Morphological differences between the eyeballs of nocturnal and diurnal amniotes revisited from optical perspectives of visual environments.

Eyes are expected to be adapted to the physical characteristics of the visual environment, yet previous analyses failed to corroborate this observation. We demonstrate that nocturnal, crepuscular/cathemeral, and diurnal activity patterns occupy distinct areas in morphospace and are identified with high accuracies based on a discriminant analysis of visual performance features. Not only nocturnal and diurnal diel activity patterns are reflected in macroscopic morphology of the eyeball, but also the crepuscular/cathemeral patterns. The eyeball morphology of the latter was believed to be undistinguishable between diurnal and nocturnal types. We show that all three categories can be delineated with high accuracies.