Geometric morphometrics and snakes: Does it work and what can be done with it

Geometric morphometric techniques have come a long way in the past twenty years. Despite the advances in methodology, this type of analysis has rarely been used on snakes. It has proven quite successful in other taxa. But despite this success it remains underutilized. The purpose of this study was to test three geometric morphometric methods on six species of snake. The analyses included testing the viability of scales as homologous landmarks, using an outlining method, and a novel grid method. The results were compared against true homologous landmark analysis. Scales had the best ability to distinguish between the snake species with outlining next and the grid method last. The lateral view and the ventral view of the head did not have good results and are not recommended for use. The dorsal view provided the best results. X-rays were used to supplement species data that was not available through osteological collections.;These successful methods were then applied to two separate studies, one involving diet and another involving sexual dimorphism. The diet study tested three different groups of dietary feeders, two specialists and a generalist. The species used were the four species of the Regina complex as well as Lampropeltis getula and L. nigra. Geometric morphometric techniques were used to evaluate the head shape of the specimens. PCA, CVA, and DFA were used to analyze the degree of shape difference. There were distinct differences in head shape between dietary groups. Regina is comprised of hard shelled crayfish specialists and soft shelled crayfish specialists and there were shape differences between the two feeding strategies. There was overlap in head shape between the generalists, L. getula and L. nigra. These differences in head shape may be associated with evolutionary history. They could be the result of phylogeny but more work needs to be done to determine this. They could also be the result of competition but this seems the less likely explanation.;Sexual dimorphism in snakes is either associated with sexual selection or ecological causes. If it is the result of ecological causes it is expected for there to be differences in diet that cause differences in trophic morphology. Agkistrodon contortrix and A. piscivorus both display diet differences associated with sex. Previous work has hypothesized head shape differences between the sexes. This study used geometric morphometrics to analyze head shape of each sex for both species. Results for both species suggest there is no sexual dimorphism in head shape. Distinct head shape differences occur between species, however, and are most likely due to diet differences.