Evolved increases in running performance in cold hypoxia in high-altitude deer mice.

The cold and hypoxic conditions at high altitude can challenge the ability of small endotherms to meet the high energy demands of locomotion and thermoregulation. We examined how high-altitude natives overcome this challenge through plastic and/or evolved improvements in locomotory performance. Deer mice (Peromyscus maniculatus) native to high and low altitude were born and raised in captivity, then acclimated to warm normoxia or cold hypoxia as adults. Running endurance was then measured in both warm normoxia and cold hypoxia across groups. Among mice acclimated to warm normoxia, endurance was greater in highlanders compared with lowlanders. Acclimation to cold hypoxia increased endurance in lowlanders, partially approaching values in highlanders. Body temperature declined while running in cold hypoxia in lowlanders, but highlanders were better at avoiding such declines. Our data suggest that evolved changes in thermoregulatory ability and muscle phenotype combine to improve locomotory performance in cold hypoxia in high-altitude deer mice.