Intrasexual competition in females: evidence for sexual selection?

TLDR

Debate remains over whether female–female competition traits are sexually selected, and female intrasexual selection also involves resource competition for mate access. This review examines female‑female aggression to elucidate mechanisms of intrasexual competition and calls for future research on mutual competition and mate‑quality effects in both sexes. The authors define sexual selection broadly, use female aggression as a model, and test four predictions across taxa based on resource abundance, male availability, and benefits to females. Females prioritize high‑quality mates over mate number, and focusing only on quantity underestimates sexual selection’s influence on female phenotype.

Abstract

In spite of recent interest in sexual selection in females, debate exists over whether traits that influence female–female competition are sexually selected. This review uses female–female aggressive behavior as a model behavioral trait for understanding the evolutionary mechanisms promoting intrasexual competition, focusing especially on sexual selection. I employ a broad definition of sexual selection, whereby traits that influence competition for mates are sexually selected, whereas those that directly influence fecundity or offspring survival are naturally selected. Drawing examples from across animal taxa, including humans, I examine 4 predictions about female intrasexual competition based on the abundance of resources, the availability of males, and the direct or indirect benefits those males provide. These patterns reveal a key sex difference in sexual selection: Although females may compete for the number of mates, they appear to compete more so for access to high-quality mates that provide direct and indirect (genetic) benefits. As is the case in males, intrasexual selection in females also includes competition for essential resources required for access to mates. If mate quality affects the magnitude of mating success, then restricting sexual selection to competition for quantity of mates may ignore important components of fitness in females and underestimate the role of sexual selection in shaping female phenotype. In the future, understanding sex differences in sexual selection will require further exploration of the extent of mutual intrasexual competition and the incorporation of quality of mating success into the study of sexual selection in both sexes.