We examined data on captive multi-mammate mice (Mastomys coucha) to assess differential maternal investment, and sex-specific resource allocation. Differences in maternal size were induced through manipulation of dietary protein in three treatment groups: low (10%), medium (15%) and high (20%) protein diets. Mothers on the 20% protein diet were significantly larger than those on the lower protein diets, and produced significantly more male than female offspring. Mothers on the lower protein diets did not produce sex-biased litters. There were no sex-specific differences in body size or body tissue composition of pups at birth or at weaning within each treatment group. At weaning, pups in the 20% protein treatment group had proportionately greater amounts of lean tissue and less body lipid reserves than pups in the 10% protein treatment group. Pups in the 20% protein treatment group were also larger, and had faster growth rates, than those in the 10% protein treatment group. Weaned pups in the 15% protein treatment group had the fastest growth rates and greatest energetic reserves of all of the treatment groups. Our results suggest that larger mothers on the high (20%) protein diet show differential investment in the sexes, not by allocating more resources to individuals of that sex, but by producing more male than female offspring.

Growth in body mass in pups of the subantarctic fur seal Arctocephalus tropicalis was studied at Amsterdam Island, southern Indian Ocean during the rearing period between December 1995 and August 1996. We examined the change in the rates of mass gain and mass loss with pup age to explain the overall change in mass throughout the rearing period, when mothers alternate foraging trips at sea and attendance periods ashore. In the absence of their mothers, pups fast and lose mass. During the maternal attendance period ashore pups suck and gain mass. There was no significant difference in the rate of growth in mass between male and female pups. The rate of loss in mass during a fasting event was positively related to the initial body mass but negatively related to the age and duration of the fast of the pup. The rate of mass loss was best related to pup age by a quadratic relationship. The rate of mass loss increased until pups were 120 days old and then decreased until the end of the study period. The rate of gain in mass following a maternal foraging trip was positively related to the initial pup mass but negatively related to the foraging trip duration, and we found that the rate of mass gain decreased linearly with the age of the pup. The concomitant decrease in the rate of mass loss and mass gain for pups > 120 days old allowed pups to maintain a positive growth until they were 220 days old. After 220 days of age the rate of mass loss exceeded the rate of mass gain and the pups start to lose mass until weaning.