Heat stress constrains milk intake and growth potential of hand-reared caribou calves

Arctic herbivores are predicted to be vulnerable to climate change through changes in available food supplies and hence nutrition. Thermal constraints associated with digestion also have the potential to affect nutrition, via effects on food intake, independently of changes to food supplies. Using hand-reared caribou (Rangifer tarandus granti) calves, I tested the hypothesis that thermal stress constrains food intake. I predicted milk intake would decrease and milk refusals would increase with increasing ambient temperature and calves would be more likely to refuse milk on warmer than cooler days. From 1 July-27 October 2023, I documented milk intake, milk refusal, and body mass of n = 5 caribou calves each day; ambient temperature was recorded at a weather station on site. For every 1 °C increase in maximum daily temperature, calves consumed ∼14 g less milk, refused ∼4% more milk, and odds of refusing a bottle were ∼14% higher. Maximum daily temperature was greater on days when caribou calves refused bottles (16.8 ± 0.4 °C; mean ± SE) versus those on which calves did not refuse bottles (13.3 ± 0.3 °C). Calves refusing the largest quantities of milk during the study were up to 7.3 kg lighter at weaning than calves refusing the least amount of milk. Findings from this study indicate that: (1) high temperatures were associated with reductions in food intake; (2) reductions in food intake are a mechanism by which high temperatures can reduce growth; and (3) nutritional effects of increased temperatures can occur independently of food supplies.