Sperm competition promotes the exploitation of rival ejaculates

Abstract

In many polyandrous species, the second male to mate with a female has fertilization priority. Strategic mechanisms generating this pattern tend to involve facultative increases in ejaculate size by informed males. Here we suggest a novel mechanism to generate second male precedence in internal fertilizers, based on the established fact that the female site of insemination is hostile to sperm, and that non-sperm components of the ejaculate frequently act to ameliorate these hostile conditions. We suggest that in species where female remating is frequent and rapid, second male sperm precedence may be due to the prior buffering of the female tract by previous males' ejaculates. In this scenario, second male costs are lower due to reduced sperm mortality and/or the redundancy of seminal fluid. Contrary to many classical predictions, second males may gain a paternity advantage despite investing less in an ejaculate.