In this introductory paper we describe the 10 papers that compose this Special Feature Theoretical and analytical advances in mammalian isotope ecology. This Special Feature contains 6 review-style papers, written by some of the leaders in the field. These are followed by 4 data-driven articles. The review-style papers were presented at a symposium we organized during the 90th annual meeting of the American Society of Mammalogists. The symposium was dedicated to Donald M. Schell (1940–2009), who pioneered research in isotopic ecology.

We open this Special Feature on stable isotopes in mammalian research with a beginner's guide, an introduction to the novice and a refresher to the well-versed. In this guide we provide the background needed to understand the more advanced papers that follow. We describe the basic principles of isotopic fractionation and discrimination, briefly explain the processes that govern isotopic incorporation into animal tissues, list some innovative studies, and provide cautionary notes and caveats. In addition to discussing the uses of natural abundance we present the concepts and applications of enriched isotopes and the potential combination of these 2 methodologies. We end with descriptions of analytical and conceptual developments that we believe will be cardinal to the future of isotopic analyses in mammalian research.

Ecologists are increasingly using stable isotope analysis to inform questions about variation in resource and habitat use from the individual to community level. In this study we investigate data sets from 2 California sea otter (Enhydra lutris nereis) populations to illustrate the advantages and potential pitfalls of applying various statistical and quantitative approaches to isotopic data. We have subdivided these tools, or metrics, into 3 categories: IsoSpace metrics, stable isotope mixing models, and DietSpace metrics. IsoSpace metrics are used to quantify the spatial attributes of isotopic data that are typically presented in bivariate (e.g., δ¹³C versus δ¹⁵N) 2-dimensional space. We review IsoSpace metrics currently in use and present a technique by which uncertainty can be included to calculate the convex hull area of consumers or prey, or both. We then apply a Bayesian-based mixing model to quantify the proportion of potential dietary sources to the diet of each sea otter population and compare this to observational foraging data. Finally, we assess individual dietary specialization by comparing a previously published technique, variance components analysis, to 2 novel DietSpace metrics that are based on mixing model output. As the use of stable isotope analysis in ecology continues to grow, the field will need a set of quantitative tools for assessing isotopic variance at the individual to community level. Along with recent advances in Bayesian-based mixing models, we hope that the IsoSpace and DietSpace metrics described here will provide another set of interpretive tools for ecologists.

A common use of stable isotope analysis in mammalogy is to make inferences about diet from isotope values (typically δ¹³C and δ¹⁵N) measured in tissues and food sources of a consumer. Mathematical mixing models are used to estimate the proportional contributions of food sources to the isotopic composition of the tissues of a consumer, which reflect the assimilated diet. This paper reviews basic mixing models and how they work; additional refinements also are described that include addressing uncertainty, larger numbers of sources, combining sources, concentration effects, and Bayesian statistical frameworks. Information is provided on where to access software for the various models. Numerous examples are cited to show application of these models in the mammal research literature.

The interpretation of isotopic data gathered in the field often demands knowing the rate at which isotopes are incorporated into different tissues and species, and the discrimination factor between tissues and diet. These 2 quantities are estimated in laboratory experiments on diet shifts in which results are interpreted using simple mathematical models, which we describe here. The simplest of these models assumes that each tissue can be represented as a well-mixed, single compartment that obeys 1st-order kinetics. Fitting this model to experimental data allows estimating discrimination factors and the instantaneous rate of isotopic incorporation, λ (the reciprocal of λ, 1/λ, equals the average residence time, τ, of an atom in the tissue). In 1-compartment models the magnitude of λ equals the sum of catabolic turnover and mass-specific growth rate. Examination of available data suggests that the magnitude of λ scales with body mass to an exponent equal to approximately −0.25, differs between endotherms and ectotherms, and could be a useful feature in isotopic incorporation studies. We outline suggestions for the design and analysis of isotopic incorporation experiments and suggest that an increased data set of species and tissues can allow field researchers to estimate rates of incorporation from body size and growth rate data.

Stable isotopes in metabolically inert tissues of migratory animals can be used to infer migratory and dispersal histories. The general approach for estimating geographic origins of migratory animals based on stable isotope values of their keratinous tissues is to develop or calibrate an assignment model based on tissues of known geographic origin. This paper reviews the general forms and evaluates the application of the 3 assignment approaches. Two of these approaches are considered as nominal assignment frameworks because they require prior declaration of named locations as the set of candidate origins. Individual samples can be sorted into the most likely location using a classification tree or a likelihood-based assignment test. The 3rd and more recent approach is considered a continuous assignment framework because it does not require a predetermined list of candidate locations. This approach depends on an underlying mechanistic geographic model of variation in isotope values. Such models can be developed directly from spatially intensive sampling of keratins or by calibrating a spatial model for isotopes in physical (water or soil) or biological (dietary species) resources. Productive approaches to increase spatial resolution of assignment models will use experiments designed to identify specific geographic-based, variance-generating mechanisms, especially if the contributing factors can be quantified for animals that are released back to the wild.

Stable isotope analysis of fossil materials has become an increasingly important method for gathering dietary and environmental information from extinct species in terrestrial and aquatic ecosystems. The benefits of these analyses stem from the geochemical fingerprint that an animal's environment leaves in its bones, teeth, and tissues. Ongoing study of living mammals has found the stable isotopic composition of several light (hydrogen, carbon, nitrogen, oxygen, and sulfur) and even a few heavy (calcium and strontium) elements to be useful tracers of ecological and physiological information; many of these can be similarly applied to the study of fossil mammals. For instance, the carbon isotopic composition of an animal's tissues tracks that of the food it eats, whereas the oxygen isotopic compositions of the carbonate and phosphate in an animal's bones and teeth are primarily controlled by that of the surface water it drinks or the water in the food it ingests. These stable isotope proxies for diet and habitat information are independent of inferences based on morphological characters and thus provide a means of testing ecological interpretations drawn from the fossil record. As such, when well-preserved specimens are available, any dietary study of fossil species should seriously consider including this approach. To illustrate the potential benefits associated with applying these methods to paleontological research, a review of current work on the ecological and evolutionary history of fossil mammals through geochemical analysis is presented. After a brief introduction to issues associated with the preservation of stable isotopic information in soft and mineralized tissues, a series of case studies involving the application of stable isotope analysis to fossil mammal research is discussed. These studies were selected to highlight the versatility of this analytical method to paleontological research and are complemented by a discussion of new techniques and instrumentation in stable isotope analysis (e.g., laser ablation and compound-specific isotope ratio mass spectrometry, and calcium and clumped isotopes), which represent the latest advances in the extension of these geochemical tools to the paleontology of fossil mammals.

Habitat use and feeding behaviors of cryptic animals are often poorly understood. Analyses of stable isotope ratios in animal body tissues can help reveal an individual's location and resource use during tissue growth. We investigated variation in stable isotope ratios of 4 elements (H, C, N, and S) in the hair of a sedentary species of insectivorous bat (Eptesicus fuscus) inhabiting a chemically complex urban landscape. Our objective was to quantify population-level isotopic variation and test for evidence of resource specialization by individuals. Bats were sampled over 3 annual molt cycles at maternity roosts in buildings and variance components analysis was used to test whether intraindividual isotopic variation among molts differed from interindividual variation, after controlling for year and roost-group effects. Consistent with prior evidence that E. fuscus is opportunistic in its habitat use and foraging at the population level, we observed wide population-level variation for all isotopes. This variation likely reflects the chemical complexity of the urban landscape studied. However, isotopic variation among years within marked individuals was lower than variation among marked individuals within year for all isotopes, and carbon signatures indicated resource specialization by roost groups and individuals. This is the 1st study to examine variation in stable isotope ratios of individual wild bats over multiple years. Although our results suggest this population tends toward opportunistic habitat use or prey selection, or both, during molt periods, results also indicate that individuals and groups of bats composing the population might be habitat or dietary specialists—a novel finding for insectivorous bats.

Advances in the application of stable isotopes have allowed the quantitative evaluation of previously cryptic ecological processes. In particular, researchers have utilized the predictable spatial patterning in natural abundance of isotopes to better understand animal dispersal and migration. However, quantifying dispersal via natural abundance alone has proven to be of limited utility for species exhibiting short- or mid-distance dispersal events, including most mammalian species. In previous experimental work, we demonstrated that consumption of 1 dose of isotopically enriched baits elicited a distinct “mark” in hair of captive martens (Martes spp.). Herein, we report findings from a field test of our isotopic enrichment approach to mark free-ranging animals and quantify dispersal of martens across forest stands at sites in southeastern Alaska and northern British Columbia. In the field, we supplemented bait used in single-capture hair traps with the amino acid glycine artificially enriched in ²H, ¹³C, or ¹⁵N. By applying unique combinations of artificially enriched isotopic markers within discrete forest stands, the isotopic signature of collected hair reflected the forest patch where the individual originated. From our isotopic marks, we were able to infer dispersal events between forested stands and, thus, estimate rates and approximate dispersal distances. Our findings demonstrate that isotopic enrichment can be a cost-effective method to mark the hair of midsized mammals for the quantification of dispersal.

The incorporation of dietary macronutrients and associated isotopic signatures of carbon (δ¹³C) and nitrogen (δ¹⁵N) into animal tissues is a result of the interaction between growth, nutritional status, and the composition of the diet. In mammalian carnivores incorporation is further complicated by lack of carbohydrates in the diet and allocation of large quantities of dietary macronutrients to fetuses and milk production. In this study, we explored the effects of diet composition, growth, pregnancy, and milk production on isotopic incorporation of ¹³C and ¹⁵N in captive mink (Neovison vison) fed 3 experimental diets (Beef, Fish, and a Mixture of the 2) that differed in lipid and amino acid composition. In nursing kits, growth was the main factor influencing isotopic incorporation rates into muscle. Similarly, in adults, changes in body mass influenced the dynamics of isotopic incorporation in red blood cells, although the rates differed for δ¹³C and δ¹⁵N, as well as among the 3 experimental groups. Effects of allocation of dietary macronutrients to fetuses and milk did not differ from body mass changes, potentially because those macronutrients were assimilated in tissues other than blood cells. Although incorporation of δ¹³C followed the expected exponential form, δ¹⁵N incorporation lagged in the Beef and Mixture diet treatments, and both δ¹³C and δ¹⁵N incorporation rates differed substantially for the Fish-fed mink. These differences in isotopic incorporation can be attributed to the differential oxidation of dietary amino and fatty acids. Thus, we advocate the development of compound-specific isotopic analyses to estimate dietary contributions through the incorporation of essential dietary fatty and amino acids.

Dietary carbon is oxidized and exhaled as CO₂, thus δ¹³C_breath values can provide information on diet and substrate use for energy. However, physiological phenomena such as fat deposition and fasting can alter values of δ¹³C_breath such that interpretation of source contributions may be unclear. Consequently, before application to free-ranging animals, inferences about feeding and nutritional states based on δ¹³C_breath should be validated with controlled experiments using captive individuals. Here, we report δ¹³C_breath values for 4 captive brown bears (Ursus arctos) under different conditions: the bears were 1st given a diet containing carbohydrate, lipid, and protein; they were then switched to a carbohydrate-free diet consisting of salmon and fish oil; and finally they were placed on a fast leading to winter hibernation. Following the switch to the carbohydrate-free diet, values of δ¹³C_breath and δ¹³C_plasma suggested that although oxidation included a substantial portion of dietary proteins, dietary lipids were preferentially utilized. After the onset of fasting, δ¹³C_breath values did not change, possibly because of selective mobilization of endogenous fatty acids accreted by bears while consuming the experimental diet. Examination of our data suggests that because CO₂ production and exhalation are influenced differentially by fasting state and diet composition, additional measurements such as respiratory exchange ratio should be used to aid interpretation of carbon isotope analysis of breath.

One of the most important factors influencing the foraging of animals is predation risk. As a result, animals do not utilize landscapes uniformly. Vervet monkeys (Chlorocebus aethiops) use both arboreal and terrestrial habitats, and thus forage across a 3-dimensional landscape. To explore how both aerial and terrestrial predators influenced small-scale foraging patterns, we measured giving-up densities (GUDs) in artificial food patches across a woodland–grassland interface. Patch transects ran vertically through trees, and horizontally into open grasslands. As vervets move up trees to avoid terrestrial predators and down out of the canopies to avoid aerial predators, we predicted that there would be a safe zone with higher patch use extending from just below the canopy to the base of the tree. Lower GUDs in this area showed this to be the case. Another factor that influenced patch use was group size. The different sized groups displayed a similar overall pattern of patch selection (i.e., they all fed more in specific patches). However, larger groups fed more intensively in all patches. As a result, larger groups achieved lower GUDs in all patches compared to the smaller groups both vertically and horizontally. Possible explanations include differences in predator abundance between the locations of the groups, group benefits lowering predation risk, competition forcing individuals to feed in less safe patches, or a combination of these factors. Ultimately, our results indicate that predation risk from both aerial and terrestrial predators shapes the vervets' 3-dimensional landscape of fear.

Knowledge of animal movement is fundamental to several biological disciplines, including wildlife conservation, yet animal movement remains one of the least understood behaviors and is critical for establishing and implementing effective management strategies. Thus, our goal was to compare the spatial, temporal, and biological factors that may influence the home ranges of Preble's meadow jumping mouse (Zapus hudsonius preblei), a federally threatened subspecies with relatively little known about its movement behavior. We radiotracked 187 individuals for 3 monthly sessions from 1998 to 2002 at 3 sites with varying hydrologic and vegetation characteristics in Colorado. A nonparametric kernel home-range estimator, local convex hull (LoCoH), was applied to estimate 50% and 95% home-range sizes for each individual. The 50% home ranges varied by session, site, and annual precipitation (P < 0.05). The broader, 95% home ranges were influenced by session and sex (P < 0.001) and slightly influenced by year. The 95% home ranges for males (X¯  =  6,342 m², SE  =  520 m²) were greater than for females (X¯  =  4,026 m², SE  =  358 m²). Individuals located at the site with the widest 100-year floodplain used twice the area compared to those at the 2 narrower 100-year floodplain sites. This information improves our ability to design management strategies to conserve and restore populations of Z. h. preblei by increasing our understanding of how biological, spatial, temporal, and environmental factors may influence movement patterns and ultimately persistence of this subspecies.

We studied the association between space sharing and kinship in a solitary rodent, the dusky-footed woodrat (Neotoma fuscipes). Genetic relatedness was inversely correlated with geographic distance for female woodrats but not for males, a pattern consistent with female philopatry and male dispersal. However, some female neighbors were unrelated, suggesting the possibility of female dispersal. Relatedness of female dyads was positively correlated with overlap of their home ranges and core areas, indicating that females were more likely to share space with relatives, whereas males showed no correlation between relatedness and the sharing of either home ranges or core areas. However, some females that shared space were not close relatives, and some closely related males shared space. House sharing was exhibited both by close relatives and by distantly related or unrelated woodrats, and was not correlated with relatedness. The kin structuring we describe likely resulted from a pattern of female philopatry and male dispersal, but also may have resulted from kin-directed behaviors by females.

Understanding population structure can lend insight into the spread of animal-borne disease, and the effects of anthropogenic land use on habitat. Raccoons are highly adaptive to human land development and can persist in a wide range of habitat types, making them ideal subjects for investigating the level of population structure in a highly fragmented area. A total of 323 raccoons were livetrapped from 7 locations encompassing 3 distinct habitat types (agriculture, urban forest preserves, and residential) across the Chicago metropolitan region (maximum distance between 2 sites was 128 km). Genetic analyses of 14 microsatellite loci indicate that although raccoon populations across the region share up to 50% of the allelic diversity, they segregated into at least 2 distinct subpopulations, dividing the Chicago metropolitan region into northern and southern groups with further structure occurring within these larger groups. Incorporating sample sites between the identified north–south groups may provide greater resolution as to where this split occurs. Although there is evidence of population structure between all sample sites, migrant analysis suggests there is enough gene flow to preserve genetic diversity throughout the population.

Wildlife research often requires the capture, chemical immobilization, and handling of free-ranging animals. These activities have been documented to yield detrimental effects among some study subjects and may bias study results. We developed an empirical censoring protocol that investigated the postcapture movement rates of 89 white-tailed deer (Odocoileus virginianus) outfitted with global positioning system (GPS) collars to identify data that should be censored from analyses due to postcapture recovery and acclimation. Average daily postcapture movement rates were significantly lower than average daily movement rates for a given calendar day for approximately 14 days postcapture. We tested the influence of biased data in a simple random walk simulation and found that net displacement values were significantly lower after 1,000 iterations when an uncensored data set was used than for data sets that did not include the first 2 weeks of GPS locations. Our results indicate that study animals exhibited sublethal effects of capture, handling, and instrument acclimation (reduced movement rates) for approximately 14 days postcapture. We recommend that all studies involving marked animals assess the influence of capture, handling, and instrumentation on postcapture movements to guard against data sets that are not representative of normal movement behavior. Inclusion of these data in movement analyses may bias results, especially when data sets are small (<90 days).

The intervening landscape between patches of forest (i.e., matrix) has enormous potential to mitigate the negative effects of forest fragmentation. However, to release this potential requires understanding of how individual species perceive matrix. Here we investigated use of matrix by pine martens (Martes martes) in a region with low forest cover (Scotland). We radiotracked 11 martens to determine their habitat preferences, then combined our data with those published from 5 additional Scottish landscapes to examine how home-range size (i.e., population density) and diet of martens varied with forest edge density (i.e., fragmentation). Our tracking showed that although mature forest was the most preferred habitat, certain matrix habitats (scrub and tussock grassland) also were consistently selected. These 2 habitats provided martens with fundamental resources that are of limited availability within intensively managed plantation forests: den sites and primary prey (Microtus agrestis). Our synthesis of data across landscapes indicated martens benefit from supplemental resources in matrix habitats; consumption of small mammals increased with fragmentation and coincided with an initial increase in marten population densities. However, population densities of martens decreased once fragmentation passed a threshold level. Our results demonstrate that habitat complementation at the landscape-scale is essential for some forest-associated species. Resource supplementation from matrix habitats may be particularly important in regions with a long history of low-forest cover or where forest cover is now dominated by afforested plantations, which may lack essential resources.

In semiarid ecosystems ungulate densities can compound the effects of drought on forage availability, which can feed back to regulate reproduction and offspring recruitment. Climatic changes in the frequency and severity of drought could thus exacerbate these effects. Here, we use long-term data sets to examine the dual influences of population density, cohort, and climatic variation on recruitment in the Henry Mountains bison (Bison bison bison) population. We found that 1-year lagged annual precipitation had a positive effect on recruitment (β  =  0.032, SE  =  0.009) and population density had a negative effect (β  =  −0.0023, SE  =  0.0004), but did not detect cohort effects. Furthermore, the effect of population density seemed to be more severe in dry relative to wet conditions, indicating that competition for forage could be severe in drought conditions. These results could help managers gain insight into the effects of climate change on bison population dynamics, and help guide the management of bison abundance on rangelands shared with cattle.

Commercial harvest severely reduced the abundance of New Zealand fur seals (NZFSs; Arctocephalus australis forsteri), and the subspecies may have become regionally extinct in Western Australia (WA). NZFS populations are now expanding in WA and this study aimed to determine the origin of these populations and distinguish local recruitment from external recolonization. Mitochondrial cytochrome-b gene sequences were obtained from 137 NZFSs from breeding colonies in WA and South Australia (SA), and analyzed with sequences from Tasmania and New Zealand. Genetic differentiation among WA and SA populations was low, indicating extensive genetic exchange throughout this large region. Three unique haplotypes, however, were recorded from WA, supporting the local recruitment hypothesis. Moreover, a test for asymmetrical gene flow identified a predominance of migration from WA to SA, suggesting a role of WA NZFSs in the recovery of more heavily exploited SA NZFS populations. Significant genetic differentiation was evident between SA and Tasmania, indicating limited genetic exchange despite the close proximity of these populations. Examination of our data suggests NZFSs were not extirpated from WA, have retained unique genetic variants, and that peripheral, low-density populations may have had a role in the recolonization of heavily exploited populations.

Partitioning of foraging niches of insectivorous bats likely is shaped by ecological factors such as availability of prey and competition; however, these factors are difficult to measure and there is little empirical evidence for either. We examined diet of the North American big brown bats (Eptesicus fuscus) and little brown bats (Myotis lucifugus) using published works and our own data from New England to better understand intraspecific dietary variation, and to test whether diet corresponded with climate (an indicator of availability of prey) and species richness of bat communities (a measure of potential interspecific competition). Diet of both species of bat varied with climate in a manner that generally corresponded with longitude. E. fuscus increased use of moths in regions with cool arid summers (western North America), whereas both species of bat consumed more beetles in moister summer climates associated with eastern North America. Additionally, E. fuscus consumed fewer beetles and more moths and true flies in more diverse bat communities. However, correlation between richness of bat communities and mean monthly precipitation limits interpretation of whether richness of bat communities influenced foraging beyond effects of climate alone. Results suggest diet is influenced indirectly by environmental factors that limit availability of prey. Further examination of these and other factors related to intraspecific dietary variation in insectivorous bats is warranted and likely to yield valuable ecological insight.

Hibernating bats display a wide range of thermoregulatory patterns during hibernation, but to date, the majority of research on the topic has been done on cavernicolous bats. Thus, we measured skin temperature of free-living big brown bats (Eptesicus fuscus) during winter in a building hibernaculum in western Indiana. Torpor bouts were considerably shorter than bouts used by cavernicolous bats, lasting a mean of 3.3 days, whereas arousals lasted on average 5.0 h. There was a significant correlation between roost temperature and length of the euthermic period and a larger proportion of bats aroused on warm days, suggesting big brown bats might be taking advantage of warm nights to forage, although evidence for winter feeding is ambiguous in this population. As has been recorded for other noncavernicolous bats during hibernation, most individuals took advantage of partial passive rewarming during some arousals. We estimate these passive arousals could save bats, on average, 96 mg of fat over the course of a 90-day hibernation period, or enough for 16.5 days of steady-state torpor, and may therefore be crucial to survival through long periods of low temperatures and low insect availability. Overall, our results suggest that hibernation patterns in big brown bats in a building hibernaculum are more similar to those of tree-hibernating bats in the Southern Hemisphere than to cavernicolous bats in North America. Further, it appears that big brown bats display extreme flexibility in hibernating patterns, which is unsurprising given their reputation as a relatively hardy species.

Fur seals rely on pelage consisting of dense, fine underfur protected by guard hairs as their primary means of limiting thermoregulatory cost. A distinctive syndrome of alopecia occurs at high prevalence in 1 colony of Australian fur seals (Arctocephalus pusillus doriferus). It is characterized by bilaterally symmetrical hair loss on the dorsal body surface and a biased prevalence toward juvenile females. Light and scanning electron microscopy demonstrated that alopecia is due to fracture of the hair shaft above the skin level. No evidence of viral, bacterial, fungal, or parasite infection was found and histological examination of skin biopsies revealed no pathological variation between case and control seals. Affected animals had statistically significant lower tyrosine and zinc concentrations in hair than unaffected seals. This may increase hair brittleness and, therefore, predispose its fracture. Alopecia cases also had higher levels of heavy metals and persistent organic pollutants, which may indicate they forage in ecosystems where concentrations of pollutants are higher.

In Uruguay, the South American fur seal population (Arctocephalus australis) is increasing, whereas the South American sea lion (Otaria flavescens) population is declining. Previous research using fecal analysis suggested a high degree of trophic overlap between these species. In this study we used stable isotope analysis to assess whether trophic overlap occurs between female fur seals and sea lions during the breeding season. We measured δ¹⁵N and δ¹³C values in serum and skin from pups of both species (n  =  47) to reflect pre- and postpartum maternal feeding habits, respectively. Our results suggested a lack of trophic overlap between lactating females; both serum and skin samples from sea lion pups had significantly greater δ¹³C and δ¹⁵N values than samples from fur seal pups, suggesting that lactating sea lions forage near shore, whereas lactating fur seals forage offshore. The pre- to postpartum diet shift in fur seals would be mainly caused by a reduction in the diversity of the exploited trophic levels, whereas in sea lions the shift could be related to a decrease in the diversity of foraging areas used. The observed trophic segregation between these sympatric otariids is probably driven by their synchronous breeding and similar maternal strategies.

Most species of bears exhibit a reproductive strategy that includes a highly defined breeding season, delayed implantation, pseudopregnancy, and no postpartum estrus. Sun bears (Helarctos malayanus), a tropical lowland species, represent a potential departure from the typical ursid reproductive pattern. We studied reproductive timing in captive sun bears by examining fecal hormones and behavior of 13 female and 5 male sun bears, the entire North American population of H. m. euryspilus at the time of the study. Fecal estrogens and progestins in females did not vary by season or month, nor did fecal androgens in males. Estrus occurred in 11 of 12 months; breeding and births occurred year-round. However, birth rates were lower in the spring and higher in the fall and winter. Our study documented 1 complete pregnancy, in which delayed implantation did not occur; the delay between estrus and the progesterone rise (i.e., implantation) in the pregnant bear was similar to that seen in nonpregnant bears. All bears, whether nonpregnant or pregnant, exhibited a prolonged period of high progesterone after estrus, indicating obligate pseudopregnancy. Studbook records show that estrus can occur within 6 months after parturition; these cases were typically associated with loss of the previous cub. Our findings suggest that sun bears are unique among ursids, being polyestrous, nonseasonal breeders that do not appear to exhibit delayed implantation and that are capable of initiating estrus in the event of early loss of a cub.

Southern flying squirrels (Glaucomys volans) use multiple nest trees for foraging and protection, but nest trees can become scarce following harvests of hardwood forests. In northern Wisconsin, the Managed Old-growth Silvicultural Study tested techniques to remediate logging impacts on forest-dependent wildlife. Three types of canopy treatments were applied (multicohort harvest [0.4-ha and 1.2-ha irregular group shelterwoods], medium gaps [18-m- and 24-m-diameter gaps], and small gaps [11-m-diameter gaps]). To evaluate the effects of treatment on nest tree selection by southern flying squirrels, we tracked 33 radiocollared southern flying squirrels once a week for 5 weeks in late summer, locating 82 nest trees (X¯ =  2.73 nest trees per southern flying squirrel [95% confidence interval: 2.28–3.18 nest trees]). Canopy treatments were important predictors of nest tree switching. Probability of switching differed by canopy treatment (listed from lowest to highest probability): multicohort harvest: 0.29 (0.17–0.42), medium gaps: 0.44 (0.32–0.56), control: 0.57 (0.41–0.73), and small gaps: 0.73 (0.61–0.85). Lower nest tree switching in the multicohort harvest compared to the small gaps likely reflected availability of habitat resources. Spatial arrangement of canopy gaps and associated effects on southern flying squirrels should be considered when planning timber harvests in northern hardwoods.

The number, size, and survival of bear cubs emerging from winter dens depend on maternal condition prior to entering the den. We hypothesized that delayed implantation provides flexibility in timing of birth such that pregnant females are able to track environmental or body conditions long after conception to optimize reproductive output in a changing environment. We tested the hypotheses that causative links between maternal condition and size of newly emerging brown bear (Ursus arctos) cubs were females in superior condition give birth earlier and, thereby, lactate longer in the den than females in poorer condition; and females in superior condition produce more milk or higher quality milk, which accelerates cub growth relative to females in poorer condition. No brown bear with a body fat content ≤ 20% produced cubs even though breeding occurred. Brown bears that were fat gave birth earlier than those that were lean. Cubs nursing from fat mothers grew faster than those nursing from lean mothers. The combination of an earlier birth date and faster growth by cubs produced from fat mothers increased mass of brown bear and polar bear (U. maritimus) twins at den emergence by 330–360 g for each unit increase in percent maternal body fat content when entering hibernation.

The structure and genetic diversity of a widely distributed species in a recently colonized area is influenced by the colonizing lineages, life-history traits, and biotic and abiotic factors. The connection established during the Pliocene between North and South America allowed the nine-banded armadillo (Dasypus novemcinctus) to expand its distributional range northward. High levels of genetic diversity have been recorded in South America, whereas low levels have been detected in populations in the United States, perhaps due to a founder effect during colonization. By sampling animals from Mexico and a few other areas, we test the hypothesis that armadillos in North America were derived from a single founding lineage, and assess whether this newly colonized region shows demographic signatures of expansion. We sequenced the mitochondrial control region of 157 individuals and genotyped microsatellites of 116 individuals. Our mitochondrial results showed 2 divergent lineages with high genetic variation in Mexico when compared to United States populations, suggesting that this species has a higher effective population size in Mexico. Samples from Central and South America indicate that both lineages differentiated prior to their arrival in Mexico. Lineages showed a historical demographic expansion, due probably to the large area of colonization. Clear genetic structure was observed with mitochondrial DNA, whereas microsatellites showed low levels of genetic differentiation. This contrasting pattern can be caused by male-biased dispersal. We conclude that North American populations of D. novemcinctus are derived from 2 founding lineages and show the consequences of the Great American Biotic Interchange influencing genetic patterns in the nine-banded armadillo in Mexico.

We examined the phylogenetic relationships among populations of the Mexican endemic and endangered Phillips' kangaroo rat, Dipodomys phillipsii (Rodentia: Heteromyidae), using mitochondrial (cytochrome-b and 12S ribosomal RNA) and nuclear (growth hormone receptor and interphotoreceptor retinoid-binding protein) genes for a total of 3,110 base pairs. Gene sequences were analyzed under maximum-likelihood and Bayesian models of phylogenetic inference. Analyses of the mitochondrial genes converged on essentially identical gene trees in which D. phillipsii is divided into 2 well-supported clades. The Mexican Plateau clade of D. phillipsii, formerly recognized as D. p. ornatus, is herein returned to full species status as D. ornatus. The southern clade of D. phillipsii retains the subspecies D. p. oaxacae, D. p. perotensis, and D. p. phillipsii. D. ornatus, D. phillipsii, D. elator, and D. merriami form a well-supported, but unresolved polytomy. Analyses of the nuclear genes show the same fundamental subdivision within D. phillipsii but do not provide additional resolution within the 4-species polytomy. Based on analyses of divergence times, we place the separation of these 4 species near the middle Pliocene, which suggests that the morphotectonic processes that gave rise to the Trans-Mexico Volcanic Belt may have influenced diversification in Mexican species of Dipodomys. Synonymies of D. phillipsii and D. ornatus are provided along with a key to the subspecies of D. phillipsi and D. ornatus.

The South American microbiotherian marsupial Dromiciops gliroides is regarded as a sister taxon to the monophyletic Australasian marsupial clade. Mammary anlagen have been documented in male American marsupials but have not been observed in males of any Australian marsupials. The aim of this study was to determine if mammary anlagen were present in developing pouch young (PY) of Dromiciops. The developmental anatomy, particularly the presence of mammary anlagen, was investigated in 3 neonatal male (n  =  2) and female (n  =  1) Dromiciops PY (∼15–20 days postpartum). External and internal structures were examined to confirm PY gender and to provide basic information on development, particularly of the genitourinary system. In the female PY early pouch development was observed as well as 4 clearly defined abdominal mammary anlagen. No external teat or mammary anlagen development was seen in the 2 male PY and serial sections from the external genitalia to upper thorax failed to reveal any histological structures similar to mammary anlagen seen in the female. Absence of mammary anlagen in male Dromiciops PY provides further evidence that Dromiciops shares a closer relationship with the Australasian marsupial clade than with the American marsupial fauna.

The common vampire bat, Desmodus rotundus Geoffroy, 1810, is a species with an extensive geographical distribution, occurring in a wide variety of habitats. A recent phylogeographical study using molecular markers described a scenario in which this species is formed by 5 distinct geographically circumscribed mitochondrial clades. Here we studied the craniometric variation of the common vampire bat to assess the amount of subdivision within this species and to test for the possibility of distinct morphological patterns associated with geographical lineages. We used 16 measurements from 1,581 complete skulls of adult D. rotundus representing 226 localities in South America and Mesoamerica. The assessment of morphological diversity between groups was done by the estimation of minimum F_ST values. Overall, the results show that most of the within-species variation is a result of the size component. Both shape data and size data are correlated with geographic distances. Our results favor the origin of biological diversity as the outcome of genetic drift and stepping-stone pattern of gene flow instead of local adaptations to local environmental conditions. The F_ST analyses also support male-biased dispersal. The results give little evidence to support previous suggestions that the common vampire bat may be composed of 2 or more species.

Allocation of evolutionary divergence into species level versus subspecies ranks is critical for conservation and management. Urocitellus brunneus (Idaho ground squirrel; formerly Spermophilus brunneus) is currently apportioned into 2 subspecies, U. b. brunneus and U. b. endemicus, but recent studies have suggested elevation to distinct species based on differences in pelage, bacular morphology, genetic data, timing of life-history cycle, and behavior. Following recent movements toward integrated taxonomy, we use the cohesion species concept to test whether both genetic and ecological data support species-level classification of U. b. brunneus and U. b. endemicus. Eight microsatellite loci and mitochondrial DNA (mtDNA) sequence data were generated for 339 individuals from 14 localities for U. b. brunneus and 312 individuals from 11 localities for U. b. endemicus. The null hypothesis of genetic interchangeability was tested with 2 independent data sets. First, we estimated an mtDNA gene tree from control region and cytochrome-b sequences via parsimony, maximum-likelihood, and Bayesian analyses. We then tested for evidence of recent migration using Bayesian clustering and coalescence analyses of microsatellite data. The 2nd null hypothesis, that of ecological exchangeability, was tested using an ecological niche-model analysis and a review of the literature based on morphology, habitat characteristics, and behavior. Although divergence of mtDNA sequences between the subspecies was modest (<1%), there are no haplotypes shared between the 2 taxa. There is strong support for monophyly of mtDNA haplotyes of U. b. endemicus (posterior probability  =  0.94), with those from U. b. brunneus forming a basal grade. No evidence of recent gene flow was detected; the Bayesian clustering algorithms of mutlilocus genotype data indicated separate ancestry for both U. b. brunneus and U. b. endemicus. The ecological-niche model showed a nonoverlapping niche for each taxon, allowing for the possibility of differential adaptation. We reject both null hypotheses based on the data, which supports elevation of U. b. brunneus and U. b. endemicus as distinct species.

We report the discovery of a new species of Lophostoma from Panama, which we name L. kalkoae. This new species resembles L. carrikeri and L. yasuni in possessing a white venter, but is distinguishable from both by external and cranial characteristics. The new species is similar in size to L. carrikeri and L. schulzi. Lophostoma sp. nov. can be most easily recognized by its combination of white venter, postauricular patches connected by a thin line of pale hair to the white fur on the chest, elongated clitoris and swollen labia, less strongly developed lateral projection of mastoid processes, well-marked indentation on the lingual cingulum of the upper canine, well-developed P3, well-developed posterior lingual cusp on the cingulum of P4, and parastyle absent on M1 and M2. We present a dichotomous key for the genus Lophostoma and a map showing all the localities where white-bellied Lophostoma have been recorded.