The eyes of land vertebrates cope with highly different light levels, largely depending on the preferred activity time of the animal. Night-active (nocturnal) species experience much lower light levels than day-active (diurnal) species. Twilight-active (crepuscular) and day-and-night-active species (cathemeral) species are exposed to both high and low light levels. It is expected that contrasting lifestyles are reflected in the morphology of eyes.
In this study we focus on features of macromorphology that are tightly correlated with optical function. We demonstrate that nocturnal, crepuscular/cathemeral, and diurnal activity patterns occupy distinct areas in morphospace and are identified with high accuracies. The bivariate plot in (a) is derived from optical equations and shows that nocturnal species can be delineated from diurnal species. A discriminant analysis (b) reveals that cathemeral/crepuscular species occupy a distinct area in morphospace, as well. The great news for paleobiology is that this method also works with skeletal features (i.e., size of the orbit and sclera ring) of birds. Stay tuned for further results on phylogenetically informed analyses and activity pattern of fossil vertebrates.
Schmitz, L. and Motani, R. 2010. Morphological differences between the eyeballs of nocturnal and diurnal amniotes revisited from optical perspectives of visual environments. Vision Research. download pdf.
Eye morphology of nocturnal and diurnal amniotes
(a) Plot of the square of lens diameter versus the product of eyeball diameter and axial length of terrestrial amniotes. Nocturnal amniotes plot above the 95% prediction belts calculated for diurnal amniotes. (Figure 4a of Schmitz and Motani 2010)
(b) Plot of discriminant functions (DF) 1 and 2 found by RDA of eyeball diameter (ED), axial length (AL), and lens diameter (LD) of terrestrial amniotes. Circles represent avians, triangles represent mammals, and squares indicate squamates. Black fillings identify nocturnal species,
and grey fillings identify cathemeral and crepuscular species. Open symbols represent diurnal species. For each group of activity pattern 95% confidence ellipses are plotted. (Figure 4b of Schmitz and Motani 2010)
posted April 15, 2010