Ontogenetic Eye–Body Allometry in Two Sympatric Natricine Snakes, Nerodia cyclopion and N. fasciata

The function of the vertebrate visual system is constrained by eye size, with the width and length of the eye influencing sensitivity and visual acuity, respectively. Thus, eye size can be an important predictor of ecological niche, influencing predatory, defensive, and reproductive strategies and the circumstances under which those behaviors can occur. Watersnakes in the genus Nerodia are semi-aquatic, though there is interspecific variation in diel activity pattern, microhabitat use, and diet. Some species of Nerodia also exhibit an ontogenetic shift in both microhabitat use and prey preference, which may correspond with a similar shift in ocular morphology. We examined the ontogenetic allometry of eye size and shape in two sympatric watersnakes, Nerodia cyclopion and N. fasciata. We found that juveniles of both species have relatively larger eyes than their adult counterparts. Further, the eyes of N. fasciata do not increase significantly in length, but they do become significantly wider, thus potentially increasing the sensitivity of the eye as this species matures. In N. cyclopion, the eye increases in both width and length throughout ontogeny, generating little change in the overall shape, but potentially increasing the resolution of the retinal image. Increased eye length in the more behaviorally aquatic N. cyclopion allows for a greater degree of visual accommodation (via lenticular displacement or deformation), which is potentially beneficial in aquatic foraging. These findings suggest that ontogenetic scaling of eye size in these species corresponds with ontogenetic differences in microhabitat use, foraging behavior, and diet.