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Ecology and Energetics of Three Sympatric Rattlesnake Species in the Sonoran Desert
Abstract
I investigated habitat use, home ranges, activity patterns, thermal biology, and energetics of sympatric western diamondback (Crotalus atrox), black-tailed (C. molossus), and tiger rattlesnakes (C. tigris) in the Sonoran desert of southeastern Arizona. The three species differed significantly in their choice of habitats, and habitats used by snakes were significantly influenced by season. Crotalus atrox used mainly creosotebush flats, but switched to rocky slopes during winter. Crotalus molossus frequented rocky areas, but during late summer and fall used arroyos and creosotebush flats. Crotalus tigris was a strick rock dweller during winter and spring, but used edges of arroyos during summer. Crotalus atrox, C. molossus, and C. tigris had mean home range sizes of 5.42 ha, 3.49 ha, and 3.48 ha, respectively. Over the year, snakes traveled a mean distance of 12.94 km (C. atrox), 14.99 km (C. molossus), and 9.29 km (C. tigris) during approximately 95 h of surface activity. During their active seasons, 1 March to 15 October, rattlesnakes traveled, on average, 50.8 m/d (C. atrox), 42.9 m/d (C. molossus), and 32.9 m/d (C. tigris). All three species had mean activity temperatures near 29.5 C. The three species showed no significant differences in home range, activity distances, or body temperatures. Rattlesnakes had standard metabolic rates similar to those of other Crotalus species, but only 40% those of other squamate reptiles of similar mass. They could fulfill yearly maintenance energy requirements with a prey quantity equivalent to 93% of their body mass; these energy demands could be met with 2 to 3 large meals.
... Anderson and Greenbaum (2012) recently reinstated C. ornatus as a valid species using morphological and genetic data, including three mitochondrial genes (ATPases 8 and 6, cyt b, and ND4) and three nuclear genes (c-mos, EXPH5, and RAG1). Historically, ecological research on Black-tailed Rattlesnakes in the United States has been conducted on populations in Arizona (Beck 1995;Hardy and Greene 1999a;Nowak 2009), which are now recognized as a different species (C. molossus), leaving only limited ecological information available for C. ornatus. ...
... Although the literature is sparse, both sexes of C. molossus seem to exhibit smaller home range sizes than those of C. ornatus at IMRS. In the Sonoran Desert of Arizona, Beck (1995) reported male C. molossus home ranges (0.93 and 3.41 ha) that were much smaller than those of male C. ornatus on IMRS, and a female home range (6.41 ha) that was slightly smaller than the mean for female C. ornatus on IMRS. Hardy and Greene (1999a) reported that C. molossus of both sexes in the Chiricahua Mountains of southeastern Arizona have smaller home ranges (male ¼ 21 ha, female ¼ 3.75 ha) than do C. ornatus on IMRS. ...
... Additionally, Mata-Silva et al. (2018) found that C. lepidus is also more likely to be found in arroyos than expected (55% of total observations), making arroyo habitat stand out as important to rattlesnakes at IMRS. It is assumed that habitats most often inhabited by rattlesnakes contain high prey densities (Beck 1995), which are associated with elevated hunting success. Although observations of foraging and feeding behavior were limited during this study, previous literature has found that important prey species for C. ornatus, such as Dipodomys merriami (Merriam's Kangaroo Rat), Chaetodipus hispidus (Rock Pocket Mouse), and Peromyscus eremicus (Cactus Mouse), can be found in high densities in the rocky habitats preferred by the snakes in this study (Reynolds and Scott 1982;Parker 1990;Wilson and Ruff 1999;Schmidly and Bradley 2016). ...
Despite its wide distribution throughout the northern Chihuahuan Desert, extraordinarily little is known about the ecology or behavior of Eastern Black-tailed Rattlesnakes (Crotalus ornatus). The primary literature for Black-tailed Rattlesnakes was largely based on research conducted on the former conspecific (C. molossus); thus, the revalidation of C. ornatus widened the void in our understanding of the species. The aim of this study was to elucidate movement patterns, home range size, and habitat use by C. ornatus in the northern Chihuahuan Desert of far West Texas. Radiotelemetry was used to monitor individual snakes for at least one active season (March–October) from May 2015 through August 2018. Mean (±1 SE) home range size for all individuals was 22.84 ± 4.49 ha and mean daily distance moved was 9.28 ± 0.93 m/d. Male snakes had larger home range sizes, larger core use areas, and greater daily distance moved than did female snakes. On a monthly basis, male movement peaked in August and female movement was statistically similar throughout the active season. Multinomial logit models were used to analyze habitat use patterns of C. ornatus, while controlling for snake, habitat availability, and season. Despite limited availability within snake home ranges, most observations of snakes occurred in arroyos or on rocky slopes. Microhabitat was also used nonrandomly, with snakes seeking cover in rocky refugia or under dense vegetation, rather than in areas containing high proportions of gravel or plant litter. This study presents the first detailed information about habitat and microhabitat use, along with patterns of movement and home range size for the recently revalidated C. ornatus.
... Arizona is home to at least 16 species of rattlesnakes (Brennan and Holycross, 2006; see Species Accounts, Volume 1). Throughout the biomes and ecosystems of Arizona, many species of rattlesnakes occur in syntopy, which we define as two or more related species occupying the same locality in a landscape (Rivas, 1964;Beck, 1995;see Glossary). Given their important trophic position as top predators in certain terrestrial ecosystems, tendency to eat large meals, physical capacity for long-term energy storage, and speciose conditions (Greene, 1992;Nowak et al., 2008), rattlesnakes represent unconventional models in the study of syntopy. ...
... In contrast to other snake species, which more commonly coexist through dietary separation (reviewed in Luiselli, 2006), syntopic viperid snakes (Viperidae) typically co-exist through habitat separation (Pough, 1966;Reinert, 1984;Beck, 1995;Luiselli et al., 2007;Scali et al., 2011;Steen et al., 2013b), including spatial and temporal use of resources (Dugan, 2011). ...
... We found little support for overall dietary partitioning between these rattlesnakes, which is an expected outcome for viperids (reviewed in Luiselli, 2006;Luiselli et al., 2007; but see Dugan, 2011). Differences in temporal and spatial use of habitats might serve to decrease the competition that an apparent lack of dietary partitioning could otherwise foster (Hardin, 1960;Schoener, 1974;Beck, 1995;Clark, 2007;Willson et al., 2010). Although lagomorphs were captured or detected in both upland and lowland trapping sites (Nowak, 2009), it is intriguing that our small sample of C. cerberus did not consume this prey (e.g., King, 2002). ...
The species diversity of rattlesnakes in Arizona is the highest in the United States, with at least 15 species present. Multiple species are syntopic across the diverse habitats of the state. We compared the spatial and temporal ecology of the adults of two species of large-bodied rattlesnakes (Crotalus atrox and C. cerberus) co-occurring at Tonto National Monument in Arizona using radio-telemetry from 2003 to 2007. We hypothesized that these two species would exhibit niche separation in spatial, temporal, and dietary resource use, including spatial response to human-developed habitats. Further, we predicted that such niche separation would represent niche partitioning due to interspecific competition. We tested this hypothesis through quantification of niche overlap in 12 resource dimensions.
The species of rattlesnakes we studied were not equally likely to be detected, and C. cerberus appeared to be comparatively rare at this site. Activity ranges often overlapped and did not differ in home range (100% MCP) and core-use (estimated by 50% probability kernel) scales. Neither of the two species used vegetation associations in proportion to their expected frequency; C. cerberus over-utilized upland habitats and C. atrox selected lowland or bajada habitats. Neither species took rodent prey in proportion to that expected from our live-trapping procedures. We found evidence for niche separation in several dimensions, including macrohabitat use and slope during the active season, human-developed area use, aspect of overwintering sites, seasonal activity patterns, and diet. However, we found little support for niche partitioning as a result of competition. Niche overlap values were significantly less than expected but only in the frequency of use of developed areas. Differences in the timing of ingress and egress were significant during some years but not others; however, C. cerberus had a significantly lower frequency of movement in spring and fall during all years compared to C. atrox. Niche overlap values for median egress dates and seasonal movements were significantly greater than expected, possibly indicating that thermal constraints during hibernation have imposed similar patterns of temporal and spatial resource use between the species during the overwintering season.
Based on evidence of separation in some of the spatial and temporal niche dimensions, we predicted differences in physiological parameters, including body temperatures, condition, reproductive frequency in females, and ectoparasite load, yet significant differences existed only for body temperatures. Given a trend toward active and hibernation season use of cooler, moister drainages and a typically shorter hibernation season, likely resulting in cooler body temperatures, it is possible that C. cerberus at this site is at the thermal limits of its range. Interspecific competition does not seem to be driving interactions between these species; however, more data is needed to investigate any influence of human developments on species interactions at this site. We also discuss resource use by a small sample of telemetered C. molossus for comparative purposes.
... converted to common units, the mass and temperature scaling of S. catenatus appears typical of other crotaline snakes (Fig. 4). The variability in mass and temperature scaling within North American pitvipers (both intra-and interspecies) has also been documented in other studies (Beck, 1995;Beaupre, 1996;Beaupre and Zaidan, 2001;McCue and Lillywhite 2002), and while the sources of variation remain unclear, they could be the result of the geographic location the animals came from, disposition of animals used (field versus lab acclimated), maternal effects, adaptation, and/ or random error. Plots such as Fig. 4, while interesting to examine for patterns, should also be interpreted with caution when extrapolating outside of the species' normal size range. ...
... Energy requirements vary ontogenetically, with 10-g snakes needing to consume 58-102% of their body mass per year, dropping to 45-70% for a 250-g snake, and 39-54% for a 500-g snake. Similar results have been reported for Crotalus species, showing that several sympatric desert species require only 2-3 meals per year (Beck, 1995), and adult C. ruber require as little as 1 large meal per year (Dugan and Hayes, 2012). While our calculations represent the minimum energy required to merely remain alive, and do not account for the energetic demands of growth or reproduction, from a conservation standpoint it does appear that prey availability is not a limiting factor for S. catenatus population growth at Carlyle Lake. ...
Physiological functions of reptiles are generally temperature-dependent, but variation exists among taxa in the magnitude of response. Pitvipers have been identified as low-energy specialists among the Squamata and exhibit characteristic metabolic responses related to this specialization. Previous studies have measured both field and resting metabolic rates in multiple species of Crotalus and Agkistrodon piscivorous, but no studies have been conducted for the third North American pitviper genus, Sisturus. We measured resting metabolic rates across an ontogenetic series of 28 individual Eastern Massasauga (Sistrurus catenatus) from Illinois to understand the magnitude and variability of their resting metabolic requirements. We found significant effects of sex, body mass, temperature, and the time-by-temperature interaction. Mass and temperature effects were expected, as larger warmer individuals will naturally have higher resting metabolic rates. The time-by-temperature interaction indicates the presence of a temperature-dependent circadian cycle in resting metabolic rate, and interpretation of the unexpected sex effect remains unclear. We compared our results to previous studies and conclude that, overall, S. catenatus does not differ greatly from other pitvipers, except with regard to the significant effect of sex. When data were extrapolated to include an entire active season, we found a 250-g female needs to ingest 2.5–8.5 individual rodents (depending on species consumed) per year to meet her resting energy requirements, further supporting their designation as low energy specialists.
... However, studies investigating thermal ecology in larger-bodied ectotherms have tended to only use single populations of a given species and/or focus on sympatric species (Beck, 1995;Blouin-Demers & Weatherhead, 2001;Blouin-Demers & Weatherhead, 2002;Bovo et al., 2012;Lelièvre et al., 2011;Moore, 1978), potentially because these species tend to be less common, rendering the effort and expense involved in these studies prohibitive. This, in turn, limits the scope of these studies to certain localities and prevents inferences about the possibility that climate change and environmental variation will interact in their future impacts on a given species. ...
... If the metabolic rates of these snakes rise with increasing temperatures, they would need to obtain additional energetic resources. However, as our calculations and other studies have shown, the metabolic needs of these snakes are incredibly low (Beaupre, 1995;Beaupre & Duvall, 1998b;Beck, 1995), with current baseline maintenance energetic demands being met with less than a single large meal per year. Even if mean active T b increased 1°C or 2°C along with ambient temperature, the annual caloric requirements for maintenance would still be met with a single large prey item. ...
1. Most studies on how rising temperatures will impact terrestrial ectotherms have focused on single populations or multiple sympatric species. Addressing the thermal and energetic implications of climatic variation on multiple allopatric populations of a species will help us better understand how a species may be impacted by altered climates.
2. We used eight years of thermal and behavioral data collected from four populations of Pacific rattlesnakes (Crotalus oreganus) living in climatically distinct habitat types (inland and coastal) to determine the field-active and laboratory-preferred body temperatures, thermoregulatory metrics, and maintenance energetic requirements of snakes from each population.
3. Physical models showed that thermal quality was best at coastal sites, but inland snakes thermoregulated more accurately despite being in more thermally constrained environments. Projected increases of 1 and 2°C in ambient temperature result in an increase in overall thermal quality at both coastal and inland sites.
4. Population differences in modeled standard metabolic rate estimates were driven by body size and not field-active body temperature, with inland snakes requiring 1.6× more food annually than coastal snakes.
5. All snakes thermoregulated with high accuracy, suggesting that small increases in ambient temperature are unlikely to impact the maintenance energetic requirements of individual snakes and that some species of large-bodied reptiles may be robust to modest thermal perturbations under conservative climate change predictions. K E Y W O R D S climate change, ectotherm, energetic requirements, metabolism, rattlesnake, thermal ecology, thermal quality
... Indeed, this study took place in the Chihuahuan Desert from April to October where temperatures average 22°C and commonly go above 40°C (US Department of Commerce, 2023). These temperatures are sufficient for rattlesnakes to carry out biological activities and when temperatures are not ideal, they use refugia (Beck, 1995). Moreover, color change abilities have been found to vary between populations in other organisms. ...
Coloration is crucial for ambush mesopredators' survival as they depend on it for camouflage to hunt and avoid predation. Furthermore, coloration is especially important in ectotherms as it is linked to two essential functions for survival: thermoregulation and crypsis. In a context of trade‐offs between thermoregulation and crypsis, the comprehension of how ectotherms employ color to address conflicting demands of thermoregulation and crypsis is limited. This study investigated background matching and thermoregulation in western diamond‐backed rattlesnakes ( Crotalus atrox ). Rattlesnakes were predicted to better match selected microhabitats in terms of color, luminance, and pattern than random microhabitats within their home range, and body temperature was predicted to influence their body coloration with higher temperatures favoring lighter colors. Pictures of 14 radiotracked western diamond‐backed rattlesnakes were taken in situ with a full spectrum camera (UV/VIS) and body temperature was recovered from internal temperature‐datalogging radiotransmitters. Crotalus atrox matched the color, luminance, and pattern of the background better than a randomly selected background, thus enhancing background matching. Additionally, rattlesnake coloration varied independently of temperature indicating that rattlesnakes are behaviorally modifying crypsis regardless of thermoregulation.
... Indeed, this study took place in the Chihuahuan Desert from April to October where temperatures average 22°C and commonly go above 40°C (US Department of Commerce n.d.). These temperatures are su cient for rattlesnakes to carry biological activities and when temperatures are not ideal, they use refugia (Beck 1995). Moreover, color change abilities have been found to vary between populations in other organisms. ...
Coloration is crucial for ambush mesopredators' survival as they depend on it for camou age to hunt and avoid predation. Furthermore, coloration is especially important in ectotherms as it is linked to two essential functions for survival: thermoregulation and crypsis. In a context of trade-offs between thermoregulation and crypsis, the comprehension of how ectotherms employ color to address con icting demands of thermoregulation and crypsis is limited. This study investigated background matching and thermoregulation in western-diamond-backed rattlesnakes (Crotalus atrox). Rattlesnakes were predicted to better match selected microhabitats in terms of color, luminance, and pattern than random microhabitats within their home range, and temperature was predicted to in uence their body coloration. Pictures of fourteen radiotracked western diamond-backed rattlesnakes were taken in situ with a full spectrum camera (UV/VIS) and body temperature was recovered from internal temperature-datalogging radio-transmitters. Crotalus atrox matched the color, luminance, and pattern of the background better than a randomly selected background, thus enhancing crypsis. Additionally, rattlesnake coloration varied independently of temperature indicating that rattlesnakes are behaviorally modifying crypsis regardless of thermoregulation.
... Diurnally active desert fauna are less common but have Chapter 1: General Introduction 6 adaptations such as gaping their mouths and fluttering their throats [e.g., nomadic desert birds (Dean 2004)], estivation [e.g., the Mohave ground squirrel, Citellus mohavensis (Bartholomew and Hudson 1960)], thermoregulatory licking and the utilisation of shade [e.g., the Macropodidae (Dawson 1977)]. 'Shuttling' or the intermittent use of subterranean structures during the extreme heat of the day is used by many desert mammals, e.g., a species of Thar Desert gerbil, Meriones hurrianae, develops hyperthermia due to sun exposure but intermittently visits cool burrow systems to unload excessive heat (Prakash 1997) and rattlesnakes, Crotalus spp., in the Sonoran desert prevent overheating by sheltering within rock crevices (Beck 1995). ...
We are experiencing a global biodiversity and climate crisis that is rapidly causing
the extinction of species. Mammal species have been disproportionately affected;
however, this trend is considerably worse in Australia. Since Australia’s occupation
by Europeans, 34 mammal species have been declared extinct. Australian
mammals in deserts are particularly at risk of extinction. Many arid zone mammals
have specialised adaptations to their hostile, unpredictable ecosystems. For
example, they use thermally insulative refuges, prefer habitats that reduce predation
risk, or have large home ranges and broad diets to maximise energy intake.
Understanding these adaptations is essential for informed conservation
management. However, little ecological data is known for the sandhill dunnart,
Sminthopsis psammophila, an endangered and charismatic marsupial that now
remains within just a few natural refugial habitats in Australia’s southern deserts. To
address conservation biology knowledge gaps, an integrated, evidence-based
approach (i) quantified the diurnal and nocturnal ecology of S. psammophila in the
Western Australian Great Victoria Desert (WAGVD), (ii) estimated the past, present
and future distributions of S. psammophila throughout Australia, (iii) examined the
key threats to S. psammophila - particularly wildfires and anthropogenic climate
change - and (iv) proposed conservation management solutions for a) S.
psammophila and b) sympatric arid zone species. Between 2015 and 2019, radio
tracking and global positioning system (GPS) technologies examined the sheltering,
foraging, dietary and habitat preferences of S. psammophila in the WAGVD. In
contrast to its previously reported habitat preferences, S. psammophila preferred
burrowing within long unburned (32+ years since a wildfire) spinifex (Triodia spp.)
grassland habitats. Dense lower stratum swale, sand plain and dune slope habitats
were preferred, whereas habitats lacking spinifex and open dune crest habitats were
rarely used. Hence, wildfires were identified as a significant threat to the species.
The sheltering preferences of S. psammophila agreed with the premise that small
desert mammals often use shelters with thermal advantages and anti-predation
benefits, such as burrows, Lepidobolus deserti hummocks and logs. Conversely,
spinifex hummocks were not found to be insulative against extreme temperatures
and were not preferred. The foraging adaptations of S. psammophila agreed with
the premise that arid zone species often have large home ranges to exploit resource
patches or islands. The 100 % home ranges of S. psammophila [mean: 70 ha;
range: 6-274 ha; minimum convex polygon (MCP)] were influenced by sex and
reproductive status. In addition, a Formicine-rich diet indicated that ants are an
important dietary resource for S. psammophila. Species distribution models (SDMs)
predicted the past, present, and future distributions of S. psammophila, evaluated
the environmental parameters that determine the species’ distribution and identified
habitats of high conservation value. The past model supported evidence that S.
psammophila was widespread but has recently contracted to more climatically
favourable areas of its geographic range. Ground-validation of the present model’s
predictions discovered a population 150 km north of the species’ known range.
Future models identified that climate change is a potential catastrophic threat for S.
psammophila. By 2050, under Representative Concentration Pathway (RCP) 8.5
(our current pathway) there is a predicted 95 % reduction in suitable habitat for S.
psammophila in the WAGVD. By 2070 (RCP 8.5), only the Eyre Peninsula
population may remain viable and the continental distribution of S. psammophila
may contract by up to 80 %. However, this contraction is predicted to be halved if
global greenhouse gas emissions peak in 2040 then reduce (RCP 4.5). Due to
specific habitat preferences for long unburned habitats, S. psammophila is further
restricted within its climatically and geographically suitable range. As a semi-arid
specialist, it is also vulnerable to drought-related population crashes. Hence, S.
psammophila should remain listed as endangered at the state and federal level, and
its status should be revised by the International Union for Conservation of Nature.
Documents predation of Aspidoscelis sonorae by Crotalus tigris in northern Sonora, Mexico.
Northern Pacific Rattlesnakes (Crotalus oreganus) are ambush-hunting predators that rely on crypsis to forage and to avoid encounters with predators, yet little is known about color variation in this ubiquitous rattlesnake species. This study addressed how coloration and contrast in tail banding in C. oreganus vary among habitat types in Washington State. We also explored whether C. oreganus exhibit sexual dimorphism in coloration or tail band contrast (sexual dichromatism). We sampled 127 rattlesnakes from seven different populations across northern and central Washington state during spring emergence from overwintering hibernacula in 2017 and 2018. We characterized snake coloration as red/blue color ratios from standardized photographs, and used GIS supervised classification schemes of satellite imagery to characterize habitat. We used generalized linear models to assess relationships among snake color and tail bands, habitat, and sex. We found that coloration (red/blue color ratios) in C. oreganus varied greatly across the landscape, both within and among populations, likely a reflection of their variable and heterogenous shrub-steppe and forest ecotone habitats. In 20% of 21 pairwise comparisons, populations differed in body color. Rattlesnake coloration was not associated with habitat (amount of forested land within 0.5 km of the snake den), but male and female rattlesnakes showed different associations between color and percent forested habitat. Male rattlesnakes did not differ in body coloration from females, but males showed greater contrast than females in the black and white banding present on the tail. We discuss several, nonmutually exclusive, hypotheses for sexual dichromatism in tail band contrast, including the possibility that tail banding constitutes warning coloration in rattlesnakes. Our results suggest that sexual dichromatism, and the role of tail banding in rattlesnake ecology, are topics worthy of further investigation.
Monitored the habitat use of timber rattlesnakes Crotalus horridus and northern copperheads Agkistrodon contortrix mokeson. Habitat was quantitatively defined on the basis of 14 structural environmental factors. A multivariate analysis of variance of 6 groups with 14 variables indicated that group centroids differed significantly. The 1st 3 discriminant functions accounted for 94% of the total variance and represented the following respective habitat gradients: 1) rocky, open sites to heavily forested sites with few rocks, 2) sites predominantly covered with leaf litter to sites with dense forest-floor vegetation, and 3) sites with large-diameter overstory trees to sites with small-diameter trees. Gravid specimens of both species were most closely associated in discriminant space and clearly separated from other segments of their respective species groups. The utilization of rocky, sparsely forested sites and decreased variation in site selection were associated with gravidity. A 2-group discriminant function analysis using melanistic and nonmelanistic male C. horridus indicated a gradient in habitat utilization from mature forest sites with numerous fallen logs to young forest sites with predominant leaf litter cover. Melanistic specimens preferred the former, nonmelanistic specimens the latter. A. c. mokeson was poorly adapted to the study site due to the preference of all population segments for open sites and young forest stands. C. horridus was better adapted, utilizing a broad range of the available habitat.-from Author
Quantitative data consisting of 14 structural habitat variables and 5 climatic variables were obtained from 505 telemetric relocations of 21 timber rattlesnakes (Crotalus horridus) and 20 northern copperheads (Agkistrodon contortrix mokeson) in eastern Pennsylvania. Multivariate analysis of variance revealed that the habitats utilized by the two species differed significantly. The single discriminant function generated by a discriminant analysis was highly correlated with variables describing structural features of the forest floor and canopy. A. c. mokeson utilized relatively open areas with higher rock density and less surface vegetation than did C. horridus. The discriminant function correctly classified 76% of the samples. The 24% overlap consisted largely of locations from gravid C. horridus whose habitat preference closely paralleled A. c. mokeson. Measurements of habitat overlap indicated that the separation of the two species along the habitat axis was theoretically sufficient for competitive avoidance. Although the observed species separation can be interpreted as reflecting interspecific competition, an alternate explanation based upon inherent physiological differences is also presented.
Although movement patterns should reflect important aspects of a species' ecology, a survey of published information on snake movements reveals few general trends among species, populations, or individuals within populations. Snakes are sometimes difficult to study, and some of the problems associated with them (e.g., crypticity of young) will not be overcome easily. However, general patterns might be more evident if: (1) studies were done over reasonably long time periods; (2) methods of data collection, analysis, and presentation were standardized to some extent; and (3) factors that might affect movement patterns (especially resource requirements and distribution) of different individuals became a focus of attention.
Free-ranging prairie rattlesnakes (Crotalus viridis viridis) exhibit lengthy vernal migrations upon emergence from winter hibernation. A series of laboratory experiments was designed to test hypotheses regarding the function and causation of vernal movements. Rattlesnakes obtained from Wyoming and Colorado populations were used. First, we hypothesized that the function of vernal movements is to locate small mammal prey. Second, we predicted that activeC. v. viridis use prey chemicals, as well as other cues, to decide whether or not rodents are present in an area. Third, we hypothesized that vernally active males would be more responsive to rodent prey and their odors than females, given observed differences in behavior in the field. Fourth, we predicted that rattlesnakes captured in Colorado would be more sensitive to prey odors than those obtained in Wyoming, because of disparate community structure and, hence, small mammal spatial distributions. As expected, snakes exhibited reduced activity, as well as certain other dependent measures reflecting predatory investigation, in arena zones containing either live rodents or their chemicals. However, responses to the latter were reduced in Wyoming rattlesnakes tested with chemicals from deer mice (Peromyscus maniculatus), relative to Colorado animals tested with chemicals obtained from house mice (Mus musculus). In contrast to patterns observed in nature, males and females exhibited almost no differences in overall responsiveness. Results are discussed in the context of simulation modeling and ongoing studies of prairie rattlesnake behavior.
Over 50 days, activity range areas (mean 9794m2), range lengths (mean 89m) and distances moved per day (mean 9.1m) did not differ significantly between sexes or study sites. There was a significantly shorter mean range length for gravid snakes (40m). Displaced specimens exhibited no homing behavior. In spring and fall, snakes frequented low, poorly drained habitats near hibernacula. During summer, habitats with low or sparse vegetation and dry soil were most frequented, particularly by gravid specimens.-from Authors
The body temperatures of six prairie rattlesnakes (Crotalus viridis) were monitored telemetrically during the winter of 1976-1977 while they inhabited a hibernaculum 8 km southeast of Moriarty, Torrance County, New Mexico. As long as their body temperatures remained about 10 C, the snakes were active in the den and often emerged to bask at the mouth of the hibernaculum on sunny days. Experimental laboratory data on the body temperature-heart rate relationship show a significant decrease in cardiac activity below 10 C, indicating a possible physiological basis for activity patterns observed at the hibernaculum. Increasing ground temperatures appear to have been responsible for emergence of the snakes in April. Within the 6-month period during which the snakes occupied the hibernaculum, their body temperatures ranged from 6-29 C, with a mean of 11.3 C.
An evaluation of the methods for determining home ranges was made, and the factors affecting home range estimates were examined. Inadequate sample size and the use of statistically based models to correct for small sample size were the most common deficiencies in home range studies. The determinant and recapture radii methods unrealistically inflated home range estimates. The assumption of normality was not confirmed by actual data. The convex polygon method is the most attractive home range measurement to field biologists because it accurately describes the area used (if based on an adequate number of sightings), and it is simple, using only the actual sightings of an animal. By examining the relationship between number of sightings and cumulative home range size, the minimum number of sightings that accurately estimates home range can be empirically determined. Additionally, these data can provide information on the use of home ranges and territories. The time interval on which home ranges are based affect size estimates greatly. Many sightings per animal and short time intervals between censuses are necessary for accurate home range estimates. Eighteen was the minimum number of sightings to accurately measure home range size in Sceloporus virgatus. Eighteen is much larger than the minimum number commonly considered adequate. Few generalizations concerning home ranges are possible from the published data on North American iguanid lizards. The data indicate that caution is necessary in interpreting the function of home ranges from correlative data, and that the same data can support alternative hypotheses regarding function.
Body temperatures (T<sub>B</sub>s) of timber rattlesnakes (Crotalus horridus) were recorded in a natural hibernaculum in northeastern New York by radiotelemetry through the winter of 1979-1980. Overall mean T<sub>B</sub> from September through May was 10.5°C (4.3-15.7°C, N = 81 records). There was no significant difference between mean T<sub>B</sub>s of individual snakes, but there was a significant difference between months. Mean rate of T<sub>B</sub> decline was 0.5°C/wk through February; in March, T<sub>B</sub> stabilized at 4.3°C after which the T<sub>B</sub> rose by 0.6°C/wk in April and May. Emergence occurred in mid-May at a T<sub>B</sub> of ca. 9°C. The profile of body temperatures over the 9-month period reflected profiles of surface soil and air temperatures.
Five adult timber rattlesnakes (Crotalus horridus) were tracked using temperature-sensitive radio transmitters during their movements away from a den and on summer habitat. The snakes dispersed a mean of 504 m (♀ ♀ 280 m, ♂ 1400 m) and gained a mean of 102 m in elevation. One gravid female was relatively sedentary (39 m/move) between 20 June-5 August. Non-gravid females moved farther. Immediately after ecdysis movements of three snakes were suddenly extensive. Two females appeared to use the same migratory routes returning to the den in autumn as they did at spring egress. Minimum and maximum voluntary temperatures were 12.5 and 33.3°C, respectively. The overall mean body temperature (T<sub>B</sub>) was 26.9°C (N = 157). On the surface in clear weather, T<sub>B</sub> averaged 30.1°C while mean air temperature (T<sub>A</sub>) was 26.0 and substrate (T<sub>S</sub>) 20.6°C. On the surface in cloudy weather and underground in clear weather, mean T<sub>B</sub> was 27.8°C. Snakes underground in cloudy weather had a mean T<sub>B</sub> of 20.2°C, not significantly different from mean T<sub>A</sub> and T<sub>S</sub>. Thermoregulation was accomplished by moving in and out of rock crevices and by moving slightly within the sunshade interface in grassy-shrub locations. Body color, reproductive condition, stage of ecdysis, and feeding are four factors that may have important effects on thermal relationships of C. horridus. Controlled experiments to examine these factors are needed.
Radiotelemetry was used to monitor the movements and habitat use of timber rattlesnakes (Crotalus horridus) in the Pine Barrens of southern New Jersey. Reproductive condition strongly influenced both aspects of behavior. Males generally exhibited the largest activity ranges, and the sizes of their ranges were positively correlated with the number of days the snakes were monitored. This was not true for gravid or non-gravid females. Time series analyses indicated that movement patterns of males and non-gravid females consisted of constantly shifting, non-overlapping activity areas. In most cases these snakes moved in a looping pattern during the active season that returned them to the same hibernation site from which they departed. Gravid females exhibited more static, overlapping activity areas and shorter dispersal distances from hibernacula. Males and non-gravid females utilized forested habitat with greater than 50% canopy closure, thick surface vegetation (approx. 75%), and few fallen logs. This habitat occurred with high frequency throughout the study area. Gravid snakes utilized less densely forested sites with approx. 25% canopy closure, an equal mixture of vegetation and leaf litter covering the surface, frequent fallen logs, and warmer climatic conditions. This habitat occurred in low frequency on the study area and was largely restricted to the edge of sand roads.