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.

配偶竞争是塑造生物性状演化的强大驱动力。例如，更优异的认知能力可能在雄性间竞争中占据优势，因此会在同性选择作用下被保留。反之，为提升配偶竞争表现而对躯体性状进行投入，可能会压缩可用于脑部发育的资源，进而导致雄性配偶竞争越激烈，其脑容量往往越小。截至目前，相关关联仅存在间接证据，且绝大多数研究均偏向于灵长类与其他恒温脊椎动物，存在明显的类群偏差。本研究针对30种中国无尾目物种，检验了雄性相对脑容量（相对于躯体尺寸）与雄性间竞争强度之间的关联。雄性配偶竞争强度的三项衡量指标——操作性别比（operational sex ratio, OSR）、产卵场密度以及雄性前肢肌肉质量——均与相对脑容量呈正相关，而产卵集群的绝对规模则无此关联。相较于雌性脑容量，雄性脑容量与操作性别比、雄性前肢肌肉质量的关联强度更强。综合来看，本研究结果表明，更大脑容量所带来的认知能力提升，在雄性间竞争中具有适应性优势。本研究为“性选择在脊椎动物脑演化中发挥关键作用”这一论点补充了更广泛的类群证据，进一步充实了相关研究成果。