The limits of sexual conflict in the narrow sense: new insights from waterfowl biology

Abstract

Sexual conflict occurs when the evolutionary interests of the sexes differ and it broadly applies to decisions over mating, fertilization and parental investment. Recently, a narrower view of sexual conflict has emerged in which direct selection on females to avoid male-imposed costs during mating is considered the distinguishing feature of conflict, while indirect selection is considered negligible. In this view, intersexual selection via sensory bias is seen as the most relevant mechanism by which male traits that harm females evolve, with antagonistic coevolution between female preferences and male manipulation following. Under this narrower framework, female preference and resistance have been synonymized because both result in a mating bias, and similarly male display and coercion are not distinguished. Our recent work on genital evolution in waterfowl has highlighted problems with this approach. In waterfowl, preference and resistance are distinct components of female phenotype, and display and coercion are independent male strategies. Female preference for male displays result in mate choice, while forced copulations by unpreferred males result in resistance to prevent these males from achieving matings and fertilizations. Genital elaborations in female waterfowl appear to function in reinforcing female preference to maintain the indirect benefits of choice rather than to reduce the direct costs of coercive mating. We propose a return to a broader view of conflict where indirect selection and intrasexual selection are considered important in the evolution of conflict.

Mechanisms and Evidence of Genital Coevolution: The Roles of Natural Selection, Mate Choice, and Sexual Conflict

“…However, sexually antagonistic coevolution (SAC) that occurs as a consequence of sexual conflict between males and females can result in coevolutionary processes that reduce fit, or that function to decrease ease of copulation, or to mitigate male damage to the female. SAC occurs when genital adaptations coevolve in each sex to advance control over mating and fertilization initiating a coevolutionary arms race (Chapman et al 2003;Parker 2006;Brennan and Prum 2012). Both direct and indirect selection are important in shaping evolutionary responses to sexual conflict including conflict over mating, fertilization, and mate identity (Arnqvist and Rowe 2005;Parker 2006;Brennan and Prum 2012).…”

Sexual conflict over mating in red-sided garter snakes (Thamnophis sirtalis) as indicated by experimental manipulation of genitalia

“…Traits involved in sexual conflict may have originally evolved as a result of male-male competition [60]. Male - rspb.royalsocietypublishing.org Proc.…”

“…Furthermore, the epithelial tissue of the female cloaca in which the basal spine embeds is thickly stratified and keratinized [64], which may be a female counter-adaptation to minimize damage owing to sexual conflict as has evolved in bedbugs with traumatic insemination [65]. Even if males with bigger, more damaging spines achieved longer copulations, this would not represent evidence of a process of female choice, but rather an expected outcome of conflict interactions [60]. We cannot, however, rule out that males may expect resistance and interpreted the anaesthetized females as weakened and unfit leading them to reduce allocation of their ejaculate accordingly, if males mate multiply during the breeding season [66].…”

Sexual and social competition: broadening perspectives by defining female roles

“…Nonetheless, most researchers tend to agree that female choice is important in most species [3], but what constitutes choice is not always clear because the interests of males and females are often not aligned [48,49]. For example, in species where sexual conflict is intense and males forcibly copulate with mates, as Brennan & Prum [50] argue and Kazancioglu & Alonzo [51] model in this issue, female choice of male advertisement and female resistance to male coercion may not actually be the same phenomenon, even though they are often treated as such [52,53]. Choice itself is a complex process, and modern tools may allow us to dissect the decision-making progress-as well as the competitive process-at the mechanistic level.…”