Turtles should be fed as naturally as possible with a diet as close as feasible to the diet they would find in their habitat. From the point of view of their diet, turtles can be divided into herbivores, carnivores and omnivores.
HERBIVORE NUTRITION
The many herbivorous species of turtles include the Testudo spp., which have strictly and exclusively vegetarian feeding habits (Fig. 1). In nature, in the wild, these turtles find a large variety of vegetation from which they can choose in order to balance their daily intake. The vegetal matter is fermented in the colon, over a fairly long period (sometimes 3-4 weeks are needed) and so the diet must contain a large amount of fibre (20-30%), a small quantity of fats, very few carbohydrates and a good proportion of proteins (15-35%).
Calcium is the fundamental element for these reptiles since it is needed for ossification of the shell, metabolism and, in the female, production of eggs. The calcium/phosphorus ratio must be at least 2:1 and, when possible, higher. It is, however, not recommended to give calcium supplements except in particular situations, because it is easy to exceed the dose, without taking into account that turtles, like all vertebrates, need vitamin D3 to fix calcium and exposure to sunlight or artificial UV light is, therefore, very important, or actually fundamental. Animals raised in a garden can, within certain limits, choose the vegetation on which to feed, but their diet can be supplemented with alfalfa.
Garden vegetables may be given, although they should be limited, and fruit must be reduced to the absolute minimum because of its high sugar content. Commercial feeds can also be given, but only as integration and foods of animal origin must be avoided, even if these reptiles do occasionally eat insects and gastropods in nature. Much care must be taken of those animals living in a terrarium as they are entirely dependent on humans for their nutritional requirements.
CARNIVORE NUTRITION
Turtles that are purely carnivorous are mostly the semi-aquatic species such as Emys orbicularis and Chelus fimbriatus. Some young emydid turtles (for example, Trachemys spp.) are initially strict carnivores but subsequently tend towards an omnivorous diet. There are commercially available, specific, balanced feeds but despite this, a very common error is to feed these animals exclusively on dried shrimps, which are undoubtedly much enjoyed, but in the long term give rise to phenomena of hypovitaminosis A. The carbohydrate and fibre intake of these species is virtually irrelevant, while the proportion of proteins and fats is considerable, given that each accounts for between 30 and 60% of the diet. In nature these turtles feed on molluscs, crustaceans and above all small fish as well as insects and annelids on dry land. This nutritional scheme could also be maintained in captivity, if desired. Frozen food is a valid alternative, but the food must not be preserved in this way for more than a few months.
OMNIVORE NUTRITION
A large number of semi-aquatic species have an omnivorous diet, including the adults of Trachemys spp. and the Sternotherus spp. These species, each of which has rather peculiar needs, must be given a mixture of foods starting from those present in the carnivorous diet and adding a proportion of vegetation rich in fibre and calcium in order to balance the diet. As a source of fibre, the animal’s terraquarium can be enriched with water plants, which it can eat fresh, as desired. Alternatively, also in this case, appropriately formulated feeds can be given.