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Reproductive phenology is a critical element of how animals persist in their environment and affects survival of young, especially in seasonal environments. We investigated the correlation between fetal eye diameter and birth timing to determine if birthdates could be predicted by a prenatal metric. We used ultrasonography to measure eye diameters of fetuses and vaginal implant transmitters to determine birthdates of Rocky Mountain Bighorn Sheep (Ovis canadensis canadensis) in Wyoming, United States, from 2019 to 2023. Fetal eye diameter strongly predicted days until birth (β = −2.8; pseudo- R 2 = 0.88). Effective prediction of birthdates via ultrasonography can yield opportunities for logistical planning in neonatal studies, monitoring changes in reproductive phenology, and evaluating questions of resource allocation relative to reproductive processes. [ABSTRACT FROM AUTHOR]

For species that inhabit environments where resource availability may be unpredictable, balance of resource allocation to life‐history traits can have heightened consequences for survival, reproduction, and ultimately, fitness. Acquisition and allocation of energy to maintenance, capital gain and reproduction should be in tune with the landscape an animal inhabits—environmental severity, food availability and population size all influence the resources animals have and dictate the ways they should be allocated.In seasonal environments, animals that experience periods of extreme resource limitation (e.g. harsh winters) may favour allocation of resources to body reserves to secure their survival at the cost of reproduction (i.e. risk averse). In contrast, the same accumulation of body reserves may not be necessary to survive in relatively benign landscapes where instead, allocation to reproduction is favoured (i.e. risk prone).According to the theory of risk‐sensitive allocation of resources, when animals are exposed to unprecedented or life‐threatening conditions, they may shift resource allocation to favour building capital over allocation in reproduction to preempt against encountering another life‐threatening event in the future.Using data from a long‐term project on a highly site‐faithful and long‐lived species, mule deer (Odocoileus hemionus), we evaluated how a life‐threatening winter and the associated changes in resource availability resulting from a population reduction influenced how animals acquired and allocated energy to survival (i.e. fat accumulation).Per capita precipitation, and the associated reduction in population abundance after the severe winter, had a positive influence of accrual of fat over summer. After the extreme physiological stress of a hard winter, deer starting spring with low body reserves accumulated 2.8 percentage points more fat over summer compared with before the experience of a bad winter and had an increased probability of recruiting fewer offspring. Fat stores can interact with environment, life history and behaviour to influence survival during periods of resource scarcity.For a long‐lived herbivore, we documented shifts in risk tolerance associated with fat accrual in preparation for winter, supporting the notion that risk‐sensitive allocation of resources may be plastic—an essential adaptation for animals to cope with rapidly changing landscapes. Read the free Plain Language Summary for this article on the Journal blog. [ABSTRACT FROM AUTHOR]

For many species, behavioral modification is an effective strategy to mitigate negative effects of harsh and unpredictable environmental conditions. When behavioral modifications are not sufficient to mitigate extreme environmental conditions, intrinsic factors may be the primary determinant of survival. We investigated how movement behavior, and internal (i.e., nutrition and age) and external (i.e., food availability and snow depth) states affect survival over winter of a long‐lived and highly faithful species (mule deer; Odocoileus hemionus). We first tested whether animals changed their behavior during winter based on internal and external states; we subsequently investigated how behavior and state interacted to influence survival in the face of extraordinary winter conditions. Movement behavior changed minimally as a function of age and nutrition; yet, movement behavior affected survival—animals that exhibited more restricted movements were more likely to succumb to mortality overwinter than animals with less restricted movements. Additionally, nutrition and cumulative snow depth had a strong effect on survival: animals that were exposed to deep snow and began winter with low fat were much less likely to survive. Behavior was an effective tool in securing survival during mild or moderate winters, but nutrition ultimately underpinned survival during harsh winters. [ABSTRACT FROM AUTHOR]

Capture and handling techniques for individual‐based, long‐term research that tracks the life history of animals by recapturing the same individuals for several years has vastly improved study inferences and our understanding of animal ecology. Yet there are corresponding risks to study animals associated with physical trauma or capture myopathy that can occur during or following capture events. Rarely has empirical evidence existed to guide decisions associated with understanding the magnitude of capture‐related risks, how to reduce these risks when possible, and implications for mortality censoring and survival estimates. We used data collected from 2,399 capture events of mule deer (Odocoileus hemionus) via helicopter net‐gunning to compare daily survival probabilities within a 10‐week period centered on a capture event and evaluated how animal age, nutritional condition (body fat), and various handling methods influenced survival before, during, and following a capture event. Direct mortality resulting from capture efforts was 1.59%. Mean daily survival was 0.9993 ± 0.0001 (SE) during the 5‐week pre‐capture window, was depressed the day of capture at 0.9841 ± 0.0004, and rebounded to 0.9990 ± 0.0008 during the 5‐week post‐capture window. Neither capture nor handling had a detectable effect on post‐capture survival, including handling time (x¯ $\bar{x}$ = 13.30 ± 1.87 min), capture time of year (i.e., Dec or Mar), tooth extraction, and the number of times an animal had been recaptured (2–17 times). Although mortality rate was slightly elevated during capture (resulting from physical trauma associated with capture), age and nutritional condition did not influence the probability of mortality during a capture event. Following a capture event, nutritional condition influenced survival; however, that relationship was consistent with expected effects of nutritional condition on winter survival and independent of capture and handling. Overall survival rates 5 weeks before capture and 5 weeks after capture were not different. A specified window of time with depressed survival following capture and handling was not evident, which contradicts the implementation of a predetermined window often used by researchers and managers for censoring mortalities that occur after capture. Previous notions that censorship of all mortality data in the 2 weeks following capture is unwarranted and risks removal of meaningful data. With previous evidence guiding our protocols for capture (e.g., reduced chase time) and handling (e.g., temperature mitigation), low direct mortality and almost undetectable indirect mortality post capture reinforces the efficacy of helicopter net‐gunning for capture and recapture of mule deer in long‐term, individual‐based studies. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

According to risk‐sensitive foraging theory, animals should make foraging decisions that balance nutritional costs and gains to promote fitness. Human disturbance is a form of perceived risk that can prompt avoidance of risky habitat over acquisition of food. Consequently, behavioural responses to perceived risk could induce nutritional costs.Population declines often coincide with increases in human disturbance, which likely is associated with direct and indirect habitat loss. Nevertheless, behavioural and physiological responses to perceived risks associated with human disturbance could be an added nutritional deficit with population‐level repercussions.Using GPS‐collar data from three populations of migratory mule deer Odocoileus hemionus exposed to a gradient of established industrial energy development on winter ranges where direct and indirect habitat loss were well documented, we evaluated whether exposure and behavioural responses to human disturbance alter changes in nutritional condition (i.e. fat reserves) over winter.Although animals exhibited behaviours indicative of perceived risk of human disturbance, such as increased movement rates and avoidance of infrastructure, exposure and behavioural responses to human disturbance had little to no measurable effect on fat loss over a winter. Instead, catabolism of fat reserves occurred primarily as a function of the amount of fat animals had entering winter, suggesting that, in the short term, animals were able to mitigate the energetic costs of perceived risk of human disturbance over winter. Animals, however, did not appear to overcome persistent food limitations, and animals with less food availability lost more fat over winter.Our findings heed caution in using short‐term behavioural and physiological responses to inform long‐term nutritional consequences of human disturbance. Although animals appear to mitigate the energetic costs of perceived risk, food limitations exacerbated by broader‐scale avoidance of food near human disturbance may be the primary pathway causing the frequently observed population declines following human disturbance to pristine landscapes. A free Plain Language Summary can be found within the Supporting Information of this article. [ABSTRACT FROM AUTHOR]