Energy Export and Offspring Growth During Lactation in Cotton Rats (Sigmodon hispidus)

Abstract

1. Maternal energy export (in milk), food intake and mass loss were examined in lactating Cotton Rats (Sigmodon hispidus) in relation to the growth of suckling offspring in small (three offspring) and large litters (six offspring). We predicted that energy export would increase from early to late lactation to provide energy for offspring growth, and that mothers with large-sized litters may reach a limit in their ability to nourish suckling young. 2. Mothers nursing small litters showed a greater ability than those with large litters to respond to the energy demands of growing offspring. Their export of chemical energy per suckling pup increased from early to late lactation. In contrast, the energy flow to individual pups in large litters remained low throughout lactation. 3. The low rate of energy flow to individual offspring in large litters significantly affected their postnatal growth. Individual pups in large litters obtained milk with 73.7% the energy content and grew at 71.2% the rate of pups in small litters. 4. The milk produced by lactating animals with large litters was dilute (had a small fraction of milk solids) and had a lower energy content per dry mass than did the milk produced by animals supporting small litters. 5. Mothers supporting large litters did not acquire sufficient energy from food intake or tissue catabolism to offset the low rate of energy export to their offspring. Maximal rates of food intake and tissue catabolism may have been attained. 6. The energy export by mothers supporting large litters (mean mass=134 g) was uniformly high. The peak rate of energy flow was approximately 220 kJ day -1 . 7. These results indicate that physical constraints on mothers with large litters limit milk production and, consequently, offspring growth