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Review
. 2017 Sep 19;372(1729):20160322.
doi: 10.1098/rstb.2016.0322.

Adult Sex Ratios and Their Implications for Cooperative Breeding in Birds

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Free PMC article
Review

Adult Sex Ratios and Their Implications for Cooperative Breeding in Birds

Jan Komdeur et al. Philos Trans R Soc Lond B Biol Sci. .
Free PMC article

Abstract

Cooperative breeding is a form of breeding system where in addition to a core breeding pair, one or more usually non-breeding individuals provide offspring care. Cooperative breeding is widespread in birds, but its origin and maintenance in contemporary populations are debated. Although deviations in adult sex ratio (ASR, the proportion of males in the adult population) have been hypothesized to influence the occurrence of cooperative breeding because of the resulting surplus of one sex and limited availability of breeding partners, this hypothesis has not been tested across a wide range of taxa. By using data from 188 bird species and phylogenetically controlled analyses, we show that cooperatively breeding species have more male-biased ASRs than non-cooperative species. Importantly, ASR predicts helper sex ratio: in species with more male-biased ASR, helper sex ratio is also more male biased. We also show that offspring sex ratios do not predict ASRs, so that the skewed ASRs emerge during the period when individuals aim to obtain a breeding position or later during adulthood. In line with this result, we found that ASR (among both cooperatively and non-cooperatively breeding species) is inversely related to sex bias in dispersal distance, suggesting that the cost of dispersal is more severe for the further-dispersing sex. As females usually disperse further in birds, this explains the generally male-biased ASR, and in combination with benefits of philopatry for males, this probably explains why ASR is more biased in cooperatively breeding species. Taken together, our results suggest that a sex bias in helping in cooperatively breeding species relates to biased ASRs. We propose that this relationship is driven by sex-specific costs and benefits of dispersal and helping, as well as other demographic factors. Future phylogenetic comparative and experimental work is needed to establish how this relationship emerges.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'.

Keywords: adult sex ratio; cooperative breeding; helper sex ratio; offspring sex ratio; sex-biased dispersal.

Conflict of interest statement

We declare we have no competing interests.

Figures

Figure 1.
Figure 1.
Adult sex ratio (proportion of males in the population) is more male-biased in bird species (a) with female-biased dispersal and (b) in cooperatively breeding bird species. Mean (± s.e.) adult sex ratio is given in relation to (a) sex bias in dispersal in birds (cooperatively and non-cooperatively breeding species combined), and (b) whether species breed cooperatively or not. Numbers indicate the number of species. Tests statistics are provided in table 1.
Figure 2.
Figure 2.
Adult sex ratio (proportion of males in the population) in cooperatively breeding bird species predicts helper sex ratio (proportion helpers that is male). Dots show species values. Mean (±s.e.) adult sex ratio and helper sex ratio are shown using a bi-directional error bar in grey. Dashed lines depict 95% confidence intervals. The grey line provides the 1 : 1 relationship between ASR and helper sex ratio. Tests statistics are provided in the main text.

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