| I investigated the ecological origin and diversity of snakes by examining one complex structure, the eye. First, using light and transmission electron microscopy, I contrasted the anatomy of the eyes of diurnal northern pine snakes and nocturnal brown treesnakes. While brown treesnakes have eyes of similar size for their snout-vent length as northern pine snakes, their lenses are an average of 27% larger (Mann-Whitney U test, p = 0.042). Based upon the differences in the size and position of the lens relative to the retina in these two species, I estimate that the image projected will be smaller and brighter for brown treesnakes. Northern pine snakes have a simplex, all-cone retina, in keeping with a primarily diurnal animal, while brown treesnake retinas have mostly rods with a few, scattered cones. I found microdroplets in the cone ellipsoids of northern pine snakes. In pine snakes, these droplets act as light guides. I also found microdroplets in brown treesnake rods, although these were less densely distributed and their function is unknown. Based upon the density of photoreceptors and neural layers in their retinas, and the predicted image size, brown treesnakes probably have the same visual acuity under nocturnal conditions that northern pine snakes experience under diurnal conditions.; Second, I quantified the orbital area, binocular overlap, eye size, lens size, and the refractive powers of the lens and spectacle within and among colubrid snakes and pit vipers. Among colubrid snakes, the size-adjusted orbital area fit preditions based upon ecology, with nocturnal arboreal species having the largest orbits (p < 0.001). My results on the distribution of binocular overlap among colubrid snakes, however, contradicted earlier studies. Diurnal arboreal species had the smallest angle of overlap, while terrestrial nocturnal species had the greatest degree of overlap (one-way ANOVA, p < 0.001).; I used a comparative analysis of ophthalmic data among vertebrate taxa to evaluate alternative hypotheses concerning the ecological origin of the distinctive features of snake eyes. In parsimony and phenetic analyses, eye and orbital characters retrieved groupings more consistent with ecological adaptation rather than accepted phylogenetic relationships. Fossorial lizards and mammals cluster together, whereas snakes are widely separated from these taxa and instead cluster with primitively aquatic vertebrates. This indicates that snakes eyes most resemble those of aquatic vertebrates, and suggests that the early evolution of snakes occurred in aquatic environments. (Abstract shortened by UMI.)... |
