Links between contrasting ecological conditions and evolutionary shifts in neurosensory components such as brain and eye size are accumulating. Whether selection operates differently on these traits between sexes is unclear. Trinidadian killifish (Anablepsoides hartii) are located 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 if females exhibit similar size-specific brain size differences between sites that differ in predation intensity. We also quantified brain size, structure, and eye size to determine if these structures coevolve 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 (prior to maturation), but such differences disappeared in larger size-classes. Females from sites with predators exhibit similar patterns in brain growth as 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 may covary to fluctuating degrees in sex-specific ways. We conclude that differential investment in brain tissue in sites without predators may be linked to varying reproductive and cognitive demands across the sexes.

Data includes wild-caught and common garden reared specimen. Variables are Code - fish ID, River - river of origin, population of origin- HP (high predation), RO (Rivulus-only), figh length (mm), ln transformed length, fish weight (g), ln transformed weight, brain weight (g) and ln transformed brain weight, eye size (mm) and In transformed eye size. Telencephalon, dorsal medulla, cerebellum, optic tetcum width (mm) and ln transformed widths. Telencephalon, dorsal medulla, cerebellum, optic tetcum volume (mm^3) and ln transformed volumes (for wild caught only), and overall brain size residuals. Residuals were output from regressions between ln-brain and ln-length. For common garden specimen, family is included as well as family by population (families designated for each population for analysis purposes).

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