Capital or income breeding? A theoretical model of female reproductive strategies

Energy storage is an important component of life-history variation. A distinction is recognized between species that provision offspring using energy gained concurrently (income breeders) and those that provision offspring using energy stores accumulated at an earlier time (capital breeders). Although this distinction has been recognized for some time, surprisingly little attention has been paid to the general adaptive value of the 2 strategies. Here, we present a simple, general framework for modeling female reproductive strategies. We show that our framework can be applicable either to annual breeders that aim to maximize the energy delivered to their offspring before independence, or to species with shorter reproductive cycles that aim to maximize reproductive rate, given that their offspring must build up a given level of reserves before independence. For both scenarios, we show that the costs of accumulating capital can lead to pure income breeding, pure capital breeding, or a mixture of the 2 strategies. Our model allows the effects of a variety of parameters to be assessed. Length of gestation, offspring metabolism, efficiency of energy transfer from mother to offspring, and the relative rates of energy gain by females with and without offspring are all important factors. The cost associated with accumulated capital is a particularly critical determinant of the strategy adopted. More detailed approaches to specific systems may provide a greater understanding of the factors promoting different maternal strategies for offspring provisioning.