Fruit and vegetables are an essential part of many mammals’ diets; however, few reptiles ever tuck into their greens. ‘Herbivory is extremely rare in squamate reptiles’, say Kevin Kohl and colleagues from Vanderbilt University, USA, and Universidad Nacional de San Luis, Argentina, adding that most reptiles prefer to feast on insects. Although the lack of vegetation in the reptile diet is understandable – it often lacks protein and can be tough to digest – the team wondered if reptiles were simply incapable of extracting enough nutrients to survive on a diet of greens alone because their guts could not adapt to the higher fibre diet. Intrigued by the possibility, Kohl and his collaborators set about feeding Ruibal's tree iguanas on an (almost) vegetarian diet of rabbit food supplemented with a few mealworms and compared how they fared with animals fed on a mixed diet of 50:50 rabbit food and mealworms for 40 days.
Monitoring the animals’ body mass and collecting their faeces, the team found that all of the animals successfully maintained their mass and were supplied with enough protein in the diet, although the veggie lizards excreted less protein waste than the animals on the mixed diet. And when they investigated the animals’ digestive tracts, they saw that the small intestine of the veggie iguanas was almost 20% longer than that in the omnivorous iguanas and the hindgut was significantly larger, which means that the iguanas can adapt their guts to a vegetarian lifestyle. They also found more bacteria that ferment plant material in the gut flora of the animals on the veggie diet. So lizards are not prevented from adopting a diet of greens by their digestive tracts and Kohl and colleagues say, ‘We hypothesize that ecological contexts and the likely fitness benefits of feeding on energy-dense insects may be more critical in constraining the evolution of herbivory in liolaemid lizards’.