Ejaculate Cost and Male Choice

TLDR

Although promiscuous species are thought to produce virtually unlimited sperm, in reality males are constrained by the limited number of ejaculates they can deliver, and they discriminate among females when allocating these resources. This study investigates how the limited capacity for ejaculates influences male choice and allocation of sperm to females. The authors model ejaculate allocation as constrained by pregnancy initiation, sperm competition, female choice, search costs, and argue that optimal strategies depend on the operational sex ratio.

Abstract

According to much evolutionary thinking, males of promiscuous species are able to produce quantities of sperm that are virtually unlimited and should be selected to mate indiscriminately with all available females. However, sperm are generally delivered in batches (ejaculates or spermatophores) that may include many millions of gametes. Males are limited with respect to the number of ejaculates they can deliver and the time required to restore depleted reserves. Because ejaculates are a limited resource, prudence would be expected in the allocation of ejaculates to females. Among the forces acting to limit the number of females with which a promiscuous male mates are high pregnancy initiation requirements, sperm competition, female choice and control, and the costs and risks of searching. Optimal strategies for ejaculate allocation will vary with the operational sex ratio. Males can and do discriminate among females as potential mating partners. The problems of limited ejaculatory capacity and male choice merit greater attention in both theory and in empirical research.