Energy Costs of Reproduction in the Cotton Rat, Sigmodon Hispidus

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Abstract
Individual organism costs of reproduction were investigated by a series of observations and experiments with the cotton rat, Sigmodon hispidus. Energy budget techniques, including respirometry, showed that ingestion in pregnant rats was increased 25%, while that in lactating rats was increased 66%, over nonreproductive rats. Digestive efficiency of rats fed a commercial standard diet was 84% and did not change as a result of pregnancy or lactation. Respiration per gram of reproductive rats was not significantly different from nonreproductive rats. Twenty—six days of pregnancy required an average additional ingestion of 221.8 kcal [=928.0 kJ]; 12 days of lactation required an extra 270.7 kcal[=1,132.6 kJ]. Total cost per offspring 12 days of age was 100.5 kcal[=420.5 kJ]. The total cost of producing a litter was a linear function of litter size. Two—thirds of the energy accumulated by a pregnant rat was not in the neonates but deposited as storage and mobilized during lactation when the precocial young grew faster than the maternal assimilation alone could support. This use of stored energy increased the apparent efficiency of lactation above that found in other wild rodents. Neonatal litters of cotton rats contained two to seven young (Z = 5). The sizes of these individuals were not affected by litter size although their subsequent early growth rates were. Individuals from larger litters grew more slowly.