Energetics of Litter Size in Five Species of Peromyscus with Generalizations for Other Mammals

The energetic cost and efficiency of lactation in five species of Peromyscus (P. maniculatus, P. leucopus, P. polionotus, P. eremicus, and P. floridanus) were investigated in the laboratory. Among these species, litter size was significantly correlated with the mass-specific rate of energy expenditure for lactation but not with the mass-specific amount of energy allocated to each offspring, the efficiency of converting food into offspring, the extent of body reserve utilization, nor weaning time. Reproductive efficiency was correlated with the rate of energy gain of the young relative to their energy content at birth. These results are contrasted with intraspecific comparisons and do not appear to be merely secondary effects of body size variation among the species. Similar energetic patterns are seen for interspecific comparisons of various cricetid rodents and mammals in general. However, the available data for cricetid rodents do not show a correlation between reproductive efficiency and the relative energetic growth rate of the young. The general mammalian comparison also suggests that there is an upper limit for the energetic cost of lactation at large litter sizes (>5), and that the postnatal energetic cost of individual offspring (as reflected in their weaning weight and preweaning growth rate) is inversely correlated with litter size. The ecological implications of the present findings are discussed in relation to environmental resource levels, latitude, and r- and K-selection. Mammal species with large litter sizes are more likely to have more food available or to take greater risks at obtaining additional food for lactation than species with small litters.