Subspecific Differences in Metabolism, Thermoregulation, and Torpor in the Western Harvest Mouse Reithrodontomys megalotis

Abstract

Oxygen consumption and body temperature were monitored for two subspecies of western harvest mouse, Reithrodontomys megalotis, under standard conditions for resting (basal) metabolism and during shallow daily torpor. Reithrodontomys m. longicaudus (7.77 g), from southern California, had a resting metabolic rate 127%, and a heat loss coefficient 126%, that expected from body mass. Reithrodontomys m. ravus (9.81 g), from near the Great Salt Lake in Utah, had both a significantly lower resting metabolic rate (114%) and a significantly lower heat loss coefficient (91%) than longicaudus. Reithrodontomys m. longicaudus was readily induced to enter shallow daily torpor when held at temperatures below thermoneutrality and starved for 10-12 h. Despite its lower rate of metabolism, R. m. ravus rarely entered torpor under any combination of cold- or starvation-induced stress. This differential use of torpor between two closely related subspecies may be partially attributed to differences in body mass and heat loss. ...