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CONQUERING THE COLD SHUDDER:THE ORIGIN AND EVOLUTION OF SNAKE EYES

Dissertation

Presented in Partial Fulfillment for the Requirements for

the Degree of Doctor of Philosophy in the Graduate

School of The Ohio State University

By

Christopher L. Caprette, B.S., M.S.

****

The Ohio State University2005

Dissertation Committee:

Thomas E. Hetherington, Advisor

Approved byJerry F. Downhower

David L. Stetson

John W. Wenzel

AdvisorThe graduate program in Evolution,Ecology, and Organismal Biology

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ABSTRACT

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

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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