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 toward 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 for 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.

中文翻译：

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大脑的相对大小是由青蛙的内在竞争强度预测的。

对伴侣的竞争是塑造特质演变的强大力量。例如，更好的认知能力在男女竞争中可能是有益的，因此可以通过性别选择来选择。或者，对有利于男性竞争表现的身体属性的投资可能会降低可用于大脑发育的资源，而更激烈的男性竞争则会与较小的大脑相吻合。迄今为止，仅存在这种关系的间接证据，并且大多数研究严重偏向于灵长类和其他同温脊椎动物。我们测试了男性大脑大小（相对于身体大小）与跨性别竞争之间的关联$ñ=30$中华无核种。雄性配偶竞争强度的三个指标-作业性别比（OSR），产卵部位密度和雄性前肢肌肉质量-与相对大脑大小呈正相关，而绝对产卵组大小则不相关。男性与女性大脑的OSR和男性前肢肌肉质量的关系更强。综上所述，我们的发现表明，较大大脑的增强的认知能力在男女竞争中是有益的。这项研究为分类选择在脊椎动物大脑进化中的重要作用中增加了分类学上的广度。