Sex roles and sex ratios in animals

Abstract

In species with separate sexes, females and males often differ in their morphology, physiology and behaviour. Such sexspecific traits are functionally linked to variation in reproductive competition, mate choice and parental care, which have all been linked to sex roles. At the 150th anniversary of Darwin's theory on sexual selection, the question of why patterns of sex roles vary within and across species remains a key topic in behavioural and evolutionary ecology. New theoretical, experimental and comparative evidence suggests that variation in the adult sex ratio (ASR) is a key driver of variation in sex roles. Here, we first define and discuss the historical emergence of the sex role concept, including recent criticisms and rebuttals. Second, we review the various sex ratios with a focus on ASR, and explore its theoretical links to sex roles. Third, we explore the causes, and especially the consequences, of biased ASRs, focusing on the results of correlational and experimental studies of the effect of ASR variation on mate choice, sexual conflict, parental care and mating systems, social behaviour, hormone physiology and fitness. We present evidence that animals in diverse societies are sensitive to variation in local ASR, even on short timescales, and propose explanations for conflicting results. We conclude with an overview of open questions in this field integrating demography, life history and behaviour.

Pathogen transmission modes determine contact network structure, altering other pathogen characteristics

“…While we control for different social systems in our model, there is also potential for further network variation within these social systems and within species. For example, mating system dynamics and patterns of sexual promiscuity can vary widely within groups and populations of the same species, as a function of ecological variation and population sex ratios [ 43 , 65 , 66 ]. Moreover, two ungulate species Grevy's zebra ( Equus grevyi ) and onagers ( Equus hemionus khur ), which both have gregarious social systems, have been shown to have significantly different network structures, likely due to species traits that have evolved from inhabiting different environments [ 67 ].…”

Early‐life food stress hits females harder than males in insects: A meta‐analysis of sex differences in environmental sensitivity

“…Accounting for sex‐biased mortality may significantly contribute to understanding the dynamics and demography of populations (Clutton‐Brock & Sheldon, 2010; Eberhart‐Phillips et al, 2017; Kappeler et al, 2023), not least importantly in the context of ongoing insect declines (Wagner et al, 2021) and pest control (Chi et al, 2020). Our results suggest that nutritional conditions leading to elevated juvenile mortality may have the potential to affect the performance of insect populations further by reducing the proportion of females among individuals reaching reproductive age.…”

Beyond classical theories: An integrative mathematical model of mating dynamics and parental care

“…Each of these cases shapes the ASR with a different meaning and makes its dynamical use even more relative to the ecological context. The ASR is currently used as a proxy of the intensity of sexual selection in many empirical studies (Kappeler et al, 2023). However, sex ratios are indirect measures of potential causal factors, such as sexual selection, that may influence care evolution.…”