The evolution of enhanced cognitive ability has sometimes been attributed to sexual selection. An association between the mating success of males and their cognitive ability could arise either through male-male competition or through female choice. Specifically in the latter case, sexual selection would act more readily if males advertized their cognitive ability through display. Most traits involved in sexual display, however, seem unlikely to have any inherent relationship with cognition beyond that which arises through the effect of cognitive abilities on acquisition of resources and, in turn, the effect of resources on development of the display trait. In contrast, for displays whose development and expression require learning, a direct link with cognition is possible because of a shared dependence on brain function. The parallel effects of developmental stress on song-learning and cognition provide a compelling explanation for an association between attributes of the song and cognitive ability. We outline the hypothesis that sexually selected qualities of song serve as an indicator of cognitive abilities. We first present evidence that song-learning is itself a challenging cognitive task. We then give evidence that sexual selection favors well-learned song. Next, we review evidence that song and cognitive ability both are affected by developmental stresses. We consider recent experimental data testing the relationship between song and cognitive ability. Finally, we suggest that the accuracy with which songs are learned may be an optimal indicator of other cognitive abilities.
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