Altricial mammals begin to independently thermoregulate during the firstfew weeks of postnatal development. In wild rodent populations, this isalso a time of high mortality (50–95%), making the physiological systemsthat mature during this period potential targets for selection. High altitude(HA) is a particularly challenging environment for small endotherms owingto unremitting low O2and ambient temperatures. While superior thermo-genic capacities have been demonstrated in adults of some HA species, itis unclear if selection has occurred to survive these unique challengesearly in development. We used deer mice (Peromyscus maniculatus) nativeto high and low altitude (LA), and a strictly LA species (Peromyscus leucopus),raised under common garden conditions, to determine if postnatal onset ofendothermy and maturation of brown adipose tissue (BAT) is affected byaltitude ancestry. We found that the onset of endothermy correspondswith the maturation and activation of BAT at an equivalent age in LAnatives, with 10-day-old pups able to thermoregulate in response to acutecold in both species. However, the onset of endothermy in HA pups wassubstantially delayed (by approx. 2 days), possibly driven by delayedsympathetic regulation of BAT. We suggest that this delay may be part ofan evolved cost-saving measure to allow pups to maintain growth ratesunder the O2-limited conditions at HA
