Energy Allocation during Lactation in Cotton Rats (Sigmodon hispidus) on a Restricted Diet

Abstract

The effects of restricted food availability on lactating Sigmodon hispidus from Kansas (KSh) and Tennessee (TSh) populations were studied in the laboratory. Mothers received 75% of food intake required by nonreproductive females of similar body mass. KSh control and restricted females produced significantly larger litter sizes at birth (6.9 and 7.3) than TSh females (4.7 and 5.0). At weaning (12d), mean litter size of restricted KSh females (4.3) was similar to that of control (4.4) and restricted TSh females (4.5). Mean litter size for control KSh females (6.6) was significantly larger than for the other three groups. KSh females of smaller body mass suffered higher offspring mortality during lactation than did larger KSh females. Mean body mass of control KSh young (20.1 g) was significantly larger than of control TSh young (16.5 g) at age 12 d. Mean body masses of restricted KSh and TSh young (12.5 and 11.2 g) were not significantly different at 12 d of age, and young of both groups were smaller than controls. Smaller restricted KSh females produced young that grew more slowly than those of larger restricted KSh females. Variation in the growth of restricted TSh young, however, was dependent upon litter size, as was growth for both TSh and ksh controls. During lactation, restricted TSh and KSh females lost an average of 31% of parturition body mass; comparable values for control TSh and KSh were 13% (16 g) and 9% (17 g), respectively. In the nest, body temperature of young of restricted KSh and TSh females averaged °1°—2°C lower than body temperatures of control young. Development of the ability of pups to respond metabolically to cold, however, did not differ significantly between control and restriction treatments or between populations. Thus, under severe food restriction, females with larger litter size (KSh) lost young until litter sizes were similar to those for females with smaller original litters (TSh). Decreased growth and body temperatures of nestlings in food—restricted groups reduced the energy demands of lactation so that smaller litters could survive to weaning. Growth and survivorship of KSh young were largely determined by the body mass of the dam, indicating the potentially significant role of body size for small mammals that experienced periodic food shortages during lactation.