Energetics of Concurrent Pregnancy and Lactation in Cotton Rats and Woodrats

Concurrent pregnancy and lactation is common among rodents in spite of the high energetic cost of lactation alone. We compared energy allocation during concurrent pregnancy and lactation in Sigmodon hispidus and Neotoma floridana, species that differ in the extent of overlap between lactation and pregnancy. For females concurrently pregnant and lactating (pregnant group) and for females only lactating (control group), we measured resting metabolism and changes in body mass of females and litters, food consumption, digestive efficiency, and urinary-energy loss of females, and energy equivalents of pup tissue. For both species, pregnant females had significantly higher resting metabolic rates than control females, but allocated the same amount of energy to production of the suckling litter. No other differences were detected for cotton rats. Pregnant woodrats assimilated more energy and produced smaller second litters than controls. Pregnant woodrats also delayed implantation of the litter in utero.