Big eyes in deep divers?

I’m finally continuing my blog after being simply too busy over the last few months.  I want to resume with a series of small posts on the eyes and visual capabilities of marine mammals, because there have been a few quite interesting findings recently.

Ever since Motani et al. published their paper on the function of giant eyes in ichthyosaurs (fish-shaped marine reptiles of the dinosaur era) it seems there has been an increased interest in eye size as an ecological proxy. Motani et al. convincingly argued that size and shape of the eye indicated deep diving in at least some ichthyosaurs. The idea is that large eyes improve the visual light sensitivity — arguably a beneficial trait in the dim-light environment of the deep sea.

Berg and Pyenson now provide an interesting and comprehensive data set on eye size and dive depth in pinnipeds, a group of marine mammals with sleek, barrel-shaped bodies and flippers that includes seals, sea lions and walruses. Pinnipeds are visual hunters that can dive to considerable depth. For example, the Northern Elephant Seal (Mirounga angustirostris), which one can observe along the California coast, reaches water depths of more than 1300 m.

Using bony orbit size as a proxy for eyeball size, Berg and Pyenson found that pinnipeds in general have big eyes compared to their terrestrial relatives. Deep diving evolved multiple times among pinnipeds, and the deep divers (assessed by maximum dive depth) tended to have bigger eyes. According to the authors this result is only weakly supported when the analysis is performed in a phylogenetic framework, which I think is a little surprising. One could try and take another careful look at the data with a fully time-calibrated phylogeny (as soon as it becomes available…) and maybe a slightly different set of analyses (see, e.g., eye size in wrasses). Regardless, I like that Berg and Pyenson emphasize the importance of analyzing comparative data from a phylogenetic perspective. Interesting stuff!

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