Matthew D. Cameron

Wildlife agencies invest substantial resources in monitoring ungulate reproductive rates given ongoing concerns about population trends and their implications for management. For barren‐ground caribou ( Rangifer tarandus ) in the North American Arctic, data on parturition and neonate survival have traditionally been collected using visual observations of antler retention and calf presence from small, fixed‐wing aircraft. However, these surveys are weather‐dependent, expensive, and dangerous, motivating the development of movement‐based approaches that infer reproductive events from global positioning system (GPS) location data. We had the unique opportunity to use video camera collars to validate both antler‐ and movement‐based methods for estimating reproduction in barren‐ground caribou, using data from the Porcupine (2018–2023) and Western Arctic (2021, 2023) herds in Alaska, USA, and Yukon, Canada. For the movement‐based approaches, we assessed individual‐based and population‐based methods, which both inferred parturition following a sharp decline in movement and inferred calf loss following a sharp increase in movement. We investigated these approaches using rarified 1‐, 2‐, 4‐, and 8‐hour fix intervals. Based on video collar observations, we found that antler retention at the onset of the calving period was highly accurate for predicting parturition (96% accuracy) but became unreliable as calving progressed, as approximately 80% of parturient females shed their antlers during the calving period. In contrast, both movement methods performed marginally for estimating caribou parturition (~70% accuracy), with estimates from the individual‐based method exhibiting greater bias than those from the population‐based method. The individual‐based method also poorly predicted neonate survival (≤49% accuracy for detecting parturition and calf fate for the Porcupine Herd). Video data revealed that inaccuracies with the movement methods often occurred when caribou bedded during storms, rested after traversing mountainous terrain, lost a calf shortly after birth, or increased movement to evade insects. These results have important implications for management agencies using antler‐ and movement‐based methods to estimate caribou reproduction, particularly when informing assessments of population decline or recovery. Our results demonstrate the utility of video collar data for validating reproductive monitoring approaches and provide insights into how these approaches can be improved.

Adult survival is a crucial factor influencing the population dynamics of caribou ( Rangifer tarandus ), yet its relationship with environmental variables remains poorly understood. We explored linkages among lichen abundance (the primary winter forage), snow depth, and overwinter survival of adult female caribou in the Western Arctic Herd (WAH), a large population of barren-ground caribou ( R. t. granti ) in northwest Alaska. We hypothesized that caribou wintering in areas with greater lichen cover would experience higher survival rates, and that snow depth would moderate this relationship. Using GPS telemetry data from 1,067 caribou winters between 2009 and 2024, we investigated overwinter survival as a function of lichen cover and snow depth. Higher lichen cover was significantly associated with reduced overwinter mortality hazard, but this relationship was strongly and non-linearly affected by snow depth. At intermediate snow depths, caribou in low-lichen habitats experienced 9.1 times higher mortality risk compared to high-lichen areas. However, in deep snow conditions the survival benefits of lichen diminished substantially, suggesting that deep snow may limit forage accessibility. Our results underscore the importance of lichen-rich habitats for barren-ground caribou winter survival and highlight the complex interplay between forage accessibility and snow conditions. Arctic-wide declines in lichen cover could pose a threat to barren-ground caribou populations, with potential long-term demographic consequences, as our study suggests that lichen abundance plays an important role in determining the overwinter survival of adult female caribou.

Modeling the spatial distribution of wildlife abundance is paramount for management. In group‐forming species, group size and occurrence may be governed by different ecological processes. Hierarchical models can conveniently address group size and occurrence as separate processes when estimating abundance. Therefore, identifying factors influencing group size may improve models that the management of group‐forming species relies upon. We test this premise on Dall's sheep ( Ovis dalli ; henceforth sheep), a group‐forming ungulate for which spatial distance sampling models are used to inform management, but environmental features that affect the group size of sheep have yet to be explored within a spatial distance sampling framework. We first used multi‐level Bayesian models to test how spatially explicit indices of predation risk and food availability explained variation in sheep group size. We then demonstrated how including group size covariates within a spatial distance sampling model can improve model inference. Variation in sheep group size was associated with interactions between and among indices of predation risk and food availability. Larger predicted group sizes occurred in areas with higher indexed predation risk and in areas with lower risk but high indexed food availability (decreased competition). Groups tended to be smaller in steep terrain near topographical apexes and in areas with limited forage. Incorporation of these predictors of group size in our application demonstrated how our understanding of spatial patterns in abundance improved when we simultaneously modeled variation in both group occurrence and size. Our findings indicate that sheep are making complex trade‐offs between predation risk and food availability when deciding to aggregate with conspecifics. Explicitly modeling these ecological relationships within our spatial distance sampling model improved predictive performance, increased abundance estimates, and mechanistically linked ecological processes with population monitoring and management. Many wildlife species that form groups are of interest to active wildlife management and our grander understanding of wildlife ecology. Therefore, the concepts we developed here are broadly applicable across a wide range of group‐forming taxa.

Denning is a critical behavioral adaptation for brown bears Ursus arctos to cope with winter, a period of extended resource scarcity. Bears reduce their body temperature, heart rate, and metabolism during this time to minimize energy expenditures. The Arctic has among the most pronounced and longest period of resource scarcity. Thus, we predicted bears in the region would respond by having among the longest recorded denning periods. We used GPS data from brown bears to determine the den entry, den exit, and denning duration for a population living primarily above the Arctic Circle. On average, brown bears in the region denned for 206 days, the longest duration reported using GPS data of which we are aware. The longest denning duration for any individual bear was a remarkable 233 days (64% of the year), which is near the theoretical maximum of 241 days. We found that food availability in fall delayed den entrance, with bears that appeared to consume more salmon entering their dens later. Bears showed greater synchrony in den exiting than den entrance, and female bears with cubs exited their dens more than a week after other bears. Later snow melt out in spring was also associated with later den exits. Climate change has the potential to affect the denning ecology of Arctic brown bears by altering the availability of food, ambient temperature, and precipitation, all of which can alter the costs and benefits of hibernation for brown bears.

Animals living in seasonal environments have adopted a wide array of tactics used to deal with seasonal resource scarcity. Many species migrate between habitats to reach areas where food resources are more plentiful as an attempt to address energetic demands through foraging. We assessed the winter behavioral adaptations of Caribou (Rangifer tarandus), a large ungulate inhabiting Arctic and sub-Arctic regions known for seasonal resource scarcity. Movement rates of Caribou are the lowest of the year during winter, revealing 1 mechanism individuals use to reduce energy expenditures. However, migratory individuals moved nearly twice as much as nonmigratory individuals during winter, suggesting that migratory individuals rely more upon income (forage), whereas nonmigratory individuals rely more upon capital (bodily reserves). Lichens are the primary winter forage for large, migratory herds of Caribou, and migratory individuals experienced more than 2.5 times greater lichen cover than nonmigratory individuals. We documented that both groups slowed their movement in areas of greater lichen cover, suggesting increased foraging time in these areas. Movement rates were greater near villages, which may be suggestive of disturbance, but the effect was weak. Overall energy saved by reduced movement rates was modest. However, energy savings were 11% of daily body energy lost or 47% of the demands of early pregnancy, which potentially could affect individual condition and/or fetal growth if not offset by increases in forage intake.

Aggregated use along salmon‐bearing ( Oncorhynchus spp.) streams is common for coastal brown bears Ursus arctos ; however, it is much less common in non‐coastal (interior) environments, especially in the Arctic. A concurrent study of GPS‐collared brown bears in the interior Brooks Range mountains, northern Alaska, revealed that salmon‐bearing streams and their use by brown bears were more prevalent than previously known. Our goal was to estimate the number, gender, and species of bears ( Ursus spp.) using anadromous streams while salmon were present in an area of northern, interior Alaska. We deployed single‐catch breakaway hair snares on heavily‐used bear trails along 7 km sections of two anadromous streams to identify individual animals through DNA genotypes. We collected 119 hair samples from August and September in 2016 and 2017 which revealed 31 unique brown bears and five American black bears Ursus americanus across both streams. Using a capture‐with‐replacement analysis, we estimated 24 (95% CI: 22–27) brown bears used 7 km of one stream and 15 (95% CI: 9–35) brown bears used 7 km of the other. Across both streams, we detected a higher proportion of females than males, more brown bears than black bears, and greater relatedness among bears that used the same stream. The high number of brown bears estimated along these streams belies their overall low densities in the region, while relatedness patterns suggests that bears obtain behavioral specialization through social learning. A 354 km industrial road is permitted to be constructed along the southern flanks of the Brooks Range, bisecting numerous streams and rivers, including the two sampled above. Given our findings, we identify potential mitigation measures to reduce human‐bear conflicts related to aggregated brown bear use along anadromous streams.

Warming temperatures and advancing spring are affecting annual snow and ice cycles, as well as plant phenology, across the Arctic and boreal regions. These changes may be linked to observed population declines in wildlife, including barren‐ground caribou (Rangifer tarandus), a key species of Arctic environments. We quantified how barren‐ground caribou, characteristically both gregarious and migratory, synchronize births in time and aggregate births in space and investigated how these tactics are influenced by variable weather conditions. We analyzed movement patterns to infer calving dates for 747 collared female caribou from seven herds across northern North America, totaling 1255 calving events over a 15‐year period. By relating these events to local weather conditions during the 1‐year period preceding calving, we examined how weather influenced calving timing and the ability of caribou to reach their central calving area. We documented continental‐scale synchrony in calving, but synchrony was greatest within an individual herd for a given year. Weather conditions before and during gestation had contrasting effects on the timing and location of calving. Notably, a combination of unfavorable weather conditions during winter and spring, including the pre‐calving migration, resulted in a late arrival on the calving area or a failure to reach the greater calving area in time for calving. Though local weather conditions influenced calving timing differently among herds, warm temperatures and low wind speed, which are associated with soft, deep snow, during the spring and pre‐calving migration, generally affected the ability of female caribou to reach central calving areas in time to give birth. Delayed calving may have potential indirect consequences, including reduced calf survival. Overall, we detected considerable variability across years and across herds, but no significant trend for earlier calving by caribou, even as broad indicators of spring and snow phenology trend earlier. Our results emphasize the importance of monitoring the timing and location of calving, and to examine how weather during summer and winter are affecting calving and subsequent reproductive success.

A fundamental assumption of many ecological studies is that researchers are studying a representative sample of the population of interest. However, fish and wildlife studies, including many telemetry studies, often rely upon data from marked individuals that are sampled opportunistically rather than randomly and may provide little information on unmarked individuals. Data that are unrepresentative at the time of marking may need to be censored until marked animal distributions are representative of the population. In the absence of additional data on unmarked individuals, evaluating the representativeness of newly marked individuals within a population of interest can be difficult. If previously marked individuals can be assumed to be spatially representative of the population, comparing the spatial distributions of different cohorts of marked individuals can provide an alternative means to assess the representativeness of newly marked animals. We used simulations to assess the effectiveness of a randomization test that compared the spatial distribution between newly marked and previously marked individuals as an alternative to recapture‐based methods for evaluating mixing of newly marked individuals with the population of interest and compared different metrics of spatial overlap. We then tested this randomization test on radio‐collared caribou ( Rangifer tarandus ) from 2 different herds with very different marking strategies. We found that, based on simulation results, kernel‐based metrics outperformed cluster analysis metrics and that Bhattacharyya's Affinity (BA) index of kernel overlap was a good metric to identify differences in spatial distribution between marked cohorts. The time it took for newly marked caribou to become mixed with previously marked animals was primarily related to the timing of marking in relation to seasonal movement patterns (periods of aggregation and dispersion, and seasonal differences in spatial fidelity) and the spatial distribution of marking relative to seasonal distribution. In some cases, mixing did not occur until calving aggregations formed in early June. We recommend that researchers evaluate the assumption that marked individuals are spatially representative of their study population and carefully assess sampling time and location of their marking program to increase the likelihood that marked individuals represent the population of interest.

Background: Migrations in temperate systems typically have two migratory phases, spring and autumn, and many migratory ungulates track the pulse of spring vegetation growth during a synchronized spring migration. In contrast, autumn migrations are generally less synchronous and the cues driving them remain understudied. Our goal was to identify the cues that migrants use in deciding when to initiate migration and how this is updated whileen route.Methods: We analyzed autumn migrations of Arctic barren-ground caribou (Rangifer tarandus) as a series of persistent and directional movements and assessed the influence of a suite of environmental factors. We fitted a dynamic-parameter movement model at the individual-level and estimated annual population-level parameters for weather covariates on 389 individual-seasons across 9 years.Results: Our results revealed strong, consistent effects of decreasing temperature and increasing snow depth on migratory movements, indicating that caribou continuously update their migratory decision based on dynamic environmental conditions. This suggests that individuals pace migration along gradients of these environmental variables. Whereas temperature and snow appeared to be the most consistent cues for migration, we also found interannual variability in the effect of wind, NDVI, and barometric pressure. The dispersed distribution of individuals in autumn resulted in diverse environmental conditions experienced by individual caribou and thus pronounced variability in migratory patterns.Conclusions: By analyzing autumn migration as a continuous process across the entire migration period, we found that caribou migration was largely related to temperature and snow conditions experienced throughout the journey. This mechanism of pacing autumn migration based on indicators of the approaching winter is analogous to the more widely researched mechanism of spring migration, when many migrants pace migration with a resource wave. Such a similarity in mechanisms highlights the different environmental stimuli to which migrants have adapted their movements throughout their annual cycle. These insights have implications for how long-distance migratory patterns may change as the Arctic climate continues to warm.