Evolution of bower complexity and cerebellum size in bowerbirds.

To entice females to mate, male bowerbirds build elaborate displays (bowers). Among species, bowers range in complexity from simple arenas decorated with leaves to complex twig or grass structures decorated with myriad colored objects. To investigate the neural underpinnings of bower building, we examined the contribution of variation in volume estimates of whole brain (WB), telencephalon minus hippocampus (TH), hippocampus (Hp) and cerebellum (Cb) to explain differences in complexity of bowers among 5 species. Using independent contrasts, we found a significant relationship between bower complexity and Cb size. We did not find support for correlated evolution between bower complexity and WB, TH, or Hp volume. These results suggest that skills supported by the cerebellum (e.g., procedural learning, motor planning) contribute to explaining the variation in bower complexity across species. Given that male mating success is in part determined by female choice for bower design, our data are consistent with the hypothesis that sexual selection has driven enlargement of the cerebellum in bowerbirds.