Not all sex ratios are equal: the Fisher condition, parental care and sexual selection

Abstract

The term ‘sex roles’ encapsulates male–female differences in mate searching, competitive traits that increase mating/fertilization opportunities, choosiness about mates and parental care. Theoretical models suggest that biased sex ratios drive the evolution of sex roles. To model sex role evolution, it is essential to note that in most sexually reproducing species (haplodiploid insects are an exception), each offspring has one father and one mother. Consequently, the total number of offspring produced by each sex is identical, so the mean number of offspring produced by individuals of each sex depends on the sex ratio (Fisher condition). Similarly, the total number of heterosexual matings is identical for each sex. On average, neither sex can mate nor breed more often when the sex ratio is even. But equally common in which sex ratio? The Fisher condition only applies to some reproductive measures (e.g. lifetime offspring production or matings) for certain sex ratios (e.g. operational or adult sex ratio; OSR, ASR). Here, we review recent models that clarify whether a biased OSR, ASR or sex ratio at maturation (MSR) have a causal or correlational relationship with the evolution of sex differences in parental care and competitive traits—two key components of sex roles. We suggest that it is more fruitful to understand the combined effect of the MSR and mortality rates while caring and competing than that of the ASR itself. In short, we argue that the ASR does not have a causal role in the evolution of parental care. We point out, however, that the ASR can be a cue for adaptive phenotypic plasticity in how each sex invests in parental care. This article is part of the themed issue ‘Adult sex ratios and reproductive decisions: a critical re-examination of sex differences in human and animal societies’.

Reproductive competition and sexual selection

“…A fundamental limitation of the approaches that I have outlined so far is that they take as their starting point a situation where the two sexes differ in the size of their gametes and in the extent of parental care and explore how these differences are likely to affect reproductive rates, reproductive competition and the intensity of sexual selection. However, the evolution of parental care and of reproductive competition are often likely to interact in the course of evolution and there are fundamental questions that need to be asked about these processes [29,[63][64][65][66]. In particular, how does the evolution of reproductive competition affect the evolution of parental care, as well as vice versa?…”

“…It has also underlined the need for models to comply with Fisher's argument that, in diploid species, the total number of offspring produced by each sex must be identical and the mean reproductive output of individuals consequently depends on the sex ratio [67,68]. Finally, it has demonstrated the potential effects of variation in the sex ratio at different stages of the lifespan and the need to think carefully about the categories of individuals that should be included in models of the evolution of interactions between parental care and reproductive competition [65].…”

Adult sex ratios and reproductive strategies: a critical re-examination of sex differences in human and animal societies

“…Kappeler's [6] overview of various non-human primate societies underpins the significance of group structure by arguing that the social unit is the most meaningful level for studying the behavioural consequences of ASR variation. Using examples from different primates, he also highlights the different temporal dynamics associated with the operational sex ratio [52] versus the ASR.…”

Females of a blood‐sucking bug may adjust their mating decisions according to the risk of ovipositing infertile eggs

“…By being choosy, females ensure the energy input in the egg and the raising of the progeny (Kokko & Jennions, 2008;Lehtonen, Parker, & Schärer, 2016). High-quality males may provide resources or genetic benefits (Vahed, 1998) and usually have distinctive traits which differently attract females (Jennions & Fromhage, 2017;Whitney & Krebs, 1975).…”