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

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 (the proportion of males
in the adult population, ASR) 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-biased 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 towards 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.