HERNANDEZ, R/A*; SECOR, S/M; ESPINOZA, R/E; California State Univ., Northridge; California State Univ., Northridge; Univ. of Alabama, Tuscaloosa: Is a dietary jack of all trades a master of none? Adaptability of gut form and function in an omnivorous lizard.

Omnivory can be viewed as a jack-of-all-trades dietary 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 specialists, or are they constrained by the jack-of-all trades/master-of-none paradigm? We raised 5�6 wk old (22 g) omnivorous bearded dragons (Pogona vitticeps) for 20 wk on either a diet of ground alfalfa (herbivore), crickets (carnivore), or a 50/50 mix of both (omnivore). We compared aspects of gut form (histology, mass, surface area) and function (apparent digestive efficiency, rates of nutrient transport) and used rates of growth as a measure of whole-animal digestive performance. Herbivores had relatively smaller stomachs and large intestines than carnivores, but not omnivores, and herbivores had smaller small intestines than both carnivores and omnivores. Lizards raised on the herbivore diet exhibited higher rates of both nutrient transport and capacity for proline and glucose than carnivores, but similar rates as omnivores. Herbivores grew slower than omnivores, but had similar growth rates as carnivores. These results indicate that the digestive system of P. vitticeps exhibits considerable plasticity in gut function, which, at a proximate level, may be advantageous to this ontogenetic diet-shifting (carnivory to omnivory) lizard. However, the apparent lack of plasticity in gross gut morphology may be indicative of an evolutionary hurdle constraining this omnivore from specializing further on plants, and may provide insight into why herbivory is so rare in reptiles generally.