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.

Figure 1.

Mating biases originate by two different mechanisms. Female mating preferences select on male display traits through intersexual selection (top row). Male coercion evolves through intrasexual selection and selects for the evolution of female resistance (bottom row). Either intersexual mate preferences or intrasexual competition through male sexual coercion can result in mating biases (left arrows). Neither female preference and resistance nor male display and coercion are synonymous.

Figure 2.

Genital coevolution in waterfowl. In Pekin ducks (Anas domesticus), in which FEPCs are common, males have a long corkscrew-shaped phallus and females have a convoluted vagina (a), in contrast to the short phallus and simple vagina (b) of harlequin ducks (Hystrionicus hystrionicus), in which FEPCs are absent. More examples of this coevolution are reported in Brennan et al. [26]. Scale bar, 2 cm.