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Animals in northern environments contend with seasonally variable resources and have evolved life histories to cope with the flush of nutrients in summer and scarcity in winter. Ungulates cope with seasonal variation in resources by accreting fat stores when resources are plentiful during the growing season and then mobilizing these stores to meet nutritional requirements when resources are more limited during winter. While this generalization makes sense, previous work has suggested that caribou (Rangifer tarandus spp.) may break this rule; however, previous works have suffered various limitations that leave this question unanswered. As part of a larger study to understand the role of nutrition as a potential factor limiting the Mulchatna Caribou Herd (MCH) in southwestern Alaska, we initiated a longitudinal study in which we captured the same individual caribou twice annually in autumn and late winter. We aimed to test the hypothesis that winter nutrition limited the MCH and predicted that caribou would lose fat between late autumn and late winter. During captures, we determined body fat percentage of lactating, adult females using validated ultrasonography and manual palpation techniques, and determined pregnancy status in late winter using transabdominal ultrasonography. Repeated measures indicated lactating females gained body fat over winter, averaging ~2.8 percentage points from 6.1% ± 0.1% (mean ± SE) in autumn to 8.9% ± 0.4% in late winter (z = 5.425, p < 0.001), with maximum individual gains of up to 8.5 percentage points. Our findings provide empirical support for overwinter gains in fat among caribou and indicate that winter nutrition is not limiting the MCH. Our findings illustrate the value of longitudinal studies in ecology, have implications for research designs for studies of nutrition in northern ungulates, and highlight important considerations for caribou relative to unique nutritional aspects of their life history.

Food quality and availability, when combined with energetic demands in seasonal environments, shape resource acquisition and allocation by animals and hold consequences for life‐history strategies. In long‐lived species with extensive maternal care, regulation of somatic reserves of energy and protein can occur in a risk‐sensitive manner, wherein resources are preferentially allocated to support survival at the cost of investment in reproduction. We investigated how Rocky Mountain bighorn sheep (Ovis canadensis), an alpine mammal in a highly seasonal environment, allocates somatic reserves across seasons. In accordance with the hypothesis of risk‐sensitive resource allocation, we expected accretion and catabolism of somatic reserves to be regulated relative to preseason nutritional state, reproductive state, and variation among populations in accordance with local environmental conditions. To test that hypothesis, we monitored seasonal changes in percent ingesta‐free body fat (IFBFat) and ingesta‐free, fat‐free body mass (IFFFBMass) in three populations of bighorn sheep in northwest Wyoming between 2015 and 2019 through repeated captures of female sheep in December and March of each year in a longitudinal study design. Regulation of somatic reserves was risk‐sensitive and varied relative to the amount of somatic reserves an animal had at the beginning of the season. Regulation of fat reserves was sensitive to reproductive state and differed by population, particularly over the summer. In one population with low rates of recruitment of young, sheep that recruited offspring lost fat over the summer in contrast to the other two populations where sheep that recruited gained fat. And yet, all populations exhibited similar changes in fat catabolism and risk sensitivity over winter. The magnitude of body fat and mass change across seasons may be indicative of sufficiency of seasonal ranges to meet energetic demands of survival and reproduction. Risk‐sensitive allocation of resources was pervasive, suggesting nutritional underpinnings are foundational to behavior, vital rates, and, ultimately, population dynamics. For species living in alpine environments, risk‐sensitive resource allocation may be essential to balance investment in reproduction with ensuring survival.

The use of ultrasonograhic measurements of muscle and body fat represent a relatively new data stream that can be used to address questions regarding ungulate condition. We have learned that measurements of body fat and presumably overall body condition among individual animals, even those taken from the same herd at that same time, are highly variable. Relatively little consideration has been given to the sources of variation in body fat and other physiological parameters in wildlife populations. We evaluated the components of variation in late-winter mule deer (Odocoileus hemionus) body fat estimates: sampling variation (i.e., variation induced by the particular set of individuals that were sampled) and process variation (i.e., variation stemming from biological processes) with a long-term data set (2002–2015) from Colorado, USA. We collected our data from across Colorado as part of historical research, ongoing research, and periodic population monitoring programs. Mean percent ingesta-free body fat (%IFBF) for sampled mule deer was 7.20 ± 1.20% (SD). Covariates related to individual deer explained approximately 4% of the total variation in %IFBF and annual effects explained an additional 13% of the variation. Substantial residual variation in %IFBF (83%) remained unexplained. The source of the 83% of unexplained variation is partially linked to fine-scale spatial dynamics but also additional individual metrics we were unable to capture, primarily the presence or absence of dependent young. We speculate that the primary factors influencing late-winter mule deer body fat and overall condition are individual in nature. These results present a cautionary check on herd-level inference that can be made from individual late-winter body fat estimates and we postulate that for mule deer, alternative and additional body condition metrics may offer added utility in management scenarios. However, an important next step to better understand wildlife population health is to evaluate the sources and magnitude of variation within other body condition metrics, with the goal of further refining data that can better allow biologists to incorporate herd health into population management recommendations.

Animals may partially overcome environmental constraints on fitness by behaviorally adjusting their exposure to costs and supplies of energy. Few studies, however, have linked spatiotemporal variation in the energy landscape to behaviorally mediated measures of performance that ostensibly influence individual fitness. We hypothesized that strength of selection by North American elk (Cervus elaphus) for areas that reduced costs of thermoregulation and activity, and increased access to high-quality forage, would influence four energetically mediated traits related to fitness: birth mass of young, nutritional condition of adult females at the onset of winter, change in nutritional condition of females between spring and winter, and neonatal survival. We used a biophysical model to map spatiotemporally explicit costs of thermoregulation and activity experienced by elk in a heterogeneous landscape. We then combined model predictions with data on forage characteristics, animal locations, nutritional condition, and mass and survival of young to evaluate behaviorally mediated effects of the energy landscape on fitness-related traits. During spring, when high-quality forage was abundant, female elk that consistently selected low-cost areas before parturition gave birth to larger young than less-selective individuals, and birth mass had a strong, positive influence on probability of survival. As forage quality declined during autumn, however, lactating females that consistently selected the highest quality forage available accrued more fat and entered winter in better condition than less-selective individuals. Results of our study highlight the importance of understanding the dynamic nature of energy landscapes experienced by free-ranging animals.

Interactions between physiological and behavioural adaptations to seasonal environments may be key to unravelling the mystery of partial migration. Physiological state, as indicated by body fat, influenced probability of migration in bighorn sheep as well as migration distance and probability of switching migratory tactics across years. Abstract Accretion of body fat by animals is an important physiological adaptation that may underpin seasonal behaviours, especially where it modulates risk associated with a particular behaviour. Using movement data from male Sierra Nevada bighorn sheep (Ovis canadensis sierrae), we tested the hypothesis that migratory behaviours were risk-sensitive to physiological state (indexed by body fat). Sierra bighorn face severe winter conditions at high elevations and higher predation risk at lower elevations. Given that large body fat stores ameliorate starvation risk, we predicted that having small body fat stores would force animals to migrate to lower elevations with more abundant food supplies. We also predicted that body fat stores would influence how far animals migrate, with the skinniest animals migrating the furthest down in elevation (to access the most abundant food supplies at that time of year). Lastly, we predicted that population-level rates of switching between migratory tactics would be inversely related to body fat levels because as body fat levels decrease, animals exhibiting migratory plasticity should modulate their risk of starvation by switching migratory tactics. Consistent with our predictions, probability of migration and elevational distance migrated increased with decreasing body fat, but effects differed amongst metapopulations. Population-level switching rates also were inversely related to population-level measures of body fat prior to migration. Collectively, our findings suggest migration was risk-sensitive to physiological state, and failure to accrete adequate fat may force animals to make trade-offs between starvation and predation risk. In complex seasonal environments, risk-sensitive migration yields a layer of flexibility that should aid long-term persistence of animals that can best modulate their risk by attuning behaviour to physiological state. Graphical Abstract Graphical Abstract

Nutrition underpins survival and reproduction in animal populations; reliable nutritional biomarkers are therefore requisites to understanding environmental drivers of population dynamics. Biomarkers vary in scope of inference and sensitivity, making it important to know what and when to measure to properly quantify biological responses. We evaluated the repeatability of three nutritional biomarkers in a large, iteroparous mammal to evaluate the level of intrinsic and extrinsic contributions to those traits. During a long-term, individual-based study in a highly variable environment, we measured body fat, body mass, and lean mass of mule deer (Odocoileus hemionus) each autumn and spring. Lean mass was the most repeatable biomarker (0.72 autumn; 0.61 spring), followed by body mass (0.64 autumn; 0.53 spring), and then body fat (0.22 autumn; 0.01 spring). High repeatability in body and lean mass likely reflects primary structural composition, which is conserved across seasons. Low repeatability of body fat supports that it is the primary labile source of energy that is largely a product of environmental contributions of the previous season. Based on the disparate levels in repeatability among nutritional biomarkers, we contend that body and lean mass are better indicators of nutritional legacies (e.g., maternal effects), whereas body fat is a direct and sensitive reflection of recent nutritional gains and losses.

Augmentation is a viable method of increasing the size of small populations if habitat quality is adequate. We examined a hierarchy of demographic responses to habitat quality, including individual nutritional condition, juvenile survival, adult fecundity, and adult survival to determine if the habitat of the Nooksack elk herd, a remnant elk population located in the northern Cascade Mountains of Washington, could support additional elk. Total ingesta-free body fat levels of lactating cow elk (11.5–13.6%), the most nutritionally challenged segment of the population, indicated that elk consumed diets at the high end of marginal quality. Juvenile survival during summer was moderate (0.66) relative to other elk populations, while annual adult survival was high (0.94). Pregnancy averaged 0.89, and early autumn calf:cow ratios ranged from 60 ± 9 to 69 ± 15 calves/100 cows. Levels of condition (approximately 56% of potential), juvenile survival (73% of potential), adult fecundity (99% of potential), and adult survival (equal to or greater than previously published survival rates) indicated habitat impacts only on the population vital rates most sensitive to nutritional stress in the Nooksack area. We conclude that the Nooksack area is capable of supporting additional elk, albeit at less than optimal levels of individual and population productivity. Further, because only a small portion of the Nooksack area that was used historically by elk is currently used, colonization of these areas used formerly by elk may also have the potential to increase numbers of elk in the Nooksack area.