Relative brain size is predicted by the intensity of intrasexual competition in frogs

Competition over mates is a powerful force shaping trait evolution. For
instance, better cognitive abilities may be beneficial in male−male
competition and thus be selected for by intrasexual selection.
Alternatively, investment in physical attributes favoring male performance
in competition for mates may lower the resources available for brain
development, and more intense male mate competition would coincide with
smaller brains. To date, only indirect evidence for such relationships
exists and most studies are heavily biased towards primates and other
homoeothermic vertebrates. We tested the association between male brain
size (relative to body size) and male−male competition across N=30 species
of Chinese anurans. Three indicators of the intensity of male mate
competition—operational sex ratio (OSR), spawning-site density and male
forelimb muscle mass—were positively associated with relative brain size,
whereas the absolute spawning-group size was not. The relationship with
the OSR and male forelimb muscle mass was stronger for the male than the
female brains. Taken together, our findings suggest that the increased
cognitive abilities of larger brains are beneficial in male−male
competition. This study adds taxonomic breadth to the mounting evidence
for a prominent role of sexual selection in vertebrate brain evolution.