Reproductive Effort Tactics: Balancing Pre- and Postbreeding Costs of Reproduction

Costs of reproduction are most frequently evaluated in terms of postbreeding survival and fecundity costs. Such demographic costs are expected to follow as the female drains her somatic resources into reproduction. However, some reproductive tactics may lead to costs of reproduction that are expressed in terms of prebreeding survival. We propose an optimality model where total absolute effort may originate from two different components. The first component measures the amount of resources that the female accumulates for reproduction during the prebreeding period. The second component represents the amount of resources drained from somatic demands relative to non-reproductive individuals. While our model allows both components to imply pre- and postbreeding survival costs, we mainly focus on the case where accumulation effort implies costs on prebreeding survival and somatic implies costs on postbreeding survival. Effective fecundity is assumed to be a function of both components. We consider accumulation and somatic effort as alternative options of an organism's reproductive effort tactic, and solve for optimal tactics by maximizing fitness over a single breeding season, assuming a constant total investment in reproduction. The present analysis suggests that the evolution of accumulation effort requires that marginal prebreeding costs due to accumulated resources remain low relative to marginal postbreeding costs implied by somatic effort. When the total investment in reproduction increases, optimal somatic effort increases relative to optimal accumulation effort. Our analysis demonstrates that natural selection may well favour different effort tactics satisfying the energy demands of reproduction, some of which may involve optimization of the balance between pre- and postbreeding costs of reproduction. For organisms relying on tactics implying prebreeding costs, empirical studies monitoring postbreeding survival only may not reveal the major costs of reproduction.