Considerations of optimal litter size in mammals

Recent advances in understanding mammalian reproductive patterns mirror gen eral developments in phylogenetic, physiological, and ecological research. We provide an overview of the diverse approaches used in studying mammalian litter size and discuss recent advances in evolutionary ecological theory and experimental data concerning reproductive patterns, Extensively documen ted natural history studies and breeding data from work in applied zoology and animal husbandry constitute a rich source of information for comparativ e studies. Such studies have helped identify fundamental differences in rep roductive "strategies" depending on whether taxa produce altricial or preco cial young and whether a given reproductive effort results in a single offs pring or a litter. On the other hand, research on ecological and physiologi cal correlates of litter size is providing new insights about reproductive energetics, the relationship between resource availability and litter size, and optimality in varying environments in general. Continuing experimental approaches are broadening our understanding of the mechanistic bases of pa tterns of mammalian litter size, but such studies, especially those involvi ng manipulation of litter size. must carefully consider the potential biase s introduced by experimentally induced non-congruence between pre- and post natal litter sizes. For example, without accounting for the effects of plac ental hormones on mammogenesis and lactation, postnatal manipulations of li tter size might be based on faulty assumptions implicit in such manipulatio ns. To conclude, despite substantial advances in thc se diverse approaches. our degree of understanding of optimal litter size in mammals remains uncl ear because of inconsistencies in data often used in comparative studies an d because of physiological and ecological constraints on litter size adjust ments that hinder or bias experimental approaches. These areas should be ad dressed in future: studies.