Observations of reproductive behaviors in sexually reproducing organisms indicate that many species can be “choosy”: they tend to be selective for their partners quality. Mate choice has costs and potential benefits that are likely to vary depending on individual characteristics (e.g. sex, quality), and on social context (number of potential partners). And if you are too picky, that cost may have dire consequences: you will end up alone.
Classically, scientific literature predicts that the limiting sex (in term of gametes) – females – should be choosy, whereas the common sex – males – less so or not at all, or in very peculiar situations. Indeed, as a result of anisogamy (unbalance between gametes number and/or size between sexes), female’s reproductive rate is lower than males, making ready to mate males more numerous than ready to mate females and thus generating stronger mating competition among males. But who is really ready to mate, with which partner with regard to quality, and for how long? This is what can be described as the mating market, and it is everything but stable. Who can afford to be choosy in these conditions: males, females , or both? Individuals of high and low quality alike?
Louise Chevalier and her colleagues investigated this question using a dynamic game theory model: they assumed that all these individual choices affect the dynamics of pairings constantly, and allowed all individuals, whatever their quality or sex, to permanently readjust their choosiness, based on the balance between costs and benefits.
Their conclusions is that in fact, somewhat contrarily to what is known as the conventional sex roles wherein males compete to access the choosy females, choosiness should often evolve in both sexes, even when females are more rare than males. The results also imply that choosiness should adapt to the mating market, by being flexible over time, and can differ between individuals of different quality.
For instance, the figure above shows that choosiness differs between sexes, but almost every individuals here can be at least a bit choosy, even when their quality is poor: mutual mate choice in this example has evolved. We can also see that the choosiness is changing along time so to adapt to the dynamics of mating market. And if we look close enough, we might notice that choosiness does not increase linearly with quality: the population is in fact made of some sorts of subgroups, within which individuals have comparable fitnesses. This is an emerging property of the mating market: you might be in competition with everyone, but to various degrees. In fact, depending on the characteristics of the mating systems (latency period before returning to the mating pool, adult sex ratio), a wide range of choosiness evolution pattern is possible: you can explore these further using a Shiny Application here.
All these results and analyses can be found in The American Naturalist, and the model code is available here.