Development of Heat Production in Altricial and Precocial Rodents: Implications for the Energy Allocation Hypothesis

Abstract

The development of thermogenic abilities, evaporative water loss, and maintenance ofskin and rectal temperatures are reportedfor neonates of two cricetid rodent species difering in developmental mode. A thermogenic response to lowered air temperature developed rapidly in the precocial cotton rat (Sigmodon hispidus) and more slowly in the altricial wood rat (Neotoma floridana). At any body temperature, neonatal cotton rats generated about twice the metabolic heatper unit body mass than did similarly aged wood rats. Newborn cotton rats maintained nearly constant metabolic rates from 35° to 20° C, but metabolic rate decreased with lowered air temperature below 30° C in newborn wood rats. Individuals ofboth species frequently exhibited local interscapular skin temperatures higher than colonic temperature. Cotton rats had higher skin temperatures and maintained higher body temperatures at younger ages than did wood rats, but neither species maintained body temperatures independent of air temperatures before weaning. At higher air temperatures, cotton rats dissipated more heat by evaporative cooling than was generated metabolically. Under similar conditions wood rats dissipated slightly less heat by evaporation than was generated metabolically. Evaporative water loss was correlated with body mass, air temperature, and metabolic rate in wood rats, but was correlated with skin temperature in cotton rats. Significant individual variation in metabolic rate waspresent in both species. For cotton rats, this variation appears to be related to skin and core temperatures, but not to body mass. In wood rats, this variation is not related to diferences in core or skin temperatures. The development of homeothermy in these species appears to be limited by diferentphysiological factors. In cotton rats, homeothermic status is strongly linked to ontogenetic increases in metabolic heat production. Development of homeothermy in wood ratsproceeds along a completely different pathway from that in cotton rats, and relies on the acquisition of significant insulation rather than on metabolic capability. The combination ofhigh metabolism and rapid growth in cotton rats violates subtractive theories of energy allocation.