Although most reptiles are carnivores, some are omnivores and a few are herbivores or strictly plant eaters. Most reptiles have a rather primitive digestive tract making them in most cases very specialized feeders. Certain forms have no means of digesting plant fibres, while others have difficulty digesting high protein sources (e.g. animals). Some groups shift from a higher protein diet in their juvenile stage to a higher fibre diet in their adult stage for example iguanas. Often these higher protein diets consists of sprouts, young greens and growing foliage, but some groups fulfill their need for protein by occasionally eating insects or animal matter (omnivores). Since reptiles in captivity consume less energy than in the wild it is also necessary to adapt their diets to their life in a terrarium.