Body size, temperature, and the evolution of herbivory in reptiles

I critically review the literature on the biology of Recent herbivorous reptiles to explain why it so few herbivorous reptiles have evolved (<1% of >8000 living species) and to identify the conditions that favor the evolution of herbivory. Despite their divergent evolutionary histories, most herbivorous reptiles live in warm climates, and have converged on a common suite of morphological, physiological, and life-history characteristics. Herbivory appears to evolve from omnivory, primarily in seasonal climates, and possibly results from an energetic deficit caused by switching between an herbivorous and a carnivorous diet. The mechanistic basis of this cost is hypothesized to be the consequence of changes in the community structure of fermentative endosymbionts that are responsible for digesting plant fiber and releasing energy and nutrients into their host's gut. Most aspects of the biology of herbivorous reptiles have been poorly studied. Liolaemid lizards are likely the smallest and least studied herbivorous reptiles. A sample of 31 species of liolaemids, with diets ranging from insectivores to herbivores, were examined and found to have dental and gut characteristics similar to those of other herbivorous lizards. These characteristics also correlate with the proportion of plant matter in the diet suggesting some traits associated with the evolution of herbivory evolve in a correlated fashion. Herbivorous reptiles produce small clutches of large eggs (or neonates) relative to nonherbivores. Herbivorous turtles do not exhibit an increase in clutch size with increasing latitude, but nonherbivorous turtles do. Clutch mass is similar between herbivorous and nonherbivorous reptiles indicating that herbivory does not impose constraints on reproductive effort. Instead, selection is expected to favor large-bodied neonates (with accordingly large guts) because large digestive tracts may be a necessity of plant digestion. An energetic model was developed to understand the interplay among body size, temperature, and the biogeography of three herbivorous lizards ranging widely in body size and geographical and climatic distribution. Simulations show that lizards from warm environments with scarce food resources need to be large bodied because large animals expend energy reserves less quickly. Small lizards are able to thrive as herbivores in cool climates because of their ability to exploit unpredictable thermal resources and their low gross energy needs.