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Denning, metabolic suppression, and the realisation of ecological opportunities in Ursidae
Abstract
• Through realisation of ecological opportunities, populations and species can experience relaxed selection pressures, facilitating ecological release and leading to rapid speciation and morphological diversification. Behavioural plasticity in response to environmental change contributes to diversification by exposing individuals to novel conditions through their interactions with resources or dispersal to new areas. Despite strong theoretical support, demonstrations of this evolutionary process are rare.
• The family Ursidae is the product of one or more adaptive radiations following the Miocene–Pliocene transition. Denning behaviour associated with over-winter seclusion, fasting, and parturition coinciding with seasonally decreased food availability appears to be paraphyletic in Ursidae phylogenies. However, female bears of all species undergo varying degrees of seclusion and fasting during parturition and early post-natal care, which is not consistent with periods of seasonally decreased food availability. As denning behaviour is tightly linked to fitness through energetics and reproduction, these behaviours are suspected to be under strong selection. Mechanisms responsible for the observed variability among species, populations, and individuals have not been explored.
• In this review, we detail the evolutionary history of extinct and extant Ursidae regarding expression of denning behaviour and as a function of realised ecological opportunities. We compare behaviours across Ursidae and contextualise our results within extant species ecology.
• We demonstrate the role of relaxed extrinsic and intrinsic factors in the expression of metabolic suppression among Ursidae species, and across populations and reproductive groups, through the realisation of ecological opportunities. In doing so, we propose a more refined consideration of and perspective on this behaviour and provide a mechanism for the adaptive radiation in Ursidae.
... Use of dens allows female grizzlies to not only succumb to winter torpor, but also produce altricial young that otherwise would not survive a threatening external world (Ramsay & Dunbrack 1986). Winter torpor passed in the shelter of a den moreover allows grizzlies to escape rigors of winter that are accentuated by both their dependence on vegetal foods as well as their comparative inefficiency as predators (Fowler et al. 2021). At the same time, omnivory allows for the accumulation of adipose reserves needed to sustain winter torpor as well as the adaptive exploitation of various animal and vegetal foods rich in fat and digestible energy (Rode & Robbins 2000, Robbins et al. 2012, Erlenbach et al. 2014). ...
... Omnivory has engendered other advantages. It allowed grizzlies, as a species, to escape evolutionary constraints on body size and grow large (Fowler et al. 2021). Large size not only conveys energetic efficiencies, but also comparative invulnerability to most-but not all-predation. ...
For perhaps 30,000 years grizzly bears ranged throughout the mountains and riparian areas of what would eventually become the southwestern United States. But in a remarkably short 50-year period between 1860 and 1910 Anglo-Americans killed roughly 90% of the grizzly bears in 90% of the places they once lived. Most of the remaining grizzlies had been killed by the 1930s.
This report provides a detailed account of natural history, relations with humans, and current and future prospects for grizzly bears of the Southwest, emphasizing the millennia prior to ascendance of Anglo-Americans. The report’s narrative is essentially chronological, starting with deep history spanning the late Pleistocene up through arrival of European colonists (Section 3.1); the period of Spanish and Mexican dominance (Section 3.2); and then the period of terminal grizzly bear extirpations that began with the political and military dominance of
Anglo-Americans (Section 3.3). Section 4 examines current environmental conditions and related prospects for restoring grizzly bears to the Southwest. Section 5 completes the chronological arc by forecasting some of what
the future might hold, with implications for both grizzly bears and humans.
The background provided in Section 2 offers a synopsis of grizzly bear natural history as well as a summary of foods and habitats that were likely important to grizzlies. Throughout the Holocene there was a remarkable concentration of diverse high-quality bear foods in highlands of the Southwest, notably in an arc from the San Francisco Peaks of Arizona southeast along the Coconino Plateau and Mogollon Rim to a terminus in the White, Mogollon, and Black Range Mountains in New Mexico. Additional high-quality habitat existed in the Sacramento, San Juan, Jemez, and Sangre de Cristo Mountains of New Mexico and adjacent Colorado.
Grizzlies in the Southwest survived remarkable extremes of climate and habitats for perhaps as long as 100,000 years. They also survived substantial variation in human-propagated impacts that culminated in the Crisis of 875-1425 C.E.—a period typified by episodic drought and the highest human population densities prior to recent times. In contrast to relatively benevolent attitudes among indigenous populations, there is little doubt that the terminal toll taken on grizzly bears by Anglo-Americans after 1850 C.E was driven largely by a uniquely lethal
combination of intolerance and ecological dynamics entrained by the eradication or diminishment of native foods and the substitution of human foods, notably livestock, that catalyzed conflict.
More positively, the analysis presented here of current habitat productivity, fragmentation, and remoteness—as well as regulations, laws, and human attitudes—reveals ample potential for restoration of grizzlies to the Southwest, including three candidate Restoration Area Complexes: the Mogollon, San Juan, and Sangre de Cristo, capable of supporting around 620, 425, and 280 grizzlies each. Major foreseeable challenges for those wishing to restore grizzly bears to these areas include sanitation of human facilities, management of livestock depredation, education of big game hunters, coordination of management, and fostering of accommodation among rural residents. Climate change promises to compound all of these challenges, although offset to an uncertain extent by prospective increases in human tolerance.
But the evolutionary history of grizzly bears also provides grounds for optimism about prospective restoration. Grizzly bears have survived enormous environmental variation spanning hundreds of thousands of years, including many millennia in the Southwest. Grizzlies survived not only the inhospitable deeps of the Ice Ages in Asia and Beringia, but also the heat and drought of the Altithermal on this continent. It was only highly-lethal Anglo-Americans that drove them to extinction in the Southwest, which is why human attitudes—more than anything else—will likely determine prospects for restoring grizzly bears.
... Most carnivores use dens only occasionally or during their reproduction phase, but bears of various age and sex classes use dens for diverse reasons (Linnell et al. 2000), such as cover for concealment and hibernation (Pigeon et al. 2016, González-Bernardo et al. 2020. Denning was an important component of Ursidae evolution (Fowler et al. 2021) and is crucial for their survival and reproduction (Seryodkin et al. 2003). Bears also use caves to escape unfavorable weather conditions or threats from people (Groff et al. 1998). ...
The range of the Asiatic black bear (Ursus thibetanus) extends in its western limits to southeastern Iran where the species lives in arid mountainous landscapes in low densities. A better understanding of the adaptations of this threatened subspecies, the Baluchistan black bear (U. t. gedrosianus), to these harsh environmental conditions is necessary to devise conservation action. Here, we investigated the use and characteristics of Asiatic black bear dens in Hormozgan Province, one of the critical components of its life cycle. We detected 12 dens in caves and monitored them from 2012 to 2021 using camera traps (1,972 trap-nights). We obtained 138 bear detections, together with detections of 8 other wildlife species, in 12 dens. Our findings show that bears use dens as heat and probably anthropogenic refugia. Given the importance of dens for the Asiatic black bear, we suggest protecting these areas from human disturbances and expanding efforts to other areas with bear dens.
... To view this model in 3D, please see the online version of this article. (Fowler et al., 2021) and long hair that the shortfaced bear likely had. Although preserved skin is unknown for the short-faced bear, it seems reasonable that it would have had a full coat even though its large size may have pushed it toward gigantothermy. ...
Paleoart is an important medium that communicates scientific understanding about prehistoric life to both the public and researchers. However, despite its broad influence, the scientific and aesthetic decisions that go into paleoart are rarely described in formal academic literature or subjected to peer review. This is unfortunate, as paleoart can easily create and perpetuate misconceptions that are carried through generations of iterative popular media. As an example of what we hope will become a standard article type in paleontological journals, we describe the process and latest scientific research used to develop 13 new paleoart reconstructions of Ice Age animals found in the La Brea Tar Pits, including the saber-toothed cat, dire wolf, and teratorn. We adopted a stylized low polygon aesthetic for these three-dimensional (3D), ani-mated virtual models both to support learning objectives and to optimize performance for smartphone based augmented reality (AR) experiences. We encourage all researchers to follow the example of this article by publishing paleoart descriptions for any major new work that, at a minimum, reference the aesthetic and scientific reason-ing behind general posture and proportions, gross appearance of soft tissues, color-ation, and behavior.
Ecological plasticity may index the capacity of individuals, populations, and species to respond to stochasticity. Few studies have demonstrated mechanisms influencing expression of plasticity and less have addressed proximate and causal mechanisms at multiple scales. We used linear models and meta-analyses to investigate factors influencing among and within population plasticity in den chronology for American black bears (Ursus americanus) and brown bears (U. arctos) in North America. We found later mean den entrance date and longer mean duration of denning among populations for both species in response to apparent increasing food availability during the growing season and denning period, along with later onset and shorter duration of mean snow accumulations ≥10 cm. Greater variability in den chronology appeared to be in response to greater variability in snow accumulation and duration precluding access to forage. Observed patterns of plasticity in den chronology varied among age-reproductive groups with male bears denning later and for shorter duration than sub-adults, non-parturient females, and parturient females. This pattern supports expected allometric energetic demands and energetic expenditures required for reproduction. Furthermore, our results align with previously published works indicating that plasticity in den chronology in response to relaxed selective pressure of food availability is energetically advantageous under the cost-benefit hypothesis of energy conservation. Understanding how species respond to variation in selective pressures is central to predicting these effects and our understanding of evolutionary and ecological processes.
Polar bears (Ursus maritimus) are expected to be adversely impacted by a warming Arctic due to melting of the sea‐ice platform from which they hunt ice‐breeding seals. We evaluated the hypothesis that scavenging on stranded large whale carcasses may have facilitated polar bear survival through past interglacial periods during which sea‐ice was limited by analyzing: (1) present‐day scavenging by polar bears on large whale carcasses; (2) energy values of large whale species; and (3) the ability of polar bears, like the brown bears (Ursus arctos) from which they evolved, to quickly store large amounts of lipids and to fast for extended periods. We concluded that scavenging on large whale carcasses likely facilitated survival of polar bears in past interglacial periods when access to seals was reduced. In a future, ice‐impoverished Arctic, whale carcasses are less likely to provide nutritional refuge for polar bears because overharvesting by humans has greatly reduced large whale populations, carcass availability is geographically limited, and climate‐induced sea‐ice loss is projected to occur at a more rapid pace than polar bears have experienced at any previous time in their evolutionary history.
Little is known about Sloth bear Melursus ursinus birthing behaviour and denning. The Wildlife SOS Bannerghatta Bear Rescue Centre, India, which houses rescued bears, is visited by wild male Sloth bears during the breeding season (April–July). Two female Sloth bears have been impregnated by these wild males, giving researchers the opportunity to observe maternal behaviour closely. One female bear made use of an excavated a den to give birth and raise her cubs. A second bear gave birth in an excavated shallow cavity. Neither mother left the cubs for food or water for weeks (22 and 28 days, respectively). In the first case, the cubs eventually joined their mother leaving the den for food and water 20 days after the mother had initially left the den alone, whereas in the second case, the cub still had not joined the mother on outings for provisions after 68 days, when both were moved from the den area.
The skeletal remains of a small bear (Protarctos abstrusus) were collected at the Beaver Pond fossil site in the High Arctic (Ellesmere I., Nunavut). This mid-Pliocene deposit has also yielded 12 other mammals and the remains of a boreal-forest community. Phylogenetic analysis reveals this bear to be basal to modern bears. It appears to represent an immigration event from Asia, leaving no living North American descendants. The dentition shows only modest specialization for herbivory, consistent with its basal position within Ursinae. However, the appearance of dental caries suggest a diet high in fermentable-carbohydrates. Fossil plants remains, including diverse berries, suggests that, like modern northern black bears, P. abstrusus may have exploited a high-sugar diet in the fall to promote fat accumulation and facilitate hibernation. A tendency toward a sugar-rich diet appears to have arisen early in Ursinae, and may have played a role in allowing ursine lineages to occupy cold habitats.
Bears are iconic mammals with a complex evolutionary history. Natural bear hybrids and studies of few nuclear genes indicate that gene flow among bears may be more common than expected and not limited to polar and brown bears. Here we present a genome analysis of the bear family with representatives of all living species. Phylogenomic analyses of 869 mega base pairs divided into 18,621 genome fragments yielded a well-resolved coalescent species tree despite signals for extensive gene flow across species. However, genome analyses using different statistical methods show that gene flow is not limited to closely related species pairs. Strong ancestral gene flow between the Asiatic black bear and the ancestor to polar, brown and American black bear explains uncertainties in reconstructing the bear phylogeny. Gene flow across the bear clade may be mediated by intermediate species such as the geographically wide-spread brown bears leading to large amounts of phylogenetic conflict. Genome-scale analyses lead to a more complete understanding of complex evolutionary processes. Evidence for extensive inter-specific gene flow, found also in other animal species, necessitates shifting the attention from speciation processes achieving genome-wide reproductive isolation to the selective processes that maintain species divergence in the face of gene flow.
The Tremarctinae are a subfamily of bears endemic to the New World, including two of the largest terrestrial mammalian carnivores that have ever lived: the giant, short-faced bears Arctodus simus from North America and Arctotherium angustidens from South America (greater than or equal to 1000 kg). Arctotherium angustidens became extinct during the Early Pleistocene, whereas Arctodus simus went extinct at the very end of the Pleistocene. The only living tremarctine is the spectacled bear (Tremarctos ornatus), a largely herbivorous bear that is today only found in South America. The relationships among the spectacled bears (Tremarctos), South American short-faced bears (Arctotherium) and North American short faced bears (Arctodus) remain uncertain. In this study, we sequenced a mitochondrial genome from an Arctotherium femur preserved in a Chilean cave. Our molecular phylogenetic analyses revealed that the South American short-faced bears were more closely related to the extant South American spectacled bear than to the North American short-faced bears. This result suggests striking convergent evolution of giant forms in the two groups of short-faced bears (Arctodus and Arctotherium), potentially as an adaptation to dominate competition for megafaunal carcasses.
Mammals are often considered to be masters of homeostasis, with the ability to maintain a constant internal milieu, despite marked changes in the environment; however, many species exhibit striking physiological and biochemical plasticity in the face of environmental fluctuations. Here, we review metabolic depression and body temperature fluctuation in mammals, with a focus on the extreme example of hibernation in small-bodied eutherian species. Careful exploitation of the phenotypic plasticity of mammals with metabolic flexibility may provide the key to unlocking the molecular secrets of orchestrating and surviving reversible metabolic depression in less plastic species, including humans.
©2015 Int. Union Physiol. Sci./Am. Physiol. Soc.
Bears consuming wild fruits for fall energy accumulation are constrained by several factors, including intake rate, the physiological capacity of the gastrointestinal tract, and the metabolic efficiency of gain in body mass. We measured these relationships through foraging and feeding trials using captive and wild black bears (Ursus americanus) and grizzly bears (Ursus arctos). Four fruit types covering a range of sizes and clustering were offered to captive bears to determine the effect of density, size, and presentation on intake rate. Intake rate (in grams per minute) and bite rates (in bites per minute) increased curvilinearly with increasing fruit density in singly spaced fruits. Maximum intakes ranged from 30 g/min for 0.5-g berries to >200 g/min for 4.2-g fruits. The highest bite rates were obtained during the initial encounter with each patch as bears consumed all visually apparent fruits on the surface. Bite rates quickly dropped by 15-20% as foraging continued within the patch. Maximum bite rates were not depressed until initial fruit density fell to <50 berries/M3. Maximum daily fresh fruit intake for the captive bears averaged 34 ± 6% (mean ± 1 SD) of body mass. The dry-matter digestibility of wild fruits, particularly preferred species, was as high as 72%. While large captive bears could gain body mass very rapidly when given fruit ad libitum, foraging efficiencies increasingly constrained growth rates of wild bears >100 kg. We concluded that large bears, such as grizzlies, must depend on plants that permit large bite sizes or high bite rates through fruit clustering and bush configuration that reduce leaf-to-fruit ratios.
Most bears are opportunistic omnivores; their diets consist of fruits, other vegetative material, and in lesser amounts, mammals, fishes, and insects. Sloth bears (Melursus ursinas) are the only species of ursid specifically adapted to feed on insects, especially termites and ants, although they also feed on fruits when available. We studied diets of sloth bears in Royal Chitwan National Park, Nepal, where fruits are available for ca. 4 months (May-August) and access to colonies of termites is reduced in lowlands that are flooded during the fruiting season. We analyzed feces and observed sloth bears foraging to investigate their responses to changes in availability of food. Diets of sloth bears were dominated by insects (>90%), especially termites (≥50%), from September through April, but they relied heavily on fruits from May through August. Seasonal movements between lowland and upland habitats seemed to be prompted mainly by availability of termites. Termites were more dominant in the diets of sloth bears in our study than in a study conducted 20 years ago in Royal Chitwan National Park and in studies in India. The dietary shift of sloth bears in Royal Chitwan National Park may have been related to changes in habitat conditions associated with relocation of people out of the Park. It appears that sloth bears, like other bears but unlike other myrmecophagous mammals, can adapt their diet to changing food conditions.
1. In total, pregnant Polar Bears that enter maternity dens in late summer/autumn may fast for up to 8 months in addition to meeting the nutritional demands of gestation and lactation. We quantified the nutritional costs of this prolonged `reproductive fast' and examined the effect of variation in maternal body condition on reproductive success. 2. Prior to entering dens, pregnant females were obese, containing as much as 1 kg of fat/kg of lean body mass (LBM). Among bears, LBM increased with body fat mass. This accumulation of LBM may be necessary in order to transport the large fat stores required for fasting, and may also provide a pool of protein essential for reproduction. 3. While fasting, bears lost 43% of body mass. Of the total energy expended on maintenance and reproduction, 93% was drawn from fat stores. This dependency upon fat conforms to the pattern of nutrient metabolism seen among other species adapted to prolonged fasting. Maternal metabolic rate was less than the predicted resting metabolic rate which illustrates the effectiveness of denning as an energy-conserving strategy. 4. Body fat was critically important for reproductive success. In particular, offspring body weight was very strongly related to the size of maternal fat stores before denning. Fatter bears produced heavier cubs which would be more likely to survive. 5. Among bears, pre-denning body condition was positively associated with age. Within the observed range (4-21 years), age-specific reproductive success should thus be highest among older bears. Such an effect could arise if: (1) the body condition of individual bears tends to improve with age and experience or (2) animals of poorer quality and condition die at a younger age.
Two instances of infanticide and cannibalism in polar bears (Ursus maritimus) were observed in SE Svalbard, at Hopen Island. In the first, aft adult male killed three young cubs at a den site and consumed one of them. In the second an adult male actively pursued, killed, and consumed a dependent yearling. Infanticide of dependent polar bear offspring by adult males may be more common in Svalbard than in other populations because the population is close to carrying capacity or because geographic features reduce spatial segregation of age and sex classes.
The giant panda (Ailuropoda melanoleuca) is one of the world's most endangered mammals and remains threatened by environmental and anthropogenic pressure. It is commonly argued that giant pandas are an evolutionary cul-de-sac because of their specialized bamboo diet, phylogenetic changes in body size, small population, low genetic diversity and low reproductive rate. This notion is incorrect, arose from a poor understanding or appreciation of giant panda biology, and is in need of correction. In this review, we summarize research across morphology, ecology and genetics to dispel the idea, once and for all, that giant pandas are evolutionary dead-end. The latest and most advanced research shows that giant pandas are successful animals highly adapted to a specialized bamboo diet via morphological, ecological and genetic adaptations and co-adaptation of gut microbiota. We also debunk misconceptions around population size, population growth rate and genetic variation. During their evolutionary history spanning eight millions years, giant pandas have survived diet specialization, massive bamboo flowering and die off, and rapid climate oscillations. Now, they are suffering from enormous human interference. Fortunately, continued conservation effort is greatly reducing impacts from anthropogenic interference and allowing giant panda populations and habitat to recover. Previous ideas of a giant panda evolutionary cul-de-sac resulted from an unsystematic and unsophisticated understanding of their biology and it is time to shed this baggage and focus on the survival and maintenance of this high-profile species.
A wealth of information has been generated for the Andean bear Tremarctos ornatus during the past four decades, and a thorough review of the species' natural history, ecology and conservation is provided here.
The Andean bear is the only remaining bear species in South America. Evolutionarily, it is the youngest of all ursids and the only remnant taxon within the subfamily Tremarctinae. The species is distributed throughout the Andes mountain range from Venezuela to Bolivia, but the limits of its current and past range are uncertain.
The species is polyestrous, capable of delayed implantation and a facultative seasonal breeder. Genetic information is scarce, and species‐specific markers need to be developed for a more appropriate assessment of the genetic structure of wild populations.
Andean bears inhabit a wide variety of habitats across the Andes including different forest types and high‐elevation grasslands. They are omnivorous with a frugivorous/folivorous emphasis that is reflected by adaptations of the typical tremarctine mandible.
Andean bears are vulnerable to extinction due to land conversion and poaching. Efforts have been made to protect them, but threats have not been reduced significantly. Therefore, the species is expected to move faster towards extinction than any other carnivore in the region.
Ursine bears are a mammalian subfamily that comprises six morphologically and ecologically distinct extant species. Previous phylogenetic analyses of concatenated nuclear genes could not resolve all relationships among bears, and appeared to conflict with the mitochondrial phylogeny. Evolutionary processes like incomplete lineage sorting and introgression can cause gene tree discordance and complicate phylogenetic inferences, but are not accounted for in phylogenetic analyses of concatenated data. We generated a high-resolution data set of autosomal introns from several individuals per species and of Y-chromosomal markers. Incorporating intraspecific variability in coalescence-based phylogenetic and gene flow estimation approaches, we traced the genealogical history of individual alleles. Considerable heterogeneity among nuclear loci and discordance between nuclear and mitochondrial phylogenies were found. A species tree with divergence time estimates indicated that ursine bears diversified within less than two million years. Consistent with a complex branching order within a clade of Asian bear species, we identified unidirectional gene flow from Asian black into sloth bears. Moreover, gene flow detected from brown into American black bears can explain the conflicting placement of the American black bear in mitochondrial and nuclear phylogenies. These results highlight that both incomplete lineage sorting and introgression are prominent evolutionary forces even on time scales up to several million years. Complex evolutionary patterns are not adequately captured by strictly bifurcating models, and can only be fully understood when analyzing multiple independently inherited loci in a coalescence framework. Phylogenetic incongruence among gene trees hence needs to be recognized as a biologically meaningful signal.
When exposed to human disturbance, most large carnivores are able to move away from the source with little energetic cost. Bears represent an exception in that during winter, most individuals spend several months in an energy-saving state of hibernation in a den. This implies that disturbance of denning bears has the potential to have a large energetic cost, although data on the subject are rather diffuse. We reviewed the literature on densite selection, denning physiology, and responses to disturbance for the brown bear (Ursus arctos), black bear (U. americanus), and polar bear (U. maritimus). Generally, bears select dens one to 2 km from human activity (roads, habitation, industrial activity) and seemed to tolerate most activities that occurred more than one km from the den. Activity closer than one km and especially within 200 m caused variable responses. Some bears tolerate disturbance even inside the den, but bears will abandon dens in response to activity within this zone, especially early in the denning period. Den abandonment by brown and black bear females with cubs of the year can lead to increased cub mortality. Specific excavated or ground dens are rarely reused, whereas natural caves or hollow trees are reused with varying frequency. There is often some distance between an individual bear's consecutive dens. This indicates that loss of a single denning area following human disturbance will not always lead to deleterious effects, if alternative denning areas are available within the home range.
It may seem as if behaviour is so plastic that it would not foster the establishment of a genetically determined trait, but under some circumstances, it can greatly influence whether a novel trait, such as a new morphology, spreads in a population. If the behaviours associated with the trait's function already exist, a new variant finds a ready foothold, and selection can act accordingly. Behaviours that are particularly likely to foster novel traits include those that play a role in life history, such as antipredator behaviour, sexual signalling and foraging. Examples of behaviour facilitating novel trait establishment include the spread of a silent mutant in male Pacific field crickets, Teleogryllus oceanicus, and the propensity of juvenile fence lizards to show antipredator behaviours towards attacking fire ants, providing a selective opportunity for the evolution of longer limbs. The genetic or physiological mechanisms behind a behaviour can also influence its establishment; for example, learning may generate selection in favour of conspicuous novel traits faster, and for a wider range of traits, than genetically based sensory biases. Just as changes in behavioural traits over evolutionary time may expose populations of individuals to new adaptive zones, behavioural variation within a population may increase the diversity of environments to which individuals are exposed.
Polar bears (Ursus maritimus) face extremely cold temperatures and periods of fasting, which might result in more severe energetic challenges than those experienced by their sister species, the brown bear (U. arctos). We have examined the mitochondrial and nuclear genomes of polar and brown bears to investigate if polar bears demonstrate lineage-specific signals of molecular adaptation in genes associated with cellular respiration/energy production. We observed increased evolutionary rates in the mitochondrial cytochrome c oxidase I gene in polar but not brown bears. An amino acid substitution occurred near the interaction site with a nuclear-encoded subunit of the cytochrome c oxidase complex, and was predicted to lead to a functional change, although the significance of this remains unclear. The nuclear genomes of brown and polar bears demonstrate different adaptations related to cellular respiration. Analyses of the genomes of brown bears exhibited substitutions that may alter the function of proteins that regulate glucose uptake, which could be beneficial when feeding on carbohydrate-dominated diets during hyperphagia, followed by fasting during hibernation. In polar bears, genes demonstrating signatures of functional divergence and those potentially under positive selection were enriched in functions related to production of nitric oxide, which can regulate energy production in several different ways. This suggests that polar bears may be able to fine-tune intracellular levels of nitric oxide as an adaptive response to control trade-offs between energy production in the form of ATP versus generation of heat (thermogenesis).
Global warming has the potential to reduce arctic sea ice and thereby increase the length of summer fall fasting when polar bears (Ursus maritimus) lose access to most marine mammals. To evaluate the consequences of such changes, we compared the cost of fasting by polar bears with hibernation by brown bears (U. arctos), American black bears (U. americanus), and polar bears and made projections about tissue reserves polar bears will need to survive and reproduce as fasts become longer. Hibernating polar bears expend energy at the same rate per unit mass as do brown bears and black bears. However, daily mass losses, energy expenditures, and the losses of lean mass are much higher in fasting, active polar bears than in hibernating bears. The average pregnant polar bear living around Hudson Bay during the 1980s and 1990s could fast for 10.0 +/- 2.3 months ((X) over bar +/- SD), and the average lactating female with cubs born during the preceding winter could fast for 4.2 +/- 1.9 months. Thus, some pregnant or lactating females with lower levels of body fat content were already approaching or beyond the constraint of being able to produce cubs and survive the required 8 months of fasting if producing new offspring or 4 months if accompanied by older offspring. Pregnant or lactating females and their dependent offspring have the most tenuous future as global warming occurs. Thus, we predict a significant reduction in productivity with even modest increases in global warming for polar bears living in the very southern part of their range and are concerned about more northern populations depending on their ability to accumulate increasing amounts of fat.
The South American giant short-faced bear (Arctotherium angustidens Gervais and Ameghino, 1880) is one of five described Arctotherium species endemic to South America and it is known for being the earliest, largest, and most carnivorous member of the genus. Here we report an extraordinarily large A. angustidens individual exhumed from Ensenadan sediments (early to middle Pleistocene) at Buenos Aires Province, Argentina. Based on overall size, degree of epiphyseal fusion, and pathologies, this bear was an old-aged male that sustained serious injuries during life. Body mass of the bear is estimated and compared to other ursid species based on a series of allometric equations. To our knowledge, this specimen now represents the largest bear ever recorded. In light of this discovery, we discuss the evolution of body size in Arctotherium (from large-to-small) and compare this to bears that exhibited different evolutionary trajectories. We suggest that the larger size and more carnivorous nature of A. angustidens, compared to later members of the genus, may reflect the relative lack of other large carnivores and abundance of herbivores in South America just after the Great American Biotic Interchange.
We reviewed worldwide spatial patterns in the food habits of the brown bear Ursus arctos in relation to geographical (latitude, longitude, altitude) and environmental (temperature, snow cover depth and duration, precipitation, primary productivity) variables.
We collected data from 28 studies on brown bear diet based on faecal analysis, covering the entire geographical range of this widely distributed large carnivore. We analysed separately four data sets based on different methods of diet assessment.
Temperature and snow conditions were the most important factors determining the composition of brown bear diet. Populations in locations with deeper snow cover, lower temperatures and lower productivity consumed significantly more vertebrates, fewer invertebrates and less mast. Trophic diversity was positively correlated with temperature, precipitation and productivity but negatively correlated with the duration of snow cover and snow depth. Brown bear populations from temperate forest biomes had the most diverse diet. In general, environmental factors were more explicative of diet than geographical variables.
Dietary spatial patterns were best revealed by the relative biomass and energy content methods of diet analysis, whereas the frequency of occurrence and relative biomass methods were most appropriate for investigating variation in trophic diversity.
Spatial variation in brown bear diet is the result of environmental conditions, especially climatic factors, which affect the nutritional and energetic requirements of brown bears as well as the local availability of food. The trade‐off between food availability on the one hand, and nutritional and energetic requirements on the other hand, determines brown bear foraging decisions. In hibernating species such as the brown bear, winter severity seems to play a role in determining foraging strategies. Large‐scale reviews of food habits should be based on several measures of diet composition, with special attention to those methods reflecting the energetic value of food.
We examined relationships befween reproductive performance of female Minnesota black bears (Ursus americanas) and various potential indicators of nutritional condition during late hibernation. Litter size (n = l0l litters) was influenced more by litter order (first or subsequent) than by matemal condition, except perhaps in very large females. An increased proportion of male cub births corresponded with increased maternal weight and serum alkaline phosphatase (ALKP) and decreased serum creatinine (CR). Weight and growth of cubs and yearlings were closely related to mother's size; they also conelated positively with maternal ALKP, and negatively with serum total protein (TP), and mean corpuscular volume (MCV). Cub survival was affected only when mother's weight 2 months postpartum was below about 65 kg. No juvenile females (2-8 years old) weighing <41 kg in March produced their first cubs the following spring, but 57% of those above this threshold weight produced cubs. Litter frequency and yearling recruitment were unrelated to maternal condition. Life history parameters of black bears appear to respond to declining nutrition in the following sequence: (l) litter size declines, then stabilizes across a broad range of maternal weights; (2) age of first reproduction increases; (3) juvenile survival decreases; (4) first-year cub survival decreases; and (5) litter frequency decreases. Better definition of these relationships, particularly at nutritional extremes, will likely require collaborative efforts of researchers studying diverse populations.
Cranial and postcranial elements of at least eight individuals of Arctodus pristinus (Ursidae: Tremarctinae) are reported from Leisey Shell Pit, an early Irvingtonian vertebrate locality within a marine shell bed. Dental measurements indicate that two distinct sizes, presumably representing males and females, are recognizable in the collection. This bear tentatively is referred from 13 localities in Florida, ranging in age from late Blancan to late Irvingtonian, and is a biochronological indicator for this period. Another tremarctine bear, Tremarctos floridanus, so far is known only from Rancholabrean localities in Florida and may have ecologically replaced A. pristinus.
The ancestral panda Ailurarctos lufengensis, excavated from the late Miocene, is thought to be carnivorous or omnivorous [1]. Today, giant pandas exclusively consume bamboo and have distinctive tooth, skull, and muscle characteristics adapted to a tough and fibrous bamboo diet during their long evolution [1, 2]. A special feature, the pseudo-thumb, has evolved to permit the precise and efficient grasping of bamboo [3, 4]. Unlike those of extant pandas, little is known about the diet and habitat preferences of extinct pandas. Prevailing studies suggest that the panda shifted to specialized bamboo feeding in the Pleistocene [5, 6]; however, this remains questionable. Pandas now survive in a fraction of their historical habitat [7], but no specific information has been reported. Stable isotope analyses can be used to understand diet- and habitat-related changes in animals [8]. Isotopic signals in bone collagen reflect dietary compositions of ancient human diets [9, 10] and dietary changes between historical and modern animal populations [11, 12]. Here, we conduct stable isotope analyses of bone and tooth samples from ancient and modern pandas and from sympatric fauna. We show that pandas have had a diet dominated by C3 resources over time and space and that trophic niches of ancient and modern pandas are distinctly different. The isotopic trophic and ecological niche widths of ancient pandas are approximately three times larger than those of modern pandas, suggesting that ancient pandas possibly had more complex diets and habitats than do their modern counterparts. Our findings provide insight into the dietary evolution and habitat contraction of pandas.
Many factors influence whether mammals reproduce seasonally or continuously but disentangling them can be challenging in free-living species that are hard to observe. We described the seasonality of reproduction in Andean bears (Tremarctos ornatus) in NW Peru (6°26'S, 79°33'W) to test for phenotypic plasticity in response to extrinsic cues. To do so, we compared the mating behavior and birthdates of free-living bears to the birthdates of captive bears housed over a broad range of latitudes. Free-living bears were observed on 302 occasions over 6 years (967 field-days), and mating behaviors recorded 61 times from late Dec to Jan. The mean birthdate of 12 wild-born litters was 17 August (range = 23 Jun - 15 Oct), 57 ± 10 (SD) days after the winter solstice. Birthdates for 367 captive litters varied widely by comparison (range = 1 Jan - 31 Dec; mean = 14 ± 49 days after the winter solstice). However, captive bears in the tropics had fewer births in autumn and winter (71.4% of births) than captive bears at higher latitudes (96.8% of births; P < 0.001). Differences in seasonal reproduction among captive bears at high and low latitudes and captive and a free-living at tropical latitudes suggest that Andean bears display phenotypic plasticity in reproductive timing but influenced by photoperiod at high latitudes. Because photoperiodic effects were less evident at tropic latitudes, we suggest that seasonality in the timing of reproduction in the free-living bears we observed was influenced by seasonal variation in food abundance. The observed effect of photoperiod on reproduction in captive Andean bears at high latitudes may also imply that free-living bears at the southern edge of the range may be constrained in their ability to adjust reproductive timing to resource availability as environments change.
Climate change and human impacts are often implicated in Quaternary megafaunal extinctions. The discovery of associated remains of extinct giant short-faced bears (Arctodus simus) and invading brown bears (Ursus arctos) raises the possibility of competition as another potential factor. We describe fossil remains of both genera from Pellucidar Cave, Vancouver Island, Canada. Analyses of ancient mitochondrial DNA support the identifications of post-cranial brown bear specimens and assign these bears to Clade 4. Our results are consistent with the migration of brown bears from Eastern Beringia to the contiguous United States before the Last Glacial Maximum (LGM) and to Vancouver Island as environmental conditions became favorable after the LGM. Radiocarbon age estimates on these specimens indicate the presence of giant short-faced bears approximately 13.5 thousand calibrated years before present (cal. ka BP; uncalibrated 11,775. ±. 30, 11,720. ±. 50, and 11,615. ±. 30 BP) and of brown bears immediately preceding (~14.5 cal. ka BP; 12,440. ±. 35, 12,425. ±. 30 BP) and following this time (~13 cal. ka BP; uncal. 11,100. ±. 30 BP), suggesting niche partitioning to reduce competition among these species. We suggest that shifts in food availability or quality due to post-glacial vegetation and faunal changes were probably of primary importance in the arrival and the disappearance of giant short-faced bears on Vancouver Island. This study focuses on a key time period and geographic location that is useful in understanding Pleistocene extinctions in North America.
Daybeds are essential for the survival of brown bears (Ursus arctos L., 1758) and may represent a population-limiting resource in human-dominated landscapes. In this study, we demonstrate which land-cover types and bear characteristics affect daybed selection in north-central Slovakia. We used the positional and activity data of 21 bears acquired by GPS–GSM telemetry to identify 3864 daybeds. By use of K-select analysis and linear mixed-effects modelling, we explored how bears chose these places for their daytime resting. The most important drivers for daybed selection were the presence of dense regenerating forests and forest–shrubbery belts in farmland. Bears avoided resting in older forests without suitable undergrowth. Females selected daybeds differently depending on the presence of dependent cubs. During spring – early summer, females with cubs of the year avoided other bears by selecting more rugged terrain. These females also selected daybeds significantly closer to human settlements than adult males, possibly to avoid the risk of infanticide. In late summer – autumn, all bears selected daybeds closer to human settlements than in spring, probably because they were attracted by maize (Zea mays) fields and fruit trees. Many daybeds were located outside protected areas in farmland closer to people, which could increase bear–human conflicts.
Evolutionary hypotheses to explain the greater numbers of species in the
tropics than the temperate zone include greater age and area, higher
temperature and metabolic rates, and greater ecological opportunity. These
ideas make contrasting predictions about the relationship between speciation
processes and latitude, which I elaborate and evaluate. Available data suggest
that per capita speciation rates are currently highest in the temperate zone,
and that diversification rates (speciation minus extinction) are similar
between latitudes. In contrast, clades whose oldest analyzed dates precede the
Eocene thermal maximum, when the extent of the tropics was much greater than
today, tend to show highest speciation and diversification rates in the
tropics. These findings are consistent with age and area, which is alone among
hypotheses in predicting a time trend. Higher recent speciation rates in the
temperate zone than the tropics suggest an additional response to high
ecological opportunity associated with low species diversity. These broad
patterns are compelling but provide limited insights into underlying
mechanisms, arguing that studies of speciation processes along the latitudinal
gradient will be vital. Using threespine stickleback in depauperate northern
lakes as an example, I show how high ecological opportunity can lead to rapid
speciation. The results support a role for ecological opportunity in
speciation, but its importance in the evolution of the latitudinal gradient
remains uncertain. I conclude that per-capita evolutionary rates are no longer
higher in the tropics than the temperate zone. Nevertheless, the vast numbers
of species that have already accumulated in the tropics ensure that total rate
of species production remains highest there. Thus, tropical evolutionary
momentum helps to perpetuate the steep latitudinal biodiversity gradient.
Studies of species with continental distributions continue to identify intraspecific lineages despite continuous habitat. Lineages may form due to isolation by distance, adaptation, divergence across barriers, or genetic drift following range expansion. We investigated lineage diversification and admixture within American black bears (Ursus americanus) across their range using 22k SNPs and mitochondrial DNA sequences. We identified three subcontinental nuclear clusters which we further divided into nine geographic regions: Alaskan (Alaska-East), eastern (Central Interior Highlands, Great Lakes, Northeast, Southeast), and western (Alaska-West, West, Pacific Coast, Southwest). We estimated that the western cluster diverged 67 kya, before eastern and Alaskan divergence 31 kya; these divergence dates contrasted with those from the mitochondrial genome where clades A and B diverged 1.07 Mya, and clades A-east and A-west diverged 169 kya. We combined estimates of divergence timing with hindcast species distribution models to infer glacial refugia for the species in Beringia, Pacific Northwest, Southwest, and Southeast. Our results show a complex arrangement of admixture due to expansion out of multiple refugia. The delineation of the genomic population clusters was inconsistent with the ranges for 16 previously described subspecies. Ranges for U. a. pugnax and U. a. cinnamomum were concordant with admixed clusters, calling into question how to order taxa below the species level. Additionally, our finding that U. a. floridanus has not diverged from U. a. americanus also suggests that morphology and genetics should be reanalyzed to assess taxonomic designations relevant to the conservation management of the species.
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Brown bears (Ursus arctos) are a long-lived and widely distributed species that occupy diverse habitats, suggesting ecological flexibility. Although inferred for numerous species, ecological flexibility has rarely been empirically tested against biological outcomes from varying resource use. Ecological flexibility assumes species adaptability and long-term persistence across a wide range of environmental conditions. We investigated variation in population-level, coarse-scale resource use metrics (i.e., habitat, space, and food abundance) in relation to indices of fitness (i.e., reproduction and recruitment) for brown bears on Kodiak Island, Alaska, 1982–97. We captured and radiocollared 143 females in 4 spatially-distinct segments of this geographically-closed population, and obtained ≥30 relocations/individual to estimate multi-annual home range and habitat use. We suggest that space use, as indexed using 95% fixed kernel home ranges, varied among study areas in response to the disparate distribution and abundance of food resources. Similarly, habitat use differed among study areas, likely a consequence of site-specific habitat and food (e.g. berries) availability. Mean annual abundance and biomass of spawning salmon (Oncorhynchus spp.) varied >15-fold among study areas. Although bear use of habitat and space varied considerably, as did availability of dominant foods, measures of fitness were similar (range of mean litter sizes = 2.3–2.5; range of mean number of young weaned = 2.0–2.4) across study areas and a broad range of resource conditions. Our data support the thesis that brown bears on Kodiak Island display ecological flexibility. This adaptability is likely representative of the entire species and has helped facilitate its wide geographic distribution and abundance. We suggest variation in brown bear resource use necessitates area-specific management strategies to ensure suitable conditions for their long-term persistence.
Reconsiders the evidence relating body size and fasting endurance, considers the evolution of both large and small sizes, and examines the relationship between fasting endurance and other environmental correlates of body size. -from Authors
From April 1989 through June 1992, we studied the denning ecology of 45 female polar bears in the Canadian Arctic archipelago, around M'Clure Strait and Viscount Melville Sound. Activity level of pregnant females was significantly lower in July-August compared to solitary, nonpregnant females. All maternity dens (n = 25) were located on land and were well dispersed throughout the study region. On average, females selected sites of maternity dens on 17 September and departed from dens on 21 March for a total den tenure of 186 +/- 3 (SE) days. Middle-aged females left dens earliest, and young and old females had the longest den tenures. Activity level of females in maternity dens suggested that by 15 December most births already had occurred. Six of 20 nival shelters used by nonparturient females were located on the sea ice. In contrast to maternity dens, shelters were occupied for an average of only 53 +/- 9 (SE) days, mainly from mid-December to late January. We hypothesize that shelters are used strictly for conserving energy during the coldest period of winter when conditions for hunting seals generally are unfavorable.
Ninety-four black bear (Ursus americanus) bedding sites were located between May 1982 and August 1984. Locations were identified by radiotracking 14 adult females. Sampling from sites of females with and without cubs showed both bedding and feeding activity at 39% of the sites. Multiple daybeds were found at 28% of the sites and all sites were within 0.8 km of water. Bedding sites occurred on canyon walls 81% of the time, the slopes of which averaged 39%. Daybeds were on the uphill side of a tree 74% of the time. Bed trees averaged 73 cm dbh. Chewing and scratching of daybed trees was recorded at 38% of the sites, and scats were found at 69% of the sites. Removal of vegetative cover and large trees in black bear bedding habitat could reduce overall habitat quality.
The distribution of polar bears (Ursus maritimus) is circumpolar in the Northern Hemisphere, but known locations of maternal dens are concentrated in relatively few, widely scattered locations. Denning is either uncommon or unknown within gaps between known denning concentration areas. The Beaufort Sea region of Alaska and Canada lies in the largest of those gaps. To understand effects of industrial development and proposed increases in hunting, the temporal and spatial distribution of denning in the Beaufort Sea must be known. We captured and radiocollared polar bears between 1981 and 1991 and determined that denning in the Beaufort Sea region was sufficient to account for the estimated population there. Of 90 dens, 48 were on drifting pack ice, 38 on land, and 4 on land-fast ice. The proportion of dens on land was higher (P = 0.029) in later compared with earlier years of the study. Bears denning on pack ice drifted as far as 997 km (x̄ 385 km) while in dens. There was no difference in cub production by bears denning on land and pack dice (P = 0.66). Mean entry and exit dates were 11 November and 5 April for land dens and 22 November and 26 March for pack-ice dens. Female polar bears captured in the beaufort Sea appeared to be isolated from those caught east od Cape Bathurst in Canada. Of 35 polar bears that denned along the mainland coast of Alaska and Canada 80% denned between 137°00′W and 146°59′W. Bears followed to > 1 den did not reuse sites and consecutive dens were 20-1,304 km apart. However, radio-collared bears were largely faithful to substrate (pack-ice, land, and land-fast ice) and the general geographic area of previous dens. Bears denning on land may be vulnerable to human activities such as hunting and industrial development. However, predictable denning chronology and lack of site fidelity indicate that many potential impacts on denning polar bears could be mitigated.
We investigated the effect of hibernation and reproductive status on changes in body mass and composition of adult female brown bears (Ursus arctos) on the Kenai Peninsula, Alaska. This information is fundamental to understanding nutritional ecology of wild brown bear populations. Six adult females handled in the fall and following spring (paired samples) lost 73 ± 22 kg (x̄ ± SD; 32 ± 10%) of fall body mass over 208 ± 19 days. Of this mass loss, 56 ± 22% (55 ± 22 kg) was lipid and 44 ± 22% (43 ± 21 kg) was lean body mass. Catabolism of lipid stores accounted for 88.4 ± 8.1% of the body energy used to meet maintenance demands. Overwinter differences in body composition of adult females assessed only once in either the fall (n = 21) or spring (n = 32) were similar to those of paired samples. Relative fatness of bears entering the den was positively related to the contribution of fat (%) to body mass (P < 0.01) and body energy (P < 0.01) losses during hibernation. Thus, relative fatness at the onset of fasting influences the relative proportion of lipid stores and lean body mass catabolized to meet protein and energy demands during hibernation. In the spring, lone females had greater body and lean masses than females with cubs of the year or yearlings. Lipid content was greatest in lone females in the fall. Studies using body mass and composition as indices of population health should consider season or reproductive class.
Loss of a dominant competitor can open ecological opportunities. Ecological opportunities are considered prerequisites for adaptive radiations. Nonetheless, initiation of diversification in response to ecological opportunity is seldom observed, so we know little about the stages by which behavioural variation either increases or coalesces into distinct phenotypes.
Here, a natural experiment showed that in a tropical island's guild of army ant‐following birds, a new behavioural phenotype emerged in subordinate spotted antbirds ( Hylophylax naevioides ) after the socially dominant ocellated antbird ( Phaenostictus mcleannani ) died out.
Individuals with this behavioural phenotype are less territorial; instead, they roam in search of ant swarms where they feed in locations from which dominant competitors formerly excluded them. Roaming individuals fledge more young than territorial individuals.
We conclude that ecological opportunity arising from species loss may enhance the success of alternative behavioural phenotypes and can favour further intraspecific diversification in life‐history traits in surviving species.
A study of the behavior and ecology of the spectacled bear, Tremarctos ornatus, in Perú revealed that they occur in all three ranges of the Peruvian Andes, with the majority in the “ceja de selva,” or eastern slope of the Cordillera Oriental. As evidenced by feeding sign and contents of scats, 83 foods were confirmed in the diet of spectacled bears. These include insects, rodents, livestock, corn, berries, tree wood, bamboo hearts, palm frond petioles, 22 species of Bromeliaceae, 11 species of Cacta-ceae, and the fruits of 31 species of trees. Spectacled bears feed predominately on bromeliad hearts, particularly during the months when fruits are not ripe. From February to July, when most of the fruits are ripe, the animals move to the sources of these preferred foods. Direct observations provided information on the bear's corn-eating behavior. Certain other behavioral activities, such as the building of tree nests, were confirmed. Humid forests between 1,900 and 2,350 m in elevation, and coastal thorn forests are preferred habitats when water is available. Spectacled bears enjoy the most protection in the saturated rain forests in the “ceja de selva” and in the seasonally occupied areas at the upper extreme of the animal's elevational range. The spectacled bear is not in immediate danger of extinction in Peru owing to its adaptation to a diversity of habitats and the difficulty of access to bear areas with more than 45-degree slopes.
Herbivores face various nutritional challenges in their life cycles, challenges that may become increasingly acute under ongoing environmental changes.
Here, focusing on calcium, phosphorus and nitrogen, we used nutritional geometry to analyse individual‐based data on foraging and extraction efficiencies, and combined these with data on reproduction and migratory behaviour to understand how a large herbivorous carnivore can complete its life cycle on a narrow and seemingly low quality bamboo diet.
Behavioural results showed that pandas during the year switched between four main food categories involving the leaves and shoots of two bamboo species available. Nutritional analysis suggests that these diet shifts are related to the concentrations and balances of calcium, phosphorus and nitrogen. Notably, successive shifts in range use and food type corresponded with a transition to higher concentrations and/or a more balanced intake of these multiple key constituents.
Our study suggests that pandas obligatorily synchronize their seasonal migration and reproduction with the disjunct nutritional phenologies of two bamboo species. This finding has potentially important implications for habitat conservation for this species and, more generally, draws attention to the need for understanding the nutritional basis of food selection in devising management plans for endangered species.
Winter denning is a critical component of American black bear (Ursus americanus) ecology. Mississippi has a small recolonizing population (about 50 individuals), including the federallythreatened Louisiana black bear (U. a. luteolus), and knowledge of den use is needed to effectively conserve the species. We quantified black bear denning chronology (n = 15) and den use (n = 18) in Mississippi during 2005-2011. Denning was highly variable and females entered dens earlier than males and emerged later; multiple den use by both sexes in a single winter was common. We recorded equal numbers of tree and ground dens, with ground dens at higher elevations surrounded by dense vegetation. With the exception of all bears denning each winter, black bears in Mississippi exhibited denning chronology and characteristics similar to other black bear populations in the southeastern United States.
Hibernation is widely regarded as an adaptation to seasonal energy shortage, but the actual influence of energy availability on hibernation patterns is rarely considered. Here we review literature on the costs and benefits of torpor expression to examine the influence that energy may have on hibernation patterns. We first establish that the dichotomy between food- and fat-storing hibernators coincides with differences in diet rather than body size and show that small or large species pursuing either strategy have considerable potential scope in the amount of torpor needed to survive winter. Torpor expression provides substantial energy savings, which increase the chance of surviving a period of food shortage and emerging with residual energy for early spring reproduction. However, all hibernating mammals periodically arouse to normal body temperatures during hibernation. The function of these arousals has long been speculated to involve recovery from physiological costs accumulated during metabolic depression, and recent physiological studies indicate these costs may include oxidative stress, reduced immunocompetence, and perhaps neuronal tissue damage. Using an optimality approach, we suggest that trade-offs between the benefits of energy conservation and the physiological costs of metabolic depression can explain both why hibernators periodically arouse from torpor and why they should use available energy to minimize the depth and duration of their torpor bouts. On the basis of these trade-offs, we derive a series of testable predictions concerning the relationship between energy availability and torpor expression. We conclude by reviewing the empirical support for these predictions and suggesting new avenues for research on the role of energy availability in mammalian hibernation.