Development of Thermoregulation in Merriam's Kangaroo Rat, Dipodomys merriami

Abstract

Body temperatures (Tb) of nursing Dipodomys merriami are lower than the average Tb of adults until the nurslings are about 14-15 days old. For the first 3 wk after birth there is a positive correlation between mean Tb of the nurslings and their age. Although D. merriami is a highly specialized species, nurslings develop Tb's of the level of adults at the same rate as more generalized rodent species. Newborn kangaroo rats lack pelage, and the rate of heat exchange with the environment is high. During exposure to 21 C for 30 min the cooling constants of the newborn kangaroo rats are high, averaging 0.0374 In C · min−1. By an age of 15-16 days, the cooling of nurslings is not significantly different from zero. As heat-exchange rates decrease with age, nurslings are more able to maintain a high temperature differential (ΔT) between body and ambient temperatures. By the age of 15-16 days, nursing kangaroo rats were capable of maintaining a ΔT of 14 C when exposed to 21 C. Species of rodents from more temperate regions develop this capacity at about the same rate as the kangaroo rat. However, three species from the tundra develop this capacity faster. Rapid development of the capability to maintain a high ΔT may be advantageous in a tundra climate where exposure to cold at an early age is probable. Within 2 wk after birth, young kangaroo rats develop 90% of the thermoregulatory ability that they will have as adults. Circumstantial evidence suggests that endothermy develops at an early age. Homeothermy does not develop until the pelage has reached a depth sufficient to reduce heat exchange to a level that will make homeothermy an energetically practical strategy.