JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org. This content downloaded from 128.235.251.160 on Mon, Abstract. Energy costs of reproduction were measured in the laboratory for individual largelittered (x = 7.0) cotton rats, Sigmodon hispidus, from Kansas, and less extensively for individual small-littered (x = 4.7) cotton rats from Texas. These estimates were compared with previously studied small-littered (x = 5.0) S. hispidus from Tennessee.Reproductive Kansas S. hispidus (KSh) increased ingestion above nonreproductive rates by 19% during pregnancy and 111% during lactation. This represents extra ingestion of 766 kJ during 26 d of pregnancy and 2111 kJ during 12 d of lactation. Extra ingestion throughout reproduction was dependent on litter size. Total extra ingestion cost per 12-d-old KSh offspring was 415 kJ, a constant unaffected by litter size. Digestive efficiency of KSh rats fed a commercial standard lab chow was 85% and did not change as a result of pregnancy and lactation. Metabolic rate of reproductive KSh rats was not significantly different from that of nonreproductive rats. The average body mass of KSh females on the day of conception was 175 g, vs. 169 g at the end of 12 d of lactation, a loss of 6 g. This loss of body mass during reproduction increased with increasing litter size: KSh females giving birth to litters of eight to ten young lost an average of 19 g below conception mass, while females giving birth to five to seven young gained an average of I g. KSh young averaged 6.8 g at birth and 21.7 g at 12 d of age.Reproductive Texas S. hispidus (HSh) increased ingestion over nonreproductive rates by 22% during pregnancy and 70% during lactation. This represents extra ingestion of 854 kJ during pregnancy and 1161 kJ during lactation. Total extra ingestion cost per 12-d-old offspring was 434 kJ for HSh litters. The average body mass of females on the day of conception was 134 g and, at the end of 12 d of lactation, averaged 138 g. HSh young averaged 6.7 g at birth and 18.2 g at 12 d of age.Kansas cotton rats produced relatively large litters and large individual young primarily by markedly increasing ingestion and drawing upon maternal storage during lactation. Both of these responses were probably facilitated by the KSh female's large body size. These results and comparisons among similarly studied species suggest the importance of the common constraints and correlated responses associated with changes in body size and litter size when considering energy utilization during reproduction.

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 thermoregula...

Mothers of eastern wood rats (Neotoma floridana) normally invest their lactation energy equally in male and female offspring, but alter that investment when their food is severely restricted during lactation. The effect of the altered investment is a significant bias against males in both mortality and growth.

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