Sex-specific evolution of brain size and structure and covariation with eye size in Trinidadian killifish

Abstract

Abstract Links between contrasting ecological conditions and evolutionary shifts in neurosensory components, such as brain and eye size, are accumulating. Whether selection operates in a different manner on these traits between sexes is unclear. Trinidadian killifish (Anablepsoides hartii) are found in sites with and without predators. Male killifish from sites without predators have evolved larger brains and eyes than males from sites with predators. These differences in brain size are present early in life but disappear in adult size classes. Here, we evaluated female brain growth allometries to determine whether females exhibit similar size-specific differences in brain size between sites that differ in predation intensity. We also quantified brain size and structure and eye size to determine whether these structures co-evolved in a sex-specific manner. We found that female brain growth allometries did not differ across populations. Yet, female killifish from sites without predators exhibited a larger cerebellum, optic tectum and dorsal medulla early in life (before maturation), although such differences disappeared in larger size classes. Females from sites with predators exhibited similar patterns in brain growth to males in those sites; therefore, shifts in brain size and structure are driven by differences between sexes in sites without predators. We also found evidence for covariation between brain and eye size in both sexes despite different levels of variation in both structures, suggesting that these structures might covary to fluctuating degrees in sex-specific ways. We conclude that differential investment in brain tissue in sites without predators might be linked to varying reproductive and cognitive demands between the sexes.