While many lizards and snakes include plants in their diet, less than 4 percent of them are actually herbivores. Researchers trying to figure out how these reptiles can survive on a low-protein diet of mostly fibrous plants have discovered that iguanas fed vegetarian diets have longer small intestines. Their findings were published in the Journal of Experimental Biology this week.
Herbivory is a difficult strategy. Plants are low in essential nutrients like proteins, and all that fiber makes them tough to digest. Not to mention, they often come with toxic defensive compounds produced by the plants against plant-eaters. Yet 43 percent of mammal species are herbivores. They’re able to break down plant matter and absorb more nutrients thanks to their gut anatomy and function as well as helpful gut microbes. For lizards, insectivory is a much more successful feeding strategy.
A team led by Vanderbilt University’s Kevin Kohl wanted to see if reptiles are incapable of extracting enough nutrients from a plant-only diet and if they can maintain a proper nitrogen balance. Ruibal’s tree iguana (Liolaemus ruibali) is a naturally omnivorous lizard: In the wild, plants make up 16 to 20 percent of their diet. Also, its family, Liolaemidae, has evolved herbivory 18 times independently.
The team collected Ruibal’s tree iguanas from Quebrada Vallecito in the Andes Mountains west of Calingasta town in Argentina. Back in the lab, they fed some iguanas a mostly vegetarian diet of alfalfa-based rabbit chow supplemented with ground mealworms. Another group of iguanas were fed a mixed diet of equal parts rabbit food and mealworms. The team monitored the iguanas’ weight and collected fecal samples.
After 40 days, the (mostly) vegetarian iguanas successfully maintained the same weight as iguanas that feasted on a mixed diet – though their low-protein diet meant they excreted lower concentrations of protein.
When the team dissected the iguanas, they found that the small intestines of the vegetarian iguanas had grown 17 percent longer than that of the omnivorous iguanas. Additionally, their hindgut was much larger, suggesting that they’ve adapted to their new vegetarian lifestyle. Furthermore, the community of gut microbes in the veg iguanas were more diverse and included bacteria that ferment plant matter. So in the absence of energy-dense insects, lizards can adopt a vegetarian lifestyle thanks to the flexibility of their digestive tracts.