Can omnivores be diet specialists? Adaptability of gut form and function in an omnivorous lizard

Includes bibliographical references (leaves 29-38)Omnivory can be viewed as a Jack-of-all-trades diet strategy that allows some animals to exploit a variety of food items. However, the extent to which omnivores can adaptively regulate gut form and function in response to different diets has not been widely investigated. When reared on specialist diets, do the digestive systems of omnivores respond like those of specialists, or are they constrained by the Jack-of-all trades/master-of-none paradigm? I raised juvenile bearded dragons (Pogona vitticeps), a naturally omnivorous lizard, for ~135 don either a diet of ground alfalfa (herbivore), crickets (carnivore), or a 50/50 mix of both (omnivore). I compared aspects of gut form (mass, surface area, histology) and function (pH, passage rate, apparent digestive efficiency, and rates and capacities of nutrient transport) and used rates of growth as a measure of whole-animal digestive performance. After accounting for body size there was no effect of diet on surface-area measurements of gut sections. Herbivores had relatively lower small intestine mass than both carnivores and omnivores and lower large intestine mass than carnivores, but not omnivores. Microscopy revealed that carnivores had longer small intestine villi than either herbivores or omnivores. Lizards raised on the herbivore diet exhibited higher rates of nutrient transport for pro line and glucose than carnivores, but similar rates as omnivores. Estimates of total nutrient uptake capacity did not differ among diet treatments for either pro line or glucose. The pH of the large intestines of herbivores was higher than for carnivores, but neither of these treatments differed from lizards raised on the omnivore diet. Omnivores grew faster than herbivores, but not carnivores, and carnivores grew slightly faster than herbivores. Differences in growth rate indicate a selective advantage to maintain an omnivorous diet during growth. These results suggest that, on a proximate scale, these diet generalists may actually be "specialist" of omnivory, inviting further consideration and study of other diet generalists. Furthermore, the diet-specific changes in both gut form and function indicate that the digestive system of P. vitticeps exhibits modest levels of plasticity, an ability that may be important for ontogenetic shifts in diet as well as ultimate shifts toward herbivory or carnivory