Demographic causes of adult sex ratio variation and their consequences for parental cooperation

Abstract

The adult sex ratio (ASR) is a fundamental concept in population biology, sexual selection, and social evolution. However, it remains unclear which demographic processes generate ASR variation and how biases in ASR in turn affect social behaviour. Here, we evaluate the demographic mechanisms shaping ASR and their potential consequences for parental cooperation using detailed survival, fecundity, and behavioural data on 6119 individuals from six wild shorebird populations exhibiting flexible parental strategies. We show that these closely related populations express strikingly different ASRs, despite having similar ecologies and life histories, and that ASR variation is largely driven by sex differences in the apparent survival of juveniles. Furthermore, families in populations with biased ASRs were predominantly tended by a single parent, suggesting that parental cooperation breaks down with unbalanced sex ratios. Taken together, our results indicate that sex biases emerging during early life have profound consequences for social behaviour.

Successful breeding predicts divorce in plovers

“…For instance, Kentish plover Charadrius alexandrinus populations in Europe and China commonly display serial polygamy with mostly females divorcing their mate soon after the chicks hatched 55 – 57 ; whereas the island population of Kentish plovers in Cape Verde is exclusively monogamous 58 . The social mating system of all other plover species included in our study is monogamy except for the snowy plover which exhibit serial polygamy 59 , 60 . Variation in mate fidelity between closely related species and populations is also common in primates, ungulates and fishes 61 – 63 .…”

“…Further research is needed to clarify whether the different responses of males and females to environmental conditions are directly influenced by abiotic factors 65 – 67 , or indirectly influenced by social environment (e.g. ASR) 60 .…”

Fluffy feathers: how neoptile feathers contribute to camouflage in precocial chicks

“…Yet our results regarding the spatial acuity suggest that the fringed feathers could be an important component of a visual antipredator strategy against mammalian predators. Even if the reduction in detectability is only small, concealing the outline may enhance survival of precocial chicks during early life when chicks face a very high predation risk (Colwell et al 2007, Brudney et al 2013, Eberhart-Phillips et al 2018, especially as the costs for having the protruding feathers may not be high.…”

“…Precocial chicks leave their nest within a few hours of hatching. Initially, those chicks suffer from high mortality as they are limited in their mobility and hence highly vulnerable to predation (Colwell et al 2007, Brudney et al 2013, Eberhart-Phillips et al 2018. To improve their survival, chicks rely on camouflage provided by their feathers especially during the first days of their lives.…”

Sex roles and the evolution of parental care specialization

“…Major transitions in sex roles may consequently be precipitated by changes in the mortality risk due to such external factors. Interestingly, Eberhart-Phillips et al [65] found that variation in the adult sex ratio in plovers was largely driven by sex differences in juvenile survival, and that parental cooperation was most common in populations with more equal sex ratios. In a more extreme example, Jiggins et al [66] observed female lekking behaviour in populations of Acraea butterflies in which the male-killing parasite Wolbachia had led to heavily femalebiased sex ratios; we note, however, that there is no male parental care in this species.…”