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

Long-term effects of climate change on juvenile bull shark migratory patterns

Seasonal variability in environmental conditions is a strong determinant of animal migrations, but warming temperatures associated with climate change are anticipated to alter this phenomenon with unknown consequences. We used a 40-year fishery-independent survey to assess how a changing climate has altered the migration timing, duration and first-year survival of juvenile bull sharks (Carcharhinus leucas). From 1982 to 2021, estuaries in the western Gulf of Mexico (Texas) experienced a mean increase of 1.55°C in autumn water temperatures, and delays in autumn cold fronts by ca. 0.5 days per year. Bull shark migrations in more northern estuaries concomitantly changed, with departures 25-36 days later in 2021 than in 1982. Later, migrations resulted in reduced overwintering durations by up to 81 days, and the relative abundance of post-overwintering age 0-1 sharks increased by >50% during the 40-year study period. Yet, reductions in prey availability were the most influential factor delaying migrations. Juvenile sharks remained in natal estuaries longer when prey were less abundant. Long-term declines in prey reportedly occurred due to reduced spawning success associated with climate change based on published reports. Consequently, warming waters likely enabled and indirectly caused the observed changes in shark migratory behaviour. As water temperatures continue to rise, bull sharks in the north-western Gulf of Mexico could forgo their winter migrations in the next 50-100 years based on current trends and physiological limits, thereby altering their ecological roles in estuarine ecosystems and recruitment into the adult population. It is unclear if estuarine food webs will be able to support changing residency patterns as climate change affects the spawning success of forage species. We expect these trends are not unique to the western Gulf of Mexico or bull sharks, and migratory patterns of predators in subtropical latitudes are similarly changing at a global scale.

A test of the frost wave hypothesis in a temperate ungulate

Growing evidence supports the hypothesis that temperate herbivores surf the green wave of emerging plants during spring migration. Despite the importance of autumn migration, few studies have conceptualized resource tracking of temperate herbivores during this critical season. We adapted the frost wave hypothesis (FWH), which posits that animals pace their autumn migration to reduce exposure to snow but increase acquisition of forage. We tested the FWH in a population of mule deer in Wyoming, USA by tracking the autumn migrations of n = 163 mule deer that moved 15-288 km from summer to winter range. Migrating deer experienced similar amounts of snow but 1.4-2.1 times more residual forage than if they had naïve knowledge of when or how fast to migrate. Importantly, deer balanced exposure to snow and forage in a spatial manner. At the fine scale, deer avoided snow near their mountainous summer ranges and became more risk prone to snow near winter range. Aligning with their higher tolerance of snow and lingering behavior to acquire residual forage, deer increased stopover use by 1 ± 1 day (95% CI) day for every 10% of their migration completed. Our findings support the prediction that mule deer pace their autumn migration with the onset of snow and residual forage, but refine the FWH to include movement behavior en route that is spatially dynamic.

Robust Responses of Female Caribou to Changes in Food Supply

AbstractUngulates can respond to changes in food supply by altering foraging behavior, digestive function, and metabolism. A multifaceted response to an environmental change is considered robust. Short seasons of plant growth make herbivores sensitive to changes in food supply because maintenance and production must be accomplished in less time with fewer options in a more fragile response. Caribou live at high latitudes where short summers constrain their response to changes in food supply. We measured the ability of female caribou to resist and tolerate changes in the quality and quantity of their food supply during winter and summer. Caribou resisted changes in food abundance and quality by changing food intake and physical activity with changes in daily temperature within each season. Peak food intake rose by 134% from winter pregnancy to summer lactation (98 vs. 229 g kg-0.75 d-1), as digestible requirements to maintain the body increased by 85% for energy (1,164 vs. 2,155 kJ kg-0.75 d-1) and by 266% for N (0.79 vs. 2.89 g N kg-0.75 d-1). Caribou required a diet with a digestible content of 12 kJ g-1 and 0.8% N in pregnancy, 18 kJ g-1 and 1.9% N in early lactation, and 11 kJ g-1 and 1.2% N in late lactation, which corresponds with the phenology of the wild diet. Female caribou tolerated restriction of ad lib. food intake to 58% of their energy requirement (680 vs. 1,164 kJ kg-0.75 d-1) during winter pregnancy and to 84% of their energy requirement (1,814 vs. 2,155 kJ kg-0.75 d-1) during summer lactation without a change in stress level, as indicated by fecal corticosterone concentration. Conversely, caribou can respond to increased availability of food with a spare capacity to process digestible energy and N at 123% (2,642 vs. 2,155 kJ kg-0.75 d-1) and 145% (4.20 vs. 2.89 g N kg-0.75 d-1) of those respective requirements during lactation. Robust responses to changes in food supply allow caribou to sustain reproduction, which would buffer demographic response. However, herds may decline when thresholds of behavioral resistance and physiological tolerance are frequently exceeded. Therefore, the challenge for managing declining populations of caribou and other robust species is to identify declines in robustness before their response becomes fragile.