Habitat Influences Eye Size in the Evolution of Cetaceans

The transition from terrestrial to aquatic habitats of early whales beginning in the Eocene, around 50 million years ago, was one of the most critical points in evolution, as it was one of the few times terrestrial vertebrates transitioned from land to water. The disparate demands of terrestrial compared to aquatic sensory abilities has the potential to drive significant morphological change. It has been shown that eye size and therefore visual ability increased during the transition from aquatic to terrestrial environments due to longer aerial visual range providing more benefit to vision (MacIver et al., 2017). Here, we studied how orbit length, a proxy for eye size, changed during the land-to-water transition of ancient whales. We hypothesized that we would see decreased eye size during the transition due to the shorter aquatic visual range reducing the benefit of larger eyes. This short range was further accentuated by the turbidity of the coastal and fresh bodies of water that facilitated this transition. We employed phylogenetically generalized least squares (PGLS) regressions to determine if the visual environment of a taxa had a statistically significant effect on their eye size. We ran three regression models: a simple model with no visual environment as a factor, an intercept model, and an interaction model. We found the intercept model to be the best fit as determined by AIC value. From this finding, we can conclude that visual environment does affect orbit length when accounting for skull width, a common proxy of body size in cetaceans. The intercept of the best fit line in restricted visual environments, shallow coastal or fresh waters, is lower, meaning that for a given size taxa in more restricted visual environments have smaller eyes. This finding is in line with our hypothesis that early cetacean eyes became smaller as they transitioned into aquatic habitats that restricted vision. However, it is clear that the visual environment is not the only important factor to eye size. There are several examples of taxa in unrestrictive visual environments with relatively small eyes and vice versa. These discrepancies could be due to behavioral differences making eyesight more or less important, but more research is needed in this area. Ultimately, our data is not sufficient to claim that the early cetacean transition to aquatic habitats was accompanied by a decrease in eye size. More specific analysis would need to be done to uncover this relationship