ESPINOZA, ROBERT E.; California State Univ., Northridge: Why herbivory shouldn�t evolve in reptiles and a hypothesis for why it does.

Herbivory is rare in reptiles and for more than 40 years researchers have postulated why so few herbivorous reptiles have evolved. Yet, a better question might be why strict herbivory should evolve at all given the challenges associated with eating fibrous plant tissues. Diets consisting of plants are less energy and nutrient rich than are diets of animals. Eating plants necessitates long processing times, which apparently select for larger guts and larger body sizes. Despite numerous adaptations for eating plants, herbivorous reptiles appear to function as efficiently as carnivores on a diet of animals. In fact, when fed a carnivorous diet, herbivorous reptiles digest more efficiently and grow more quickly than when fed plants. So why don�t herbivorous reptiles �cheat� in nature by switching to a diet of animals (e.g., insects) when these high-quality foods become seasonally available? I propose a model wherein strict herbivory evolves in reptiles because the cost of switching from herbivory to carnivory, and back to herbivory is energetically costly. This cost is predicted to result from a substantial reduction in digestive efficiency when returning to a diet of plants, following a period of eating animals. The mechanistic basis of this proposed reduction is a change in the community structure of hindgut microsymbionts (bacteria and protozoa). These fermentative microbes convert plant fiber into useable energy for the host. Disruption of this community by dramatic changes in diet may result in severe lags in digestive performance. Thus, once herbivorous reptiles make the �evolutionary commitment� to herbivory, switches back to herbivory, following bouts of carnivory, may be energetically disadvantageous, particularly in the seasonal environments where herbivorous reptiles appear to have evolved.