On the evolution of visual female sexual signalling.

A long-standing evolutionary puzzle surrounds female sexual signals visible around the time of ovulation. Even among just primates, why do some species have substantial sexual swellings and/or bright colorations visible around females' genital regions, while other species are like humans, with no signs of ovulation visible? What is the evolutionary purpose behind not just these signs, but also this great variation seen among species? Here, we examine the evolutionary trade-offs associated with visual ovulation signalling using agent-based modelling. Our model predicts how various factors, including male genetic heterogeneity and reproductive inequality, female physiological costs, group size, and the weighting of genetic versus non-genetic benefits coming from males, each influence the strength of ovulation signalling. Our model also predicts that increasing the impacts of infanticide will increase ovulation signalling. We use comparative primate data to show that, as predicted by our model, larger group size and higher risk of infanticide each correlate with having stronger visual ovulation signs. Overall, our work resolves some old controversies and sheds new light on the evolution of visual female sexual signalling.