Costs of Lactation and Optimal Litter Size in Northern Grasshopper Mice (Onychomys leucogaster)

Abstract

A mother's lifetime reproductive success depends on the number and quality of young produced in each reproductive event and the number of such events over her lifetime. However, excessive cost of a larger litter may reduce a mother's future reproductive potential or result in smaller-than-normal young at weaning. Because lactation is the most energetically demanding segment of reproduction in mammals, assessment of its cost provides an accurate gauge of short-term reproductive investment. If high proximate reproductive costs are correlated with decreases in lifetime reproductive potential or fitness, understanding the tradeoffs between costs and benefits of lactation for different-sized litters should lead to a better understanding of mammalian reproductive patterns. In the present study, body mass of young and food intake as measures of maternal investment were measured daily in 43 litters of captive northern grasshopper mice (Onychomys leucogaster) to day 16. Litter sizes ranged from one to six with a mean of 3.4 young. Food intake for a litter of one was 155% of mean nonreproductive requirements, whereas cost for a litter of six was 293%. Relative to litter size, both total cost of lactation and total litter mass increased by day 16, whereas mean size of offspring decreased. The relationship between cost per offspring and offspring size relative to litter size showed that a litter of four is most efficient in terms of benefit per unit cost. If the best maternal strategy is to maximize both the size and number of offspring weaned while simultaneously minimizing costs of reproduction, the optimum litter size is the one that yields the largest offspring per unit cost. In grasshopper mice, this optimum is four.