Relative Allocation of Energy to Growth and Development of Homeothermy in the Eastern Wood Rat (Neotoma floridana) and Hispid Cotton Rat (Sigmodon hispidus)

Abstract

Allocation of energy to growth and heat production was measured by constructing energy budgets of male and female Sigmodon hispidus (x Adult body mass = ABM = 113 g ♂ 105 g ♀) and Neotoma floridana (ABM = 270 g ♂ 184 g ♀) from birth to maturity. Energy ingested and oxygen consumption per gram of body mass of post—weaning animals were not significantly different between sexes within species, but did differ significantly between the species. In all animals the relation between post—weaning ingestion (per gram body mass) and body mass follows a negative power function. Post—weaning metabolic rate as a function of body mass follows a different negative power function. The pattern of change in metabolic rate prior to weaning is different in the two species. In Sigmodon, metabolic rate increases linearly from birth to about 20 g (or about 20% of ABM). The rate at 20 g is about 5 cm3°g—1°h—1. In Neotoma metabolic rate remains constant at 1.7 cm3°g—1°h—1 from birth to about 26 g (or about 11% of ABM), then increases linearly to about 2.2 cm3°g—1°h—1 at 65 g (29% ABM). In both species, the peak in metabolic rate and nutritional weaning coincide with development of homeothermy, (measured as ability to maintain adult—level body temperatures at an ambient temperature of 20°C). Young Sigmodon become efficient homeotherms by 10—12 d of age whereas Neotoma require 19—22 d. Growth of each sex and species can be described by Gompertz equations. Growth rate constants, K, and asymptotic mass, A, differ between sexes within species and between species. Gompertz equations do not accurately describe growth of pre—weaning Neotoma which was linear from birth to 21—22 d of age. No such period of linearity is seen in Sigmodon growth. Analysis of litter—size dependence of growth of pre—weaning animals indicates that the early linear growth of Neotoma may not be a result of limited milk production, as it seems to be in Sigmodon. The relative total investment of energy in growth for a litter of Sigmodon (° = 4.8 young) between birth and 12 d of age was 416.7 kJ compared to 770.7 kJ for the first 24 d of life of the average Neotoma (x = 2.6 young). Over these same times, a litter of Sigmodon expended 1350.6 kJ on respiration compared with 1296.2 kJ in Neotoma. Thus the total metabolizable energy requirements of 1767.7 kJ and 2066.2 kJ are similar in the two species; however, a litter of Sigmodon requires about twice as much metabolizable energy per day each day they remain with their mother. Neotoma daily requirements are lower but protracted in time. During the pre—weaning period, Neotoma are investing proportionally more of their metabolizable energy in growth, whereas Sigmodon spend more on respiration, perhaps because they depend on their own heat production for thermoregulation while Neotoma rely on their mother's heat. These results are compared with data from the literature on growth and development of homothermy in a variety of small rodents. The hypothesis is advanced that, in general, larger species defer onset of active thermoregulation to larger body masses when lower metabolic rates occur, perhaps as an adaptation to permit more efficient early growth. Small species sacrifice growth efficiency in favor of rapid attainment of early independence and they pay the high energy costs of that speed. The different developmental physiologies of Neotoma and Sigmodon are explainable by inference from current ideas on the evolution of life histories, but much of the difference is also predictable simply from their difference in body size. This may indicate the importance of body size in the complex of life history traits.