When does it pay to be sexy? And why would you seasonally change the ratio of sons to daughters you produce?

Abstract

This is a talk in two parts. First, I describe a recent theoretical model explaining the forces the influence investment into sexual selected traits (e.g. sexual ornamentation). The model provides a framework to understand the effect that the intensity of competition for mates has on whether it is beneficial for an individual to invest into being more competitive. The answer turns out to be far subtler than that originally outlined by Emlen & Oring (1977) (a classic biology paper), and subsequently promulgated in most evolutionary textbooks. I show that the so-called Bateman gradient (which describes how a higher mating rate affects an individual’s reproductive success), the level of mating competition (the operational sex ratio, OSR, which is the ratio of sexually active males to females) and the naturally selected costs of competitive traits must all be taken into account. In combination they affect the scope for investment into traits that make their bearer more sexually competitive. As an aside, the model illuminates a recent debate about problems with variance-based measures of sexual selection. I briefly mention this technical topic because these indices are very popular in the human evolutionary literature. Second, I shift focus and describe a study about sex allocation. Specifically, how many sons to daughters should parents produce? I make the simple point that evolutionary theory is often best tested using clever ‘natural experiments’. As a biologist I recommend choosing appropriate study species to test theory - unfortunately, humans are often not the best option. I illustrate this approach with the example of seasonal changes in sex allocation in a live-bearing fish. Good news: fish also care about economics.