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1. Shed female caribou antlers extend records of calving activity on the Arctic National Wildlife Refuge by millennia

   https://doi.org/10.3389/fevo.2022.1059456  

   Miller, Joshua H. ; Wald, Eric J. ; Druckenmiller, Patrick ( February 2023 , Frontiers in Ecology and Evolution)

   Caribou ( Rangifer tarandus ) have among the longest annual migrations of any terrestrial mammal as they move from winter ranges to spring calving grounds. Biomonitoring records indicate broad consistencies in calving geography across the last several decades, but how long have herds used particular calving grounds? Furthermore, how representative are modern patterns of calving geography to periods that pre-date recent climatic perturbations and increased anthropogenic stresses? While modern ecological datasets are not long enough to address these questions, bones from past generations of caribou lying on the tundra provide unique opportunities to study historical calving geography. This is possible because female caribou shed their antlers within days of giving birth, releasing a skeletal indicator of calving. Today, the Coastal Plain of the Arctic National Wildlife Refuge (Alaska) is a key calving ground for the Porcupine Caribou Herd (PCH). To test the duration across which caribou have used this area as a calving ground, we radiocarbon dated three highly weathered female antlers collected from tundra surfaces on the Coastal Plain. Calibrated radiocarbon dates indicate that these antlers were shed between ~1,600 and more than 3,000 calendar years ago. The antiquity of these shed antlers provides the first physical evidence of calving activity on the PCH calving grounds from previous millennia, substantiating the long ecological legacy of the Coastal Plain as a caribou calving ground. Comparisons to published lake core records also reveal that dates of two of the antlers correspond to periods with average summer temperatures that were warmer than has been typical during the last several decades of biomonitoring. This finding expands the range of climatic settings in which caribou are known to use the current PCH calving grounds and suggests that the Coastal Plain of the Arctic Refuge may remain an important caribou calving ground during at least portions of predicted future warming. Discarded skeletal materials provide opportunities to assess the historical states of living populations, including aspects of reproductive biology and migration. Particularly in high-latitude settings, these insights can extend across millennia and offer rare glimpses into the past that can inform current and future management policies. 

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2. Continental synchrony and local responses: Climatic effects on spatiotemporal patterns of calving in a social ungulate

   https://doi.org/10.1002/ecs2.4399  

   Couriot, Ophélie H. ; Cameron, Matthew D. ; Joly, Kyle ; Adamczewski, Jan ; Campbell, Mitch W. ; Davison, Tracy ; Gunn, Anne ; Kelly, Allicia P. ; Leblond, Mathieu ; Williams, Judy ; et al ( January 2023 , Ecosphere)

   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.

    

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3. Mechanistic movement models identify continuously updated autumn migration cues in Arctic caribou

   https://doi.org/10.1186/s40462-021-00288-0  

   Cameron, Matthew D. ; Eisaguirre, Joseph M. ; Breed, Greg A. ; Joly, Kyle ; Kielland, Knut ( December 2021 , Movement Ecology)

   Abstract 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 while en 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. 

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4. Pronounced Fidelity and Selection for Average Conditions of Calving Area Suggestive of Spatial Memory in a Highly Migratory Ungulate

   https://doi.org/10.3389/fevo.2020.564567  

   Cameron, Matthew D. ; Joly, Kyle ; Breed, Greg A. ; Mulder, Christa P. ; Kielland, Knut ( November 2020 , Frontiers in Ecology and Evolution)

   A distinguishing characteristic of many migratory animals is their annual return to distinct calving (birthing) areas in the spring, yet the navigational mechanisms employed during migration that result in this pattern are poorly understood. Effective conservation of these species requires reliable delineation of such areas, quantifying the factors that influence their selection, and understanding the underlying mechanisms resulting in use of calving areas. We used barren-ground caribou ( Rangifer tarandus granti ) as a study species and identified calving sites of the Western Arctic Herd in Alaska using GPS collar data from 2010–2017. We assessed variability in calving areas by comparing spatial delineations across all combinations of years. To understand calving area selection at a landscape scale, we performed a resource selection analysis comparing calving sites to available locations across the herd’s range and incorporated time-varying, remotely sensed metrics of vegetation quality and quantity. We found that whereas calving areas varied from year to year, this annual variation was centered on an area of recurring attraction consistent with previous studies covering the last six decades. Calving sites were characterized by high-quality forage at the average time of calving, but not peak calving that year, and by a narrow range of distinct physiographic factors. Each year, calving sites were located on areas of above-average conditions based on our predictive model. Our findings indicate that the pattern of spring migration for pregnant females was to migrate to areas that consistently provide high-quality forage when averaged across years, and then upon arriving at this calving ground, refine selection using their perception of annually varying conditions that are driven by environmental stochasticity. We suggest that the well-documented and widespread pattern of fidelity to calving grounds by caribou is supportive of a navigational mechanism based on spatial memory at a broad scale to optimize foraging and energy acquisition at a critical life-history stage. The extent to which migrants depend on memory to reach their spring destinations has implications for the adaptability of populations to changing climate and human impacts. 

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5. Quantifying effects of snow depth on caribou winter range selection and movement in Arctic Alaska

   https://doi.org/10.1186/s40462-021-00276-4  

   Pedersen, Stine Højlund ; Bentzen, Torsten W. ; Reinking, Adele K. ; Liston, Glen E. ; Elder, Kelly ; Lenart, Elizabeth A. ; Prichard, Alexander K. ; Welker, Jeffrey M. ( September 2021 , Movement Ecology)

   Caribou and reindeer across the Arctic spend more than two thirds of their lives moving in snow. Yet snow-specific mechanisms driving their winter ecology and potentially influencing herd health and movement patterns are not well known. Integrative research coupling snow and wildlife sciences using observations, models, and wildlife tracking technologies can help fill this knowledge void.

   Here, we quantified the effects of snow depth on caribou winter range selection and movement. We used location data of Central Arctic Herd (CAH) caribou in Arctic Alaska collected from 2014 to 2020 and spatially distributed and temporally evolving snow depth data produced by SnowModel. These landscape-scale (90 m), daily snow depth data reproduced the observed spatial snow-depth variability across typical areal extents occupied by a wintering caribou during a 24-h period.

   We found that fall snow depths encountered by the herd north of the Brooks Range exerted a strong influence on selection of two distinct winter range locations. In winters with relatively shallow fall snow depth (2016/17, 2018/19, and 2019/20), the majority of the CAH wintered on the tundra north of the Brooks Range mountains. In contrast, during the winters with relatively deep fall snow depth (2014/15, 2015/16, and 2017/18), the majority of the CAH caribou wintered in the mountainous boreal forest south of the Brooks Range. Long-term (19 winters; 2001–2020) monitoring of CAH caribou winter distributions confirmed this relationship. Additionally, snow depth affected movement and selection differently within these two habitats: in the mountainous boreal forest, caribou avoided areas with deeper snow, but when on the tundra, snow depth did not trigger significant deep-snow avoidance. In both wintering habitats, CAH caribou selected areas with higher lichen abundance, and they moved significantly slower when encountering deeper snow.

   In general, our findings indicate that regional-scale selection of winter range is influenced by snow depth at or prior to fall migration. During winter, daily decision-making within the winter range is driven largely by snow depth. This integrative approach of coupling snow and wildlife observations with snow-evolution and caribou-movement modeling to quantify the multi-facetted effects of snow on wildlife ecology is applicable to caribou and reindeer herds throughout the Arctic.

    

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