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

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.