FREQUENCY OF FEMALE COPULATIONS WITH MULTIPLE MALES AND SEXUAL SELECTION
Female animals often copulate with more than a single male during one fertile period. I suggest that female propensity to engage in such multimale copulations is caused by constraints on female mate choice set by the availability of males. A simple graphic model of mate choice after formation of pair bonds suggests that the probability of female engagement in copulations with multiple males should be inversely proportional to the availability of preferred mates at the time of mate choice. This problem will be present whenever males and females establish pair bonds, and the male availability constraint on female mate choice will be more important for mating systems in which males attract only a single or few females. Monogamy thus particularly often sets the stage for sexual selection via multiple matings. Male availability to females during mate choice differs between mating systems, and the relative frequency of female copulations with multiple males should be inversely related to male availability. If females attempt to adjust their precopulatory mate choice by copulating with additional males, sexual selection resulting from choice of copulation partners should reinforce precopulatory sexual selection. Patterns of male and female extrapair copulations and patterns of extrapair paternity in birds are in accordance with this scenario. Sexual selection arises from differential mating success of the individuals of one sex. Two mechanisms may lead to differential success: male male competition for mates (intrasexual selection) and female choice of mates (intersexual selection; Darwin 1871) Intrasexual competition may arise because of an excess of members of the competing sex (usually males) relative to the abundance of preferred resources used to attract mates, or because of an excess of members of the competing sex relative to the abundance of desirable mates. Intersexual selection may arise because members of the choosing sex (usually females) exhibit a consistent pattern of choice with an effect on the average phenotype of successful members of the competing sex. The mechanisms of mate choice will be influenced by factors like time and energy constraints and possibilities for making comparisons between more than one individual male. One particular kind of constraint, which has rarely been considered, is the availability of mates as influenced by the previous choice of already mated individuals. Females may often be constrained in their choice because the most highly preferred individuals may already have become monopolized by previously mated females. This constraint will be important for all individuals of the choosy sex, except for the first individual to make a choice, and the availability constraint therefore may have important implications for female Am. Nat. 1992. Vol. 139, pp. 1089 1101. 1992 by The University of Chicago. 0003 0147/92/3905 0010$02.00. All rights reserved. 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