We review developments in the study of stress in free-ranging mammals and summarize the physiological and behavioral components of the stress response. Both the sympathetic nervous system response and the regulation and reactivity of the hypothalamic–pituitary–adrenal (HPA) axis are discussed. In particular, we describe how the activity of the HPA axis at baseline levels follows circadian and circannual rhythms in ways that allow animals to respond to predictable environmental changes, focusing largely on the endpoint of this axis, the glucocorticoid hormones cortisol and corticosterone. Superimposed upon these rhythms are the elevated glucocorticoid levels characteristic of the stress response, which allow an animal to respond to unpredictable social, physical, or environmental challenges. Methods used to explore the stress response in free-ranging mammals are described. Both inter- and intraspecific variation in the stress response as they relate to the environment are discussed. Finally, how the regulation and reactivity of the HPA axis varies by life-history stage and sex in mammals is reviewed, focusing on reproduction and development.

Habitat constrains and shapes successful ecological and physiological strategies and thus provides the context for the evolution of life-history traits. The stress axis plays a vital role in the endocrine system and is a critical component adapting mammals to particular habitat pressures. It is subject to both individual activational and organizational plasticity as well as to evolutionary modification. To illustrate, I examine the suite of traits of the stress axis associated with breeding frequency in male mammals, which varies in a continuum from semelparity to iteroparity. During the breeding season, males in species at the semelparous end of the continuum exhibit high concentrations of free corticosteroids, low concentrations of glucocorticoid-binding protein, a failure of the negative feedback system, a gonadal axis that is not inhibited by high corticosteroid concentrations, and immunosuppression. Iteroparous species exhibit the opposite traits. The evolutionary constraints selecting for the former may partially be related to phylogeny (in marsupials) as well as to an interaction of the restrictions imposed by the environment on female reproduction, the mating system, the high costs of reproduction, and the low adult survival during the nonbreeding season.

Vertebrates have regular patterns and routines that involve obtaining food and carrying out life-history stages such as breeding, migrating, molting, and hibernating. These are generally regulated by predictable changes in the environment, for example, seasons. Superimposed on these are unpredictable challenges, for example, storms and natural disasters, which have great potential for stress. The concept of allostasis, maintaining stability through change, has been introduced as a fundamental process through which organisms actively adjust to both predictable and unpredictable events. This process considers the predictable and unpredictable components of the environment as a continuum and includes the effects of body condition (parasites and injuries), experience, and habitat configuration. It combines classical homeostasis with anticipatory responses, stress, and social status. By using the balance between energy input and expenditure as the basis for applying the concept of allostasis, 2 types of allostatic overload have been proposed. Type 1 allostatic overload occurs when energy demand exceeds supply, resulting in activation of the emergency life-history stage. This serves to direct the animal away from normal life-history stages into a survival mode that decreases allostatic load and regains positive energy balance. The normal life cycle can be resumed when the perturbation passes. Type 2 allostatic overload begins when there is sufficient or even excess energy consumption accompanied by social conflict and other types of social dysfunction. The latter is the case in human society, and in some situations affecting animals in captivity and possibly in natural conditions as well. In all cases, secretion of glucocorticoids increases with allostatic load, but if it is chronically high, then pathologies develop. Curiously, type 2 allostatic overload does not trigger an escape response, and can only be counteracted through learning or changes in social structure.

In social animals, reproductive success is often related to social dominance. In cooperatively breeding birds and mammals, reproductive rates are usually lower for social subordinates than for dominants, and it is common for reproduction in subordinates to be completely suppressed. Early research with captive animals showed that losing fights can increase glucocorticoid (GC) secretion, a general response to stress. Because GCs can suppress reproduction, it has been widely argued that chronic stress might underlie reproductive suppression of social subordinates in cooperative breeders. Contradicting this hypothesis, recent studies of cooperative breeders in the wild show that dominant individuals have elevated GCs more often than do subordinates. Here, I summarize relationships between rank, aggression, and GCs from field studies of 3 cooperatively breeding carnivores: the dwarf mongoose (Helogale parvula), the African wild dog (Lycaon pictus), and the gray wolf (Canis lupus). In all 3 species, GC levels are higher in dominants than in subordinates for 1 or both sexes. Higher GCs are associated with higher rates or severity of aggression in some cases, but not all. As studies have accumulated, the patterns observed in these carnivores are emerging as typical for cooperative breeders.

In southern California the Dulzura kangaroo rat (Dipodomys simulans) occurs in shrub-dominated habitats adjacent to forb- or grass-dominated habitats, which support the endangered Stephens's kangaroo rat (D. stephensi). We monitored a number of populations of both species from 1996 through 2000, spanning the 1997–1998 El Niño Southern Oscillation (ENSO) event. Although populations of D. stephensi were not significantly impacted by this climatic event, D. simulans declined from 10–24 individuals/ha to about 7–12 individuals/ha over a 6- to 8-month period; numbers subsequently continued to decline to 1–2 individuals/ha and failed to recover in the approximately 2.5 years of post-ENSO monitoring. We noticed no signs of sickness or illness among these animals, although adult body masses of D. simulans (but not D. stephensi) declined through the ENSO event, suggesting that food resources for the former species may have been compromised. Alternatively, populations of D. simulans may have been unusually high before the ENSO event, such that the onset of rains merely hastened a predictable winter die-off. Finally, other rodents could have robbed seeds from the food caches of Dulzura kangaroo rats, influencing the ability of the latter to survive the ENSO event. Although this explanation remains speculative, cache pilferage has been demonstrated among other heteromyids, and a recent theoretical study suggests this may be an important mechanism of species coexistence. Further investigations of cache pilferage could enhance our understanding of the ecology and evolution of these diverse small mammal assemblages.

We examined the nest-tree preferences of northern flying squirrels (Glaucomys sabrinus) in an old-growth, mixed-conifer and red fir (Abies magnifica) forest of the southern Sierra Nevada of California. We tracked 27 individuals to 122 nest trees during 3 summers. Flying squirrels selected nest trees that were larger in diameter and taller than either random trees or large (>50-cm diameter at breast height) nearest-neighbor trees. Snags were used more often than live trees relative to their availability. Nest trees were usually close to riparian habitat; 86% of nest trees were <150 m from a perennial creek. Flying squirrels selected red fir and avoided incense cedar (Calocedrus decurrens). Mean distances between nest trees and size of core-nest areas were greater for males than for females. No detectable relationship was found between size of core-nest area and distance to a perennial creek. These results suggest that flying squirrels of the Sierra Nevada may require large trees and snags within 150 m of perennial creeks for their critical habitat needs.

The Indian short-nosed fruit bat (Cynopterus sphinx) is thought to use a resource-based polygynous mating system. Breeding colonies typically consist of a few harems with 1 or more solitary males nearby. Essential to understanding the mating system of C. sphinx is identification of the reasons for solitary roosting behavior in adult males. In this study, we attempt to elucidate the impetus behind solitary roosting by investigating the following 2 questions: Are solitary males less competitive and so remain isolated from breeding activities? Are trees and foliage suitable for tent-making a scarce resource? We carried out weekly censuses and mark–recapture studies from January to December 2000. We assessed impact of available resources on roosting patterns of adult males. Examination of our results suggests the following about the mating system of these bats. Female aggregation in C. sphinx cannot be attributed to scarce resources. Morphological variables did not differ between harem and nonharem males. Approximately 50% of nonharem males had scrotal testes. These results suggest that nonharem males are reproductively active, gain access to females, and presumably obtain some reproductive success.

Resource availability and reproductive activity can exert contrasting demands on movements of promiscuous or polygynous mammals. Movements of the black-eared opossum (Didelphis aurita) vary seasonally in the Atlantic Rainforest of Brazil, so our objectives were to test the relative importance of reproductive activity and resource abundance on movement areas and intensity of use by opossums. We used a spool-and-line device to map the path of individuals, to estimate daily movement areas, and to measure intensity of habitat use. Two models were compared by generalized linear models, by grouping movements by reproductive or climatic season. Males used larger areas less intensively in the breeding season, whereas the movement of females did not differ between breeding and nonbreeding season. Differences between movements of females were significant only when grouped by climatic seasons, with females using larger areas less intensively in the dry season. Movements of females were determined by resource availability, whereas movements of males were determined by reproductive season.

We captured, marked, and recaptured southern short-tailed shrews (Blarina carolinensis) during a 30-month livetrapping study in a woodlot in Jackson County, Illinois, to compare aspects of their life history with those of the northern short-tailed shrew (B. brevicauda). A total of 106,496 trap checks (15,782 trap nights) resulted in 3,430 captures of 313 B. carolinensis from February 1996 through August 1998. Trapping mortality was only 18 individuals. Sex ratio did not differ from 1:1. Estimated population density peaked at 57 individuals/ha in late summer and autumn then declined during winter. Recruitment, including birth and immigration, peaked in spring and late summer each year. Individuals entering the population in the spring and early summer had higher survival rates than those entering in the autumn. A weak correlation was found between recruitment and precipitation, and between population density and humidity. Shrew activity (timing of captures) showed significant relationships with light condition and season. During summer, shrews were caught more frequently at night. In the winter, they were captured more frequently during the day. Capture rate was negatively related to precipitation and positively related to humidity. Population dynamics and activity patterns were similar to those of B. brevicauda.

Circadian patterns of activity have important implications for numerous aspects of a species' biology, including patterns of sociality and paternal care. The activity patterns of subterranean rodents are of particular interest because of the presumed lack of environmental entrainment cues available in underground habitats. We used radiotelemetry to monitor activity of adult cururos (Spalacopus cyanus) in 2 populations of this species from north-central Chile. The locations of radiocollared animals from Parque Nacional Fray Jorge (n = 10 adults) and Santuario de la Naturaleza Yerba Loca (n = 8 adults) were determined hourly for 72 consecutive hours during austral summer, 2003. Examination of these data revealed that surface and subterranean activity were largely restricted to daylight hours. Specifically, the following measures of activity were found to be significantly greater during daytime: percentage of animals outside of nest, distance from nest, and distance between successive locations at which an animal was detected. In addition, the occurrence of cururo vocalizations (typically given by animals at burrow entrances) was significantly associated with daylight. Collectively, these analyses indicate that, contrary to the behavior of captive S. cyanus, free-living cururos are diurnal. Physical and social environments in which captive animals are housed may contribute to observed differences in activity between field and laboratory populations of this species.

Annual cycles in life-history traits often are interpreted as adaptations. The Mt. Graham red squirrel (Tamiasciurus hudsonicus grahamensis) is an endangered species isolated in the Pinaleno Mountains of Arizona (United States) at the southernmost range terminus where the ecology is poorly known. I monitored annual cycles of reproduction and body mass in male and female Mt. Graham red squirrels. Males with scrotal testes were present during all months except October, with a peak in testis size in April. Lactating females were found from April to September. Male body mass was highest in winter and lowest in summer; females did not fluctuate seasonally in mass. Male body mass exceeded that of females during winter and was positively related to size of testes late in the breeding season. Annual cycles in reproduction and body mass of Mt. Graham red squirrels are similar to those of other red squirrels in similar life zones.

We investigated influences of the thermal environment on patterns of body temperature (T_b), activity, and use of burrows during the active season in a population of free-living arctic ground squirrels (Spermophilus parryii). Arctic ground squirrels normally exhibited a daily 5°C range in T_b, and had higher T_b when above ground than when in burrows (P < 0.0001). This difference decreased as standard operative temperature (T_es; an index of environmental heat load) increased. Ground squirrels entered burrows more frequently on warmer compared to average or cooler days and when T_b exceeded 39°C. On cool days with heavy precipitation, ground squirrels remained almost exclusively below ground, and peaks in T_b were associated with brief aboveground forays. Time on the surface was maximal (about 80% between 0500 and 2200 h) at T_es = 17–33°C and decreased proportionately with decreasing T_es from 17°C to −2°C. Forty-six percent of variation in timing of presence above ground could be explained by a series of thermal and nonthermal variables related to environmental heat transfer. This suggests that diurnal activity patterns in this arctic environment with 24-h daylight result from a strategy that minimizes thermoregulatory costs.

Incremental dentin and associated enamel, features visible on the surface of lower incisors, were characterized for 3 species of ground squirrels (Spermophilus): Pleistocene and Recent S. elegans, Recent S. richardsonii, and Recent S. parryii. A hibernation mark was evident in incisor dentin and enamel, most characteristically as a sleeve of enamel terminating basally adjacent to medially depressed dentin with indistinct and often very fine increments. This mark was absent in juveniles but present in older animals of both sexes for at least 6 weeks after hibernation, eventually being lost through growth and wear of the incisor. Temporal association with hibernation was confirmed from specimens of S. richardsonii with known dates of hibernation. Parturition and onset of lactation were usually associated with reduction in thickness of dentin increments but could not be recognized unambiguously. Combining wear stage of cheek teeth with the presence and location of the hibernation mark allowed placement of many specimens into age and season categories at time of death (young of year, early-season adults, and late-season adults). Examination of lower incisors of Pleistocene S. elegans from Porcupine Cave in central Colorado showed that hibernation was recorded in fossils and confirmed the utility of event-anchored incremental dentin in elucidating taphonomic questions.

The gray short-tailed opossum (Monodelphis domestica) began tooth eruption at 32 days with the deciduous premolars (dp3 and dP3). All but 5 teeth completed eruption by 56 days, with only p3, m4, P3, M3, and M4 unerupted at that age. Eruption was complete by 134 days with eruption of P3 and M4. We found no significant differences between sexes in tooth eruption timing, but significant differences occurred among litters at some tooth positions. Timing and sequence of tooth eruption differed somewhat as seen in live young versus that seen in a series of skulls of known age. Live juveniles can be placed into a series of 6 age classes based on emergence of teeth through the gingiva, whereas juvenile skeletal material is best placed into age classes based solely on eruption of upper molars. Other systems of age classes used in didelphid marsupials that are based on sequence of eruption of P3 and M4 are not generally applicable because of variation in this sequence. In didelphids delayed eruption of I1 may be functionally related to weaning, whereas weaning and 1st reproduction are not obviously correlated with age class based on molar eruption.

Bowhead whales (Balaena mysticetus) hauled out on shore-fast ice by Inupiat whale hunters off Barrow, Alaska were examined. Anatomical observations confirmed the occurrence of a large, well-muscled tongue. Temporomandibular articulations were synovial. The mandibular symphysis was unfused. Standard measurement of baleen plates and close-range photogrammetry of a baleen rack confirmed that the anterior portion of the baleen rack is strongly convex, in contrast to its shape in balaenopterids. Moderate force applied to the lower lip in a lateral direction caused a pronounced abduction of the lower jaw. These observations support a conclusion that during feeding, the tongue may deflect the incoming flow of prey-laden water from side to side in the mouth, to convey prey slurries into the postlingual recess. Abduction of the lower lip likely would establish a channel for acceleration of flow around the outside of the baleen racks, reducing external pressures and drawing water out through the baleen. Thus, the shape of the baleen rack in the bowhead appears to be an adaptation to reduce the amplitude of the bow wave projected during feeding, the simultaneous advantage being concentration of prey slurries inside the mouth. It may also impart a configuration to the bow wave that stimulates countereffective evasive effort by actively mobile prey. Final compaction of a concentrated prey slurry in the postlingual recess probably involves retraction of the tongue against the oropharyngeal wall. These insights notably enable consideration of certain threats to bowheads and right whales (Balaenidae) associated with oil spills and oral entanglement. Previous research on the “physiologic” effects of fouling of baleen with oil is judged to be inadequate with respect to this taxonomic family. Oral entanglement with nonbiodegradable marine debris is predicted to be lethal because of interference with a critical hydrostatic oral seal.

We sequenced the 12S rRNA gene of 2 elusive and morphologically plesiomorphic species of phalanger: the small Sulawesi cuscus (Strigocuscus celebensis—Gray, 1858) and the Sulawesi bear cuscus (Ailurops ursinus—Temminck, 1824). The sequences were integrated with previously existing data on the same gene in other species of phalangerids, as well as newly derived data from Wyulda Alexander, 1918. In contrast to current wisdom, we resolve S. celebensis not as a member of the tribe Trichosurini, but rather as a taxon sister to Ailurops in a reconstituted Ailuropinae in turn successively sister to Phalangerinae. Examination of our data supports an evolutionary origin for the family approximately 34 million years ago (mya), in the northwestern region of the Sahul Shelf, the continental mass underlying Australia and New Guinea. The radiation of the most plesiomorphic genera in the family, Trichosurus and Wyulda, is restricted to that region. S. celebensis, resolved as sister to A. ursinus in a clade ingroup to trichosurines, diverged from remaining ingroup lineages between 21.1 and 23.3 mya, a time when Sulawesi was available for colonization and sea currents would have enhanced the colonization potential from the east of Sulawesi and neighboring islands. We recommend Trichosurinae as a subfamilial level entity on par with Ailuropinae and Phalangerinae, circumscription of Trichosurinae to Trichosurus and Wyulda, and removal of Strigocuscus into Ailuropinae, leaving only Phalanger and Spilocuscus in Phalangerinae.

The family Heteromyidae includes 6 genera of rodents traditionally placed in 3 subfamilies endemic to the Nearctic and northern Neotropical biogeographic regions. Although several of these taxa represent intensively studied members of North and Central American ecosystems (e.g., kangaroo rats and pocket mice), phylogenetic relationships within and among subfamilies, genera, and species-groups are not well understood. Here, we used maximum-likelihood, Bayesian, and maximum parsimony analyses of sequence data from 2 mitochondrial DNA genes, the cytochrome oxidase subunit 3 gene (699 base pairs [bp]) and the cytochrome-b gene (1,140 bp), to investigate phylogenetic relationships among 55 species-level taxa. We found robust support for monophyly of genera Dipodomys, Microdipodops, Chaetodipus, and Perognathus; sampling of Liomys and Heteromys was inadequate to evaluate their reciprocal status. All analyses converge on a phylogeny that robustly resolves several historically contentious issues, including monophyly of the subfamily Dipodomyinae (Microdipodops plus Dipodomys), and a monophyletic Chaetodipus that includes C. formosus, C. baileyi, C. rudinoris, and C. hispidus. However, Perognathinae (Perognathus plus Chaetodipus) is not supported, with no basal resolution among Perognathus, Chaetodipus, Dipodomyinae, and Heteromyinae. Many intrageneric clades receive strong support and are discussed herein. Although phylogenetic resolution is limited at the basal nodes of the Heteromyidae radiation, our results provide a basis for developing a provisional hypothesis regarding the historical biogeography in combination with independent information on the Neogene geological history of western North America and the fossil record of the family.

Lemmings are a classic example of animals with strong population cycles. High-amplitude density fluctuations with low numbers during the low phase are expected to lead to strong genetic drift, which erodes genetic variability. By compiling data on mitochondrial DNA polymorphism for 72 lemming populations from 5 species, we found no evidence for this erosive mechanism. On the contrary, high levels of haplotype diversity (average h of 0.75 for samples of the genus Lemmus) were observed in many populations. Although the effective size determines the level of genetic diversity in closed populations, diversity is primarily determined by immigration in open populations. Simulations of genetic drift in open populations fluctuating in density confirmed the independence of genetic variability from local effective size, and predicted a deficit of rare alleles, as observed in lemming samples. High genetic variability thus implies high gene flow over a considerable area for lemmings, but does not provide information about the local effective size of populations. Examination of empirical data suggests that high genetic diversity may be the rule rather than the exception in cyclic populations.

Anthropogenic impacts such as habitat conversion and fragmentation, in combination with predator control and fur trapping, are responsible for substantial reductions in the ranges of many carnivores worldwide. The wolverine (Gulo gulo) is classified as vulnerable throughout the Holarctic Region by the International Union for Conservation of Nature and Natural Resources, is designated as endangered in eastern Canada, and has been petitioned twice for listing with the United States Fish and Wildlife Service. We examined genetic structure across populations in northwestern North America by using mtDNA sequences of the left domain of control region and the complete cytochrome-b gene (Cytb). Nucleotide diversity (π) and sequence divergence among haplotypes were low for both markers, whereas haplotype diversity (h) was generally high. Genetic divergence (F_st) values were significant, and high for control region (0.21), and moderate to high for Cytb (0.16). Globally, Eurasian and Scandinavian wolverines were distinguishable from North American. Within North America, the Kenai Peninsula, southeastern Alaska, and Nunavut populations were distinctive. Comparisons with studies based on nuclear markers reveal greater geographic structure in these maternally inherited mitochondrial markers, a finding consistent with male-biased dispersal in wolverines. Conservation plans for these medium-sized carnivores should emphasize maintenance of genetic diversity and recognize that successful dispersal of females between populations may be limited.

Multivariate statistical analyses of cranial and dental morphology and comparison of pelage coloration were used to describe a new subspecies of giant panda from the Qinling Mountain range of Shaanxi Province in China. Based on 2 components that were highly related to skull and molar sizes, respectively, principal components analysis grouped all samples into 2 clusters corresponding to the Qinling and Sichuan samples. The Qinling cluster was characterized by significantly smaller skulls and larger molars compared to the Sichuan cluster. Based on skull and molar sizes, discriminant function analysis also correctly identified all samples from Qinling and Sichuan populations. Comparison of pelage coloration indicated that Sichuan individuals had black chest patches and white ventral pelages, whereas Qinling pandas had dark brown chest patches and brown ventral pelages. These results reveal that the diagnostic characteristics of the new Qinling subspecies are a small skull, large molars, dark brown chest patch, and brown ventral pelage.

A new genus and species in the tribe Lonchophyllini, subfamily Glossophaginae (Chiroptera: Phyllostomidae) is described based on the analysis of 4 specimens collected in 3 different localities in a semiarid area of northeastern Brazil. The presence of a groove and filiform papillae on the lateral side of the tongue places this taxon as a member of the tribe Lonchophyllini. Fourteen cranial and dental characters distinguish the new genus from all other genera in the tribe (Lonchophylla, Lionycteris, and Platalina). The new genus is characterized by the presence of inward-facing molars on the palate; the loss of the cingulum in P3 and P4; the reduction or lack of the metastyle and preprotocrest in M1, ectoflexus, preprotocrest, and postprotocrest in M2, and postprotocrest in M3; and loss of hypoflexid, metacristid, entocristid, and entoconid on m1–3. A wider gap also occurs between lower incisors and canines than in other genera of Lonchophyllini. The palate in the new genus presents an unusual morphology. The palate of members of this genus is the longest among lonchophyllines and has 4 pits that permit the insertion of the main cusps of the lower last premolar and all 3 lower molars. Cladistic analysis suggests that the new genus is sister to Platalina.

The murine genera Celaenomys and Chrotomys comprise a group of semifossorial, vermivorous rodents endemic to the Philippines. Analysis of variation within this group supports the recognition of 4 named species occurring on the large islands of Luzon and Mindoro, as well as our description of a 5th species from Sibuyan, a small island in the central Philippines that has remained isolated from the other islands by deep water channels. Examination of cytochrome-b gene sequence data from 4 of the 5 taxa indicates that Celaenomys silaceus may be basal and sister to the other taxa but morphological and chromosomal data do not support the recognition of Celaenomys as a separate genus. Analysis of the sequence data indicates a relatively early separation of the Sibuyan species and a later split between 2 species of Chrotomys on Luzon. Diversification of Chrotomys appears to have involved speciation associated with both habitat vicariance on Luzon and overwater dispersal to neighboring islands. Most of the members of this clade are restricted to mid- or high-elevation forests, have limited geographic ranges within the archipelago, and some may be imperiled due to loss of natural habitat.