The potential use of stable-isotope analyses (6I3C and 6I5N) to estimate bear diets was assessed in 40-day feeding trials using American black bears (Ursus americanus). Bear plasma and red blood cells have half-lives of -4 days and -28 days, respectively. The isotopic signature of bear plasma is linearly related to that of the diet, and with the exception of adipose tissue, there is no isotopic fractionation across bear tissues. Isotopic analyses were used to estimate the diets of three bear populations: Pleistocene cave bears (U. speleaus) in Europe, grizzly bears (Ursus arctos horribilis) inhabiting the Columbia River drainage prior to 1931, and brown bears (U. arctos) of Chichagof and Admiralty islands, Alaska. Cave bears were omnivores with terrestrially produced meat contributing from 4.1 to 78% (58 f 14%) of their metabolized carbon and nitrogen. Salmon contributed from 33 to 90% (58 f 23%) of the metabolized carbon and nitrogen in grizzly bears from the Columbia River drainage. Finally, most brown bears on Chichagof and Admiralty islands feed upon salmon during the late summer and fall; however, a subpopulation of bears exists that does not utilize salmon.

Polar bears (PBs) are superbly adapted to the extreme Arctic environment and have become emblematic of the threat to biodiversity from global climate change. Their divergence from the lower-latitude brown bear provides a textbook example of rapid evolution of distinct phenotypes. However, limited mitochondrial and nuclear DNA evidence conflicts in the timing of PB origin as well as placement of the species within versus sister to the brown bear lineage. We gathered extensive genomic sequence data from contemporary polar, brown, and American black bear samples, in addition to a 130,000-to 110,000-y old PB, to examine this problem from a genome-wide perspective. Nuclear DNA markers reflect a species tree consistent with expectation, showing polar and brown bears to be sister species. However, for the enigmatic brown bears native to Alaska's Alexander Archipelago, we estimate that not only their mitochondrial genome, but also 5-10% of their nuclear genome, is most closely related to PBs, indicating ancient admixture between the two species. Explicit admixture analyses are consistent with ancient splits among PBs, brown bears and black bears that were later followed by occasional admixture. We also provide paleodemographic estimates that suggest bear evolution has tracked key climate events, and that PB in particular experienced a prolonged and dramatic decline in its effective population size during the last ca. 500,000 years. We demonstrate that brown bears and PBs have had sufficiently independent evolutionary histories over the last 4-5 million years to leave imprints in the PB nuclear genome that likely are associated with ecological adaptation to the Arctic environment.demographic history | hybridization | mammalian genomics | phylogenetics G enome-scale studies of speciation and admixture have become essential tools in evolutionary analyses of recently diverged lineages. For example, paradigm-shifting genomic research on archaic and anatomically modern humans has identified critical gene flow events during hominin history (1, 2). However, aside from several analyses of domesticated species and their wild relatives (e.g., ref.3), studies that use whole-genome sequencing to investigate admixture in wildlife populations are only now beginning to emerge.The bear family (Ursidae, Mammalia) represents an excellent, largely untapped model for investigating complex speciation and rapid evolution of distinct phenotypes. Although polar bears (PBs; Ursus maritimus) and brown bears (Ursus arctos) are considered separate species, analyses of fossil evidence and mitochondrial sequence data have indicated a recent divergence of PBs from within brown bears (surveyed in ref. 4). For example, phylogenetic analyses of complete mitochondrial genomes, including from a unique 130,000-to 110,000-y-old PB jawbone from Svalbard, Norway, confirmed a particularly close relationship between PB and a genetically isolated population of brown bears from the Admiralty, Baranof, and Chichagof islands in Alaska's Alexander Archipelago (hereaf...

Stable isotope signatures of lactating females and their nursing offspring were measured on 11 species, including herbivores, carnivores, hibernators, and non-hibernators. We hypothesized that: (1) nursing offspring would have stable isotope signatures that were a trophic level higher than their mothers, and (2) this pattern would be species-independent. The plasma of adult females had a δN enrichment over their diets of 4.1±0.7‰, but offspring plasma had a mean δN enrichment over maternal plasma of 0.9±0.8‰ and no C enrichment (0.0±0.6‰). The trophic level enrichment did not occur between mother and offspring because milk was depleted in both δN (1.0±0.5‰) and δC (2.1±0.9‰) relative to maternal plasma. Milk to offspring plasma enrichment was relatively small (δN enrichment of 1.9±0.7‰ and δC enrichment of 1.9±0.8‰) compared to the trophic level enrichment between the adults and their diets. While some species did have significant differences between the isotope signatures of mother and offspring, the differences were not related to whether they were hibernators or non-hibernators, carnivores or herbivores. Investigators wanting to use stable isotopes to quantify weaning or other lactation processes or diets of predators when both adults and nursing offspring are consumed must first establish the parameters that apply to a particular species/environment/diet combination.

Despite significant sexual dimorphism and differing reproductive strategies in carnivores, sexual segregation is rarely studied and is often overlooked in the management of wild populations. Potential nutritional constraints imposed by sexual dimorphism and differing reproductive strategies between the sexes have important implications, particularly when combined with differential effects of human activities on sex and age classes. We examined the effects of sexual dimorphism, reproductive strategies, and human activities (bear-viewing and hunting) on resource use by different sex and age classes of brown bears (Ursus arctos). Sexual segregation of habitat use and effects of experimental bear-viewing were quantified at a single site in south-central Alaska, U.S.A., by capturing, collaring, and observing brown bears at a salt marsh and salmon stream. Effects of salmon capture rate, availability of alternative salmon runs, harvest pressure, and numbers of annual visitors on sex and age class use were examined from data collected or previously published from 13 other sites. Bear-viewing sites on salmon streams where salmon capture rates were low (<4 salmon/hour) resulted in low use by adult males (<10% of all bears), except for sites with falls. However, maximum male use of viewing areas also depended on the availability of alternative salmon streams and harvest pressure. Use of habitats by females with dependent young was significantly related to the prevalence of adult males at the site. Thus, both sexual dimorphism and differing reproductive strategies led to sexual segregation in habitat use by bears. As a result of infanticide, females with young appear to prioritize avoidance of male bears over avoidance of humans when choosing habitats, in contrast to responses documented in herbivores. Because carnivores often exhibit both sexual dimorphism and infanticide, selection for sexual segregation is likely to be high. In these cases, the nutritional demands of large adult males, balanced with responses to human activity, drive dynamic temporal and spatial distributions of individuals in the population.

Energy maximization, time minimization, and linear programming models subject to various constraints have dominated foraging ecology ideas and methods for decades. However, animals must use very complex physiological processes and foraging decisions to ensure fitness that in many cases may not be adequately described by these approaches. An example of this problem occurs when brown bears, Ursus arctos, have access to both abundant salmon and fruit. Salmon are one of the most energy and nutrient dense foods available to bears. Fruits are often high in soluble carbohydrates, low to deficient in many required nutrients, and more difficult to efficiently exploit. Therefore, wild brown bears that fatten primarily on fruits without access to salmon are 50% smaller than salmon-feeding bears. Thus, we predicted based on a linear, energy-maximizing model without dietary interaction effects that wild brown bears with access to both abundant salmon and fruit would feed almost exclusively on salmon. However, wild adult females with or without accompanying offspring foraged three times longer per day on fruit than on salmon. Similarly, the relative dry matter intake of ad libitum apples and salmon by captive, adult brown bears averaged 7695% fruit and 2495% salmon. Captive brown bears consuming mixed diets with intermediate dietary protein levels had 60% lower maintenance energy costs, 37% to 139% higher efficiencies of mass gain, and 72% to 520% higher maximum rates of gain than when they consumed either salmon or fruit alone. These relationships were nonlinear functions of dietary protein content in which salmon and fruit provided complementary nutritional resources. Both wild and captive bears attempted to regulate total protein, energy, and carbohydrate intake within a multidimensional intake target that both maximized energy intake and mass gain.