Developmental delay in shivering limits thermogenic capacity in juvenile high-altitude deer mice (Peromyscus maniculatus)
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Many endotherms native to cold and hypoxic high altitude (HA) environments have evolved a highly vascularized and aerobic skeletal muscle. This specialized muscle phenotype contributes via shivering to an enhanced capacity for aerobic thermogenesis (cold-induced VO2max). However, it is unclear how selection at HA for shivering thermogenesis acts early in the development of small altricial mammals, which are born with immature skeletal muscles and without the capacity for homeothermic endothermy. We have previously shown that postnatal maturation of brown adipose tissue and non-shivering thermogenesis is delayed in HA native deer mouse pups (Peromyscus maniculatus). To assess if HA adaptation has also altered the developmental program of skeletal muscle and shivering thermogenesis, we used laboratory-reared descendants of deer mice native to low (430 m a.s.l) and high (4350 m a.s.l.) altitude and a low altitude (LA) congeneric outgroup (P. leucopus). We found that LA juveniles were able to shiver robustly at 2 weeks after birth. However, HA juveniles were unable to shiver at this point, resulting in a 30% lower capacity for thermoregulation compared to lowlanders. It was only at 27 days after birth that HA juveniles had established the aerobic muscle phenotype characteristic of HA adults and a superior cold-induced VO2max compared to LA mice of the same age. The capacity for shivering may be delayed in HA mice to allow energy to be allocated to other important processes such as growth.
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