Abstract
Most mammals and birds are altricial, small and naked at birth/hatching. They attain endothermic thermoregulation at a fraction of their adult size at a vulnerable stage with high heat loss when many could profit from using torpor for energy conservation. Nevertheless, detailed data on the interrelations between torpor expression and development of endothermic thermoregulation are currently restricted to <0.1% of extant endotherms. We investigated at what age and body mass (BM) desert hamsters (Phodopus roborovskii), wild-caught in Inner Mongolia and born in autumn/early winter when environmental temperatures in the wild begin to decrease, are able to defend their body temperature (Tb) at an ambient temperature (Ta) of ∼21°C and how soon thereafter they could express torpor. Measurements of surface temperatures via infrared thermometer and thermal camera show that although neonate hamsters (BM 0.9 ± 0.1 g) cooled rapidly to near Ta, already on day 15 (BM 5.5 ± 0.2 g) they could defend a high and constant Tb. As soon as day 16 (BM 5.8 ± 0.2 g), when their maximum activity metabolism (measured as oxygen consumption) approached maxima measured in vertebrates, animals were able to enter torpor for several hours with a reduction of metabolism by >90%, followed by endothermic arousal. Over the next weeks, torpor depth and duration decreased together with a reduction in resting metabolic rate at Ta 30–32°C. Our data show that development of endothermy and torpor expression in this altricial hamster is extremely fast. The results suggest that precocious torpor by juvenile hamsters in autumn and winter is an important survival tool in their vast and harsh Asian desert habitats, but likely also for many other small mammals and birds worldwide.
Highlights
Hibernation and daily torpor are used by many endothermic mammals and birds to reduce energy expenditure, often to deal with a shortage of food or adverse environmental conditions (Boyer and Barnes, 1999; Nowack et al, 2017)
Torpor is more common in small than in large endotherms and, apparently to maximize energy savings during the torpid state, small species have on average lower minimum Tbs and their metabolic rate (MR) reductions are more pronounced (Ruf and Geiser, 2015)
We tested the hypothesis that this fast-growing species is able to express torpor early during the growth phase, soon after endothermy is attained. Such data will improve the understanding of the functional significance of torpor especially in rapidly developing altricial mammals and the role of heterothermy during development in endotherms in general
Summary
Hibernation (multiday torpor) and daily torpor are used by many endothermic mammals and birds to reduce energy expenditure, often to deal with a shortage of food or adverse environmental conditions (Boyer and Barnes, 1999; Nowack et al, 2017). We tested the hypothesis that this fast-growing species is able to express torpor early during the growth phase, soon after endothermy is attained Such data will improve the understanding of the functional significance of torpor especially in rapidly developing altricial mammals and the role of heterothermy during development in endotherms in general
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