The effects of diet and metabolism on the life history of an omnivorous carnivore

Many ecological principles are built on relationships which connect an organism’s diet, space-use, and fitness. However, actualizing these theoretical relationships into reality is complicated, because an organism’s fitness is the result of many complex interactions. In the case of diet, for example, polyphagy is prevalent within the order Carnivora, yet there is a tendency to focus only on animal-derived foods when assessing diet quality. Thus, creating an oversimplified role of carnivores within ecological communities which excludes numerous trophic interactions that may be important to community stability, particularly given anthropogenic alterations to the environment or changes due to climate change. Shifts in primary productivity and other community interactions may lead to changes in species’ life histories as they adapt to changes in resource availability mediated through trade-offs among food quality and availability. Ecological theory predicts organisms will adapt their foraging behavior and energy allocation between growth and reproduction in a particular way, based on their life histories. However, there is variation among populations and individuals within species that deviate from the expectations but are locally adaptative, such as mothers altering their energy investment in reproduction. Offspring of mothers which invest more resources in reproduction tend to have higher lifetime fitness than offspring of mothers which invest fewer resources. Therefore, local adaptations to landscape changes which change maternal investments can affect population fitness metrics. Greater investment in each reproductive attempt may result in females that grow slower or reach a smaller asymptotic body size, which could decrease lifetime reproductive output and survival. Collectively, foraging patterns, as well as resource acquisition, and allocation can affect population growth and stability through reproductive output and female survival. Thus, understanding the links between the environment, growth, body size metrics, and reproductive investments provides important information for explaining population growth rates. Brown bears (Ursus arctos) are large-bodied hibernators that gestate, give birth, and begin lactating in the den during hibernation while consuming no additional foods or water. Thus, females undergo the most energetically taxing event in mammalian life history during a long fast and under extreme metabolic circumstances. Even though brown bears belong to the order Carnivora, many populations consume little meat, relying on vegetation, hard and soft mast, and invertebrates. The diverse brown bear diet suggests that they are highly adaptable to local conditions, which makes them an ideal species to study changes in climate and phenology, because we expect them to rapidly alter their behavior and foraging. Here, we determine how life history is affected by diet and physiological demands and how diet changes annually in response to resource availability. We used stable isotope and cortisol data as indices of diet and metabolic demands in addition to individual body condition indices and reproductive output from an individually marked brown bear population in southcentral Sweden. To address the larger research topic, this thesis focused on five specific research questions:1) do diet estimates from stable isotopes match previous estimates from scat analysis and ecological theory; 2) how does age and reproductive state affect cortisol concentrations in this population of brown bears; 3) how does resource availability and metabolic demands affect the life history trade-off between growth and reproduction; 4) are Scandinavian brown bears smaller than North American brown bears and what determines variation in growth; 5) how does brown bear diet change with annual variation in resource availability. We used Bayesian mixing models in R to estimate dietary proportions of the five main foods in our system (ants; Formica and Camponotus spp., bilberry; Vaccinium myrtillius, crowberry; Empetrum nigrum, lingonberry; Vaccinium vitis-idaea, and moose; Alces alces) and published data to estimate the proportion of fats, lipids, and carbohydrates, and energy content of brown bear diet. We used linear regression modelling to explain variance in cortisol, growth and body sizes, female reproductive investments, and trends in brown bear diet over time. We built models based on a priori hypotheses and used and information-theoretic framework to compare models. When we evaluated the individual model components, we used the extent to which the confidence or credible intervals overlapped zero as well as the amount of variation in the data explained by the model in question when compared to a similar model (i.e., differing by a single model component). Confidence or credible intervals with no overlap = strong evidence, some overlap (< ~10%) = moderate evidence, and intervals that overlapped zero by > 15% was weak evidence for a relationship between the covariate and the response variable in the equation. Brown diet estimated using stable isotopes did not resemble estimates from previous scat analyses and most closely resembled predictions from the optimum foraging theory, although none of the models fully described the relationship between brown bear foraging and community interactions in our system (question 1). The % dry matter proportions and metabolizable energy provided by proteins in Scandinavian brown bear diet was much lower than predicted—Scandinavian bears primarily consumed bilberries and lingonberries, followed by crowberries, ants, and moose. Cortisol concentrations measured in brown bear hair varied among demographic classes, as well as with age (question 2). Solitary bears had the lowest cortisol concentrations and females with dependent offspring had substantially higher concentrations, while dependent offspring had the highest concentrations. Cortisol is predominately a metabolic steroid hormone and reflects the energetic demands of organisms, therefore, higher cortisol concentrations are likely related to the extraordinary metabolic demands that developing offspring and reproducing females’ experience. Offspring mass was greater with heavier mothers and lower in larger litters and in years with higher bear density (question 3). Variation in the mass difference between mothers and offspring was best explained by the model that accounted for the effects of residual maternal mass, maternal δ15N, litter size, and bear density. This suggests that when energetic demands increase beyond some unknown threshold, females pass the burden onto their offspring. Brown bear size was different between North America and Scandinavia, but only for females (question 4). Alberta females were heavier and longer than Swedish females and there was a positive effect of NDVI on female size. The capture model was the most parsimonious model to explain variation in male mass between the study areas and contained no habitat variables. Through time, there is evidence that the proportion of ants and moose in the diet have decreased, whereas the proportion of bilberry in the diet has remained stable or slightly increased (question 5). Variation in δ13C values was negatively correlated with bilberry productivity and the number of moose calves observed in the year in which hair was grown. Variation in δ15N values was best described by the model that accounted for the effects of bear age, sex, an interaction between age and sex, and bilberry production the year prior to hair growth. Brown bear diet in Scandinavia seems to be driven directly by primary productivity, with bears either consuming more higher-trophic foods or catabolizing their own tissues to meet energetic requirements following years with poor bilberry production, which is surprising for an “apex predator”. These results highlight the importance of expanding our views of species and ecological principals beyond narrow human constructs, such as “carnivore” and “apex predator”. Human constructs around nature that become too narrow lead to comparisons which can minimize the views and novel research questions. No single population or life history is superior to others. Instead, each iteration is a unique result from different selection pressures. Rather than focusing on what makes one population “more fit” than the other, ecologists should focus on the environmental limitations that drive differences among individuals, populations, or species and the unique adaptations that arise due to environmental limitations. Understanding the diverse adaptations required to meet metabolic demands under extreme conditions and the evolutionary processes that shape and maintain these adaptations is critical to conserving ecosystem processes. This research project has expanded our understanding of both brown bear ecology as well as the larger ecological theories within our discipline. Yet these are only a foundation on which to build a better understanding of life-history trade-offs in the face of extreme metabolic demands, such as giving birth and lactation during a long fast.

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