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.