LAUSR

Our knowledge is limited on how weaned mammals adjust to environmental events in the absence of maternal assistance in the polar north. With the expectation that weaned offspring should express the adaptive potential of the mature phenotype, we determined their appetite and body mass responses to extreme cold. Between December and May when mean daily temperature ( T a , °C) was between − 48 and 2.3 we provided five naturally weaned reindeer calves ( Rangifer tarandus L.) ad libitum a pelleted feed of 11% crude protein with access to snow only in individual fenced outdoor pens in interior Alaska (64° N 147° W). We predicted voluntary food intake would meet maintenance of body mass, but that extreme cold would result in an increase in appetite that would be compensated for by access to warm water. A linear mixed effect model indicated a moderate daily increase in body mass from 55.9 ± 1.6 kg on 9 December to 59.2 ± 0.6 kg on 14 May and a moderate increase in intake that was evidence for maintenance of thermal homeostasis. However, at both low (≤ 30) and high (≥ 10) T a (°C) intake increased and we estimated a lower critical temperature of − 22 and upper critical temperature at − 10 that was indicative of high total-body insulation. Despite the appetite response at low T a , provision of warm water did not affect intake. Thus, whereas adult reindeer restrict intake by mobilization of endogenous reserves the calf thermogenic response was an increase in appetite that indicated the importance of access to a food base for calves in winter.