American Society of Naturalists

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“Fluctuating dynamics of mate availability promote the evolution of flexible choosiness in both sexes”

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Louise Chevalier, Jacques Labonne, Matthias Galipaud, and François-Xavier Dechaume-Moncharmont (Dec 2020)

Fluctuations in the mating market trigger the evolution of flexible and quality-dependent choosiness in males as in females

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The mating market is in constant motion, as illustrated by the issue of the "closing time effect" during a speed-dating session. <br/>Individuals must constantly adapt their choosiness in order to secure a mating partner. <br />(Credit: Louise Chevalier)
The mating market is in constant motion, as illustrated by the issue of the "closing time effect" during a speed-dating session. Individuals must constantly adapt their choosiness in order to secure a mating partner.
(Credit: Louise Chevalier)

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).

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?

The authors of this new study 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. As a result, they predict that choosiness should indeed evolve in many situations, even in the face of seemingly unfavorable situations: for instance, they show that some of the males could be choosy even when females are rare, and that the best choosiness strategy should be flexible.

In a nutshell, using a single unified approach – a dynamic game theory model – they endeavored to provide a better picture of all the variation in choosiness than can be observed in natural populations, in a wide span of mating systems.


Abstract

The evolution of choosiness has a strong effect on sexual selection, as it promotes variance in mating success among individuals. The context in which choosiness is expressed, and therefore the associated gain and cost, is highly variable. An overlooked mechanism by current models is the rapid fluctuations in the availability and quality of partners, which generates a dynamic mating market, to which each individual must optimally respond. We argue that the rapid fluctuations of the mating market are central to the evolution of optimal choosiness. Using a dynamic game approach, we investigate this hypothesis for various mating systems (characterized by different adult sex ratio and latency period combinations), allowing feedback between the choosiness and partner availability throughout a breeding season, while taking into account the fine variation in individual quality. Our results indicate that, quality dependent and flexible choosiness evolve usually in both sexes for various mating systems, and that a significant amount of variance in choosiness is observed, especially in males, even when courtship is costly. Accounting for the fluctuating dynamics of the mating market, therefore, allows envisioning a much wider range of choosiness variation in natural populations and may explain a number of recent empirical results regarding choosiness in the less common sex or its variance within sexes.