American Society of Naturalists

A membership society whose goal is to advance and to diffuse knowledge of organic evolution and other broad biological principles so as to enhance the conceptual unification of the biological sciences.

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

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Chun Lan Mai (麦春兰), Wen Bo Liao (廖文波), Stefan Lüpold, and Alexander Kotrschal (Aug 2020)

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Of brawny and brainy males in frogs

A treefrog (<i>Rhacophorus omeimontis</i>).<br />(Credit: Yiqiang Fu)
A treefrog (Rhacophorus omeimontis).
(Credit: Yiqiang Fu)

It is widely known that it takes brains for males to learn complex songs or behaviors that they use to charm females into mating. But what about species in which males fight over access to mates rather than attracting them with elaborate courtship or ornaments? Some studies suggest that even fighting males might benefit from better cognitive abilities, because they may be better able to predict when defeat becomes inevitable and it is time to back off to avoid being harmed. But others indicate that investing in physical attributes like weapons or muscles, to increase success in fights, might come at the cost of brain development because of energetic constraints. So, male mate competition could drive the evolution of both larger and smaller brains. But which is more likely? Our study tested these predictions against each other in a sample of 30 different Chinese species of frogs and toads, using several predictors of mate-competition intensity: the ratio between breeding males and females, spawning-site density and group size, and the weight of male arm muscles. More males per spawning site, and especially more males per available female, enhance the aggression and likelihood of wrestling matches between them. And stronger arms increase the success in such contests. Accounting for variation in body size and relatedness between species, we found males of species with a greater surplus of males, denser populations and stronger arms—that is, with more intense male competition—to have larger brains than those species with weaker competition. Our clearest evidence for a link between mate competition and brain size, however, was that male brains showed a stronger response than female brains to our most important indicators of mate competition: sex ratio and arm muscles. Our study thus provides strong evidence that not just wooing females but also fighting for them can drive the evolution of brainier males.


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.