimum-evolution) approaches suggest that coadaptation is tight only for genera with high Tp . Phylogenetic analyses suggest that low Tp and, thus, partial coadaptation are evolutionarily derived, indicating that low thermal preferences can evolve, even if this results in reduced performance. In one instance, thermal preferences and the thermal dependence of sprinting may have evolved in opposite directions, a phenomenon we call "antagonistic coadaptation." We speculate on factors driving partial coadaptation and antagonistic coadaptation in these skinks.
Temperature pervasively impacts the phenotypes and distributions of organisms. These thermal effects generate strong selective pressures on behaviour, physiology, and life history when environmental temperatures vary over space and time. Despite this fact, progress toward a quantitative theory of thermal adaptation has lagged behind empirical descriptions of patterns and processes. This book draws on current evolutionary paradigms (optimization, quantitative genetics, and genetic algorithms) to establish a theory of thermal adaptation. It initially focuses on simple models that describe the evolution of thermosensitivity, thermoregulation, or acclimation. Later chapters focus on more complex models describing the coadaptation of traits or the coevolution of species. Throughout the book, various lines of evidence are used to question the major assumptions of these models. Furthermore, the predictions of these models are confronted with experimental and comparative data. Empirical examples represent a wide range of taxa, including bacteria, plants, fungi, and animals. The result is a synthesis of theoretical and empirical studies of thermal biology that offers insights about evolutionary processes.
Phrynosoma douglassi brevirostre has a median body temperature (Tb) of 32.9oC in SE Alberta, with a sharply peaked and negatively skewed Tb frequency. The range of the Tb frequency distribution in daylight hours during the active season (activity temperature range) is wide (20.0oC). The realized thermal niche is approx 7oC wide, centred loosely on the median Tb. The populations examined are active over a wide range of ambient temperatures. With regard to substrate temperature and air temperature at 10cm, the populations are intermediate between thermoconformity and thermal independence, but with regard to air temperature at 1m they display more thermal independence. Habitat use is most varied on sunny days. Shuttling is an important thermoregulatory strategy. There is a significant difference in median Tb between small and large lizards, but not between either of these size groups and medium-sized lizards. Small lizards utilize significantly warmer substrates than the other 2 size groups. Large lizards are more closely coupled to substrate temperature, and less closely coupled to air temperature, than small lizards. Medium-sized lizards have a wider realized thermal niche than the other 2 size groups and are intermediate in their thermal relations relative to the other 2 size groups. -Authors
In this review we consider how small-scale temporal and spatial variation in body temperature, and biochemical/physiological variation among individuals, affect the prediction of organisms' performance in nature. For 'normal' body temperatures - benign temperatures near the species' mean - thermal biology traditionally uses performance curves to describe how physiological capabilities vary with temperature. However, these curves, which are typically measured under static laboratory conditions, can yield incomplete or inaccurate predictions of how organisms respond to natural patterns of temperature variation. For example, scale transition theory predicts that, in a variable environment, peak average performance is lower and occurs at a lower mean temperature than the peak of statically measured performance. We also demonstrate that temporal variation in performance is minimized near this new 'optimal' temperature. These factors add complexity to predictions of the consequences of climate change. We then move beyond the performance curve approach to consider the effects of rare, extreme temperatures. A statistical procedure (the environmental bootstrap) allows for long-term simulations that capture the temporal pattern of extremes (a Poisson interval distribution), which is characterized by clusters of events interspersed with long intervals of benign conditions. The bootstrap can be combined with biophysical models to incorporate temporal, spatial and physiological variation into evolutionary models of thermal tolerance. We conclude with several challenges that must be overcome to more fully develop our understanding of thermal performance in the context of a changing climate by explicitly considering different forms of small-scale variation. These challenges highlight the need to empirically and rigorously test existing theories.
© 2015. Published by The Company of Biologists Ltd.
An increase in flight distance with a decrease in body temperature in 32 male Anolis lineatopus at Mona, Jamaica, supports more casual field observations that these lizards are shier when they are cold than when they are warm. It is suggested that this increased shyness compensates for the slowing effects of lower temperatures on biological processes and so helps protect these arboreal lizards from warm-blooded predators.
Body temperature relations of nine species of lizards from Baja California were investigated. Both endogenous and exogenous factors appear to contribute to body temperature variation. Positive correlations between air and body temperatures were found for four species. Besides such environmental effects, observations and experiments suggest that species of lizards occurring in thermally diverse habitats have greater tolerances to extreme body temperatures than do forms occurring in less diverse habitats. A survey of the literature points to a correlation between the number of species and the variance of the mean body temperatures within a group of closely related forms.
I studied various aspects of the life history of Cnemidophorus tigris over much of its geographic range, from southern Idaho through southern Arizona and into northern Sonora. In the northern parts of its range, Cnemidophorus usually emerge from hibernation in May, and most adults aestivate during the midsummer months, but in the south the animals are active from April through late August. The seasonal period of activity is therefore considerably shorter in the north. Daily periods of activity are of similar duration from north to south, although the time of emergence tends to be later on northern areas. There is a significant positive correlation between estimated abundance and the total precipitation during the last 5 years, suggesting that the abundance of this species is determined by food supply. There is a latitudinal cline in the mean body temperature of active lizards, with northern populations being active at lower air and body temperature. Whereas termites constitute the major food of southern lizards, beetles and grasshoppers are the primary food of northern lizards. Some seasonal dietary trends are described. The numbers and biomass of predatory lizards, birds, and snakes increase from north to south; correlated with this is a latitudinal increase in the percentage of lizards with broken regenerated tails. Hence there is probably greater predation on southern lizards. The length of fat bodies in C. tigris is not correlated with latitude but shows an inverse correlation with the long-term average annual precipitation. It is suggested that lizards from less productive areas must allow themselves a greater margin of safety due to the more probable occurrence of drought. Northern lizards breed only once during the short northern season but lay significantly larger clutches than southern lizards, which lay at least two clutches annually. Clutch size appears to be flexible in response to feeding conditions; in one study area with a low long-term mean precipitation, females laid larger clutches during the second year, after supranormal rainfall. There is a significant correlation between mean number of eggs per clutch and the deviation of the short term (last 5 years) mean precipitation from the long-term mean precipitation. A number of facts suggest that there was a decided population @'crash@' on one study area during the period of study. Competition is briefly discussed and it is suggested that if there is any latitudinal trend, it is most likely in the direction of greater competition among southern lizards. In the conclusions, I suggest that the ecological challenges facing the northern populations are primarily physical, largely climatic ones, biotic interactions (particularly predation) assume relatively greater importance to southern lizards.
We discuss seasonal variation in thermoregulatory behavior and its consequences on body temperature for 12 species of diurnal lizards in the southern Kalahari semidesert of Africa and also evaluate several methods of attempting to document thermoregulatory behavior using a descriptive data base. Lizards vary time of activity among seasons, which limits the variation in ambient conditions actually experienced. Ground-dwelling lizards and probably arboreal lizards move nonrandomly with respect to sun and shade; thus the percentage of lizards in sun in inversely proportional to air temperature. Arboreal lizards shift to higher perches at midday in summer and to logs or ground in winter thus decreasing and increasing incident heat loads, respectively. Both juveniles and adults of 3 species, only juveniles of 2 species, and only adults in 1 species are active in winter: both adults and juveniles of 6 species brumate [= hibernate]. Mean body temperature (T"b) varies within days and among months and is positively correlated with corresponding mean air temperature (T"a) in almost all species. Nonetheless, correlation and regression analysis suggests that thermoregulatory behaviors reduce the impact of variations in ambient conditions on Kalahari lizards. The mean T"b of different species reflect evolutionary relationships. In summer, mean T"b is proportional to the percentage of lizards in sun and with the tendency of lizards to be active only in summer. Thus, lizards with inferred low optimal temperatures are active during more months of the year.
Modern comparative methods allow the examination of the probable course of evolution in a lineage of lizards (family Varanidae, genus Varanus). Within this genus, body mass varies by nearly a full five orders of magnitude. The fossil record and present geographical distribution suggest that varanids arose over 65 million yr ago in Laurasia and subsequently dispersed to Africa and Australia. Two major lineages have undergone extensive adaptive radiations within Australia: one evolved dwarfism (subgenus Odatria, pygmy monitors), whereas the other Australian lineage (subgenus Varanus) remained large, and several of its members evolved gigantism. Body sizes of extant varanid species are plotted on a phylogeny, and probable sizes at ancestral nodes are inferred from those of their descendants. Felsenstein's method of phylogenetically independent contrasts, coupled with information on branch lengths, is exploited to identify several likely instances of relatively rapid evolution of body size, both between and within clades. Numerous questions about the evolution of size in this genus within a historical/geographical perspective remain to be answered.
The tolerance ranges (total ranges of temperatures over which lizards can locomote) of tropical lizards are narrower than those of temperate-zone lizards, apparently because tropical lizards experience a narrower total range of body temperatures, and tolerance range is correlated with total body-temperature range. In contrast, the performance breadths (ranges of temperatures over which sprint speed is high) of tropical lizards are not consistently narrower than those of temperate-zone lizards. This may reflect many lizards' use of behavior for thermoregulation when they are active in the field, with the result that their body-temperature variabilities are independent of latitudinal patterns in ambient temperatures. Even so, performance breadths are usually correlated with the variability in body temperatures of lizards that are active in the field (when high sprint speeds should be important). -from Author
Species-rich adaptive radiations typically diversify along several distinct ecological axes, each characterized by morphological, physiological, and behavioral adaptations. We test here whether different types of adaptive traits share similar patterns of evolution within a radiation by investigating patterns of evolution of morphological traits associated with microhabitat specialization and of physiological traits associated with thermal biology in Anolis lizards. Previous studies of anoles suggest that close relatives share the same "structural niche" (i.e., use the same types of perches) and are similar in body size and shape, but live in different "climatic niches" (i.e., use habitats with different insolation and temperature profiles). Because morphology is closely tied to structural niche and field active body temperatures are tied to climatic niches in Anolis, we expected phylogenetic analyses to show that morphology is more evolutionarily conservative than thermal physiology. In support of this hypothesis, we find (1) that thermal biology exhibits more divergence among recently diverged Anolis taxa than does morphology; and (2) diversification of thermal biology among all species often follows diversification in morphology. These conclusions are remarkably consistent with predictions made by anole biologists in the 1960s and 1970s.
A transformation to approximate normality of the null distribution of g1, the standardized third sample moment, is given. Monte Carlo simulations and checks with an existing table indicate it is very accurate. It is suggested it can be used in performing tests of normality against skewed alternatives.
The view that behavior and physiological performance are tightly coadapted is a central principle of physiological ecology. Here, we test this principle using a comparative study of evolutionary patterns in thermal preferences and the thermal dependence of sprinting in some Australian skinks (Lygosominae). Thermal preferences (To) differ strikingly among genera (range 24⚬ to 35⚬C), but critical thermal maxima (CTMax) (range 38⚬ to 45⚬C) and optimal temperatures for sprinting (To, 32⚬ to 35⚬C) vary less. Diurnal genera have relatively high Tp, To, and CTMax. In contrast, nocturnal genera have low To but have moderate to high To and CTMax. Both nonphylogenetic and phylogenetic (minimum-evolution) approaches suggest that coadaptation is tight only for genera with high Tp. Phylogenetic analyses suggest that low To and, thus, partial coadaptation are evolutionarily derived, indicating that low thermal preferences can evolve, even if this results in reduced performance. In one instance, thermal preferences and the thermal dependence of sprinting may have evolved in opposite directions, a phenomenon we call "antagonistic coadaptation." We speculate on factors driving partial coadaptation and antagonistic coadaptation in these skinks.
Adult brown trout were captured near a thermal discharge, tagged with radiotransmitters, released at the point of capture and tracked from a stationary position to determine plume-use and temperature selection over the annual cycle. Patterns of residence and temperature selection in the thermal gradient were highly variable both within and among seasons. Residence in the gradient and dispersion of body temperatures (T
b
) decreased with increased ambient temperature while selected temperatures and skewness of body temperature distributions were directly related to ambient temperature. The variability of responses and the apparent partitioning of thermal habitat among adult brown trout were functions of sex, date and forage density, as well.On an annual basis, brown trout selected a modal temperature of 12C, the reported optimum metabolic temperature for the species. The upper preferred temperature for adult brown trout in Lake Michigan is estimated to be 16C. Male and female trout often selected similar temperatures, but in late summer females selected 18C and demonstrated negative skewness while males selected 15C with high positive skewness. The reverse was true in October when the selected temperature of males was 18C (females, 12C), and the distribution of male body temperatures was negatively skewed.Brown trout spent more time in the heated gradient during winter, spring, and fall, resulting in high dispersions of body temperature compared to fish at ambient temperature. Thermal niche breadth (dispersion of T
b
) of plume-resident fish was 3.5C on an annual basis, while the annual dispersion of water temperatures in ambient areas (T
a
) was 4.1C. Over the short term (months) dispersion in T
b
was considerably higher than dispersion in T
a
.
Many biologists now recognize that environmental variance can exert important effects on patterns and processes in nature that are independent of average conditions. Jensen’s inequality is a mathematical proof that is seldom mentioned in the ecological literature but which provides a powerful tool for predicting some direct effects of environmental variance in biological systems. Qualitative predictions can be derived from the form of the relevant response functions (accelerating versus decelerating). Knowledge of the frequency distribution (especially the variance) of the driving variables allows quantitative estimates of the effects. Jensen’s inequality has relevance in every field of biology that includes nonlinear processes.
Accurately assessing free-ranging animals’ patterns of surface activity and refuge use is critical, yet fundamentally challenging for biologists and wildlife managers. We evaluate the accuracy of an automated technique—temperature-based activity estimation (TBAE)—in estimating surface activity and refuge use patterns of two sympatric reptiles, the western diamond-backed rattlesnake (Crotalus atrox) and the Gila monster (Heloderma suspectum) in the Sonoran Desert. TBAE derived from a comparison of body temperature to shaded air temperature was effective in estimating the overall percent surface activity for both rattlesnakes (observed surface activity 51.8%, TBAE estimated surface activity 48.2%) and Gila monsters (observed 22.3%, TBAE 24.5%). There was, however, considerable interspecific difference in the effectiveness of TBAE in predicting surface activity at specific time points; TBAE was far more accurate for Gila monsters than for rattlesnakes (96% vs. 66% time point-specific accuracy, respectively). We assert that, when validated, TBAE can be used to yield concurrent and accurate body temperatures and activity estimates for multiple free-ranging animals, particularly in arid environments, which improves our understanding of animal biology and can be used to inform management decisions.
Comparative studies of the relationship between 2 phenotypes, or between a phenotype and an environment, are frequently carried out by invalid statistical methods. Most regression, correlation, and contingency table methods, including nonparametric methods, assume that the points are drawn independently from a common distribution. When species are taken from a branching phylogeny, they are manifestly nonindependent. Use of a statistical method that assumes independence will cause overstatement of the significance in hypothesis tests. Some illustrative examples of these phenomena are given, and limitations of previous proposals of ways to correct for the nonindependence discussed. A method of correcting for the phylogeny is proposed. It requires that we know both the tree topology and the branch lengths, and that we be willing to allow the characters to be modeled by Brownian motion on a linear scale. Given these conditions, the phylogeny specifies a set of contrasts among species, contrasts that are statistically independent and can be used in regression or correlation studies. -from Author
The body temperature of desert iguanas implanted with miniature temperature-sensitive radio transmitters was continuously monitored in their natural habitat. Extensive thermoregulatory behavior occurred in retreat burrows prior to morning emergence. Such behavior permits the igluana to emerge from below ground at its preferred body temperature rather than suboptimal temperature at which activity in the burrow is initiated.