Evolution of large males is associated with female-skewed adult sex ratios in amniotes

András Liker, Veronika Bókony, Ivett Pipoly, Jean Francois Lemaître, Jean Michel Gaillard, Tamás Székely, Robert P. Freckleton

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Body size often differs between the sexes (leading to sexual size dimorphism, SSD), as a consequence of differential responses by males and females to selection pressures. Adult sex ratio (ASR, the proportion of males in the adult population) should influence SSD because ASR relates to both the number of competitors and available mates, which shape the intensity of mating competition and thereby promotes SSD evolution. However, whether ASR correlates with SSD variation among species has not been yet tested across a broad range of taxa. Using phylogenetic comparative analyses of 462 amniotes (i.e., reptiles, birds, and mammals), we fill this knowledge gap by showing that male bias in SSD increases with increasingly female-skewed ASRs in both mammals and birds. This relationship is not explained by the higher mortality of the larger sex because SSD is not associated with sex differences in either juvenile or adult mortality. Phylogenetic path analysis indicates that higher mortality in one sex leads to skewed ASR, which in turn may generate selection for SSD biased toward the rare sex. Taken together, our findings provide evidence that skewed ASRs in amniote populations can result in the rarer sex evolving large size to capitalize on enhanced mating opportunities.

Original languageEnglish
Pages (from-to)1636-1649
Number of pages14
Issue number7
Early online date22 May 2021
Publication statusPublished - 19 Jul 2021


  • Comparative method
  • mating competition
  • mating opportunity
  • sex-biased mortality
  • sexual selection

ASJC Scopus subject areas

  • Ecology, Evolution, Behavior and Systematics
  • Genetics
  • Agricultural and Biological Sciences(all)


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