Michelle Barrows, BSc, BVMS, MRCVS
The Tortoise Clinic, Holly House Veterinary Surgery, 468 Street Lane,
Moortown Corner, Leeds LS17 6HA.
Presented to the B.C.G. Northern Symposium at Chester 14th October 2000.
Nutritional Management in Captivity:
The species of chelonian commonly kept in captivity display a variety of dietary strategies and preferences in the wild. These are not always clear cut and in some species, such as the red-eared terrapin, preferences may change as the animal matures. In general though, most captive chelonians can be treated as completely herbivorous, e.g. Mediterranean tortoises (Testudo spp), Leopard tortoises; completely carnivorous, e.g. snapping turtles, soft-shelled turtles (Trionyx spp); or as omnivorous, e.g. American and Asian box turtles, African hingebacks. All require water, dietary energy and essential nutrients such as vitamins and minerals. Ideally these should be provided in a diet which mimics that of the species in the wild as closely as possible, A wide variety of available substitutes should be given, avoiding reliance on a few easily available items.
Nutritional problems are an avoidable but common cause of disease and usually result from deficiency or excessive amounts of protein or various essential vitamins and minerals. Signs of nutritional disease are often seen at times of metabolic stress, for example growth in juveniles, or gravidity in mature females and are more likely to develop if the chelonian is not kept under the correct environmental conditions for the species. As poikilothermic reptiles, tortoises are only able to digest their food efficiently if kept at the correct temperature. They should be given the opportunity to regulate their body temperature by providing a gradient of temperatures within their preferred optimum temperature zone (POTZ) - 20-32°C for a Testudo species. If kept outside this range, physiological stress, reduced activity, poor food intake, inefficient digestion, failure to thrive and disease can result.
Wild herbivorous tortoises such as Testudo species feed on a wide range of vegetation, which is high in fibre and calcium, and low in fat, protein and phosphorus. Captive tortoises will do best if fed on similar vegetation such as weeds, flowers and grasses; however, most keepers will find themselves having to rely on grocery greens and vegetables during at least part of the year. In comparison to natural forage, grocery greens are generally higher in protein and lower in fibre with, in many cases, an unsuitable calcium: phosphorus ratio (see below). For this reason these tortoises require an artificial vitamin and mineral supplement. The supplements available vary in nutrient content and quantity but should contain the following fat and water-soluble vitamins and trace minerals: vitamin A, vitamin D3 (note that vitamin D2 is not suitable for reptiles), vitamin E, vitamin K, vitamins BI, B2, B6 and B12, folic acid, nicotinic acid, pantothenic acid, calcium, phosphorus, sodium, iron, copper, cobalt, iodine, manganese and selenium.
Calcium metabolism and control is a complicated process relying on several organ systems and the interrelated actions of various hormones. Various homeostatic mechanisms act to maintain plasma calcium levels within a narrow range. Calcium is obtained from the food but how much of the calcium content is available to the tortoise depends on the ratio of calcium to phosphate in the food and the presence of calcium and phosphate binding chemicals such as oxalates. Ingested calcium is absorbed from the digestive tract under the influence of parathyroid hormone and vitamin D3. In most vertebrates, vitamin D3 is produced via a complex biochemical pathway involving the action of (AV light on cholesterol in the skin and then further processing by the liver and kidney. Carnivorous species also obtain vitamin D3 from the diet. Herbivorous reptiles such as Testudo species however, do not naturally ingest sources of vitamin D3, relying on their ability to synthesize their own after exposure to UV radiation from sunlight. A tortoise that has been maintained outside in a warm sunny climate during the summer will probably synthesize and store enough vitamin D3 to last it through the winter (Innis 1994); however in Britain where warm sunny days are fewer and most tortoises need to be maintained inside for a significant proportion of the year, it is usual to provide some supplemental vitamin D3 on the food. In addition, since it is not known how well this vitamin D3 is absorbed in many herbivorous species, it is recommended that UVB lighting is provided. Parathyroid hormone, as well as aiding calcium absorption from the gut, also has the ability to increase plasma calcium levels by initiating resorption of calcium from bone, which can result in decreased bone density (metabolic bone disease). Prolonged high levels of parathyroid hormone also have damaging effects on the kidneys.
When considering the calcium content of the diet, it is not the total amount of calcium but the ratio of calcium to phosphorus that is important. It is generally recommended that herbivorous reptiles should be fed a diet with a calcium: phosphorus ratio of at least 1.5-2:1 (Scott, 1996). However, natural diets of wild tortoises typically contain a calcium: phosphorus ratio of at least 4:1 (Highfield, 1994) and it is possible that chelonia have a higher dietary calcium requirement than some other reptiles. This is important for a variety of physiological processes, including growth of the shell and skeleton, egg production and muscular function. As mentioned earlier, many grocery greens have an unsuitable and often inverse calcium: phosphorus ratio. For example, whereas dandelions have a calcium: phosphorus ratio of 3:1, iceberg lettuce has an inverse ratio of 0.8:1 (Donoghue, 1995). This means that calcium supplementation is essential. Unfortunately, many of the supplements available do not have a high enough calcium: phosphorus ratio to balance deficiencies in the diet (Donoghue,1996). For example Reptivite (ZooMed) has a ratio of 2:1 and Vionate (Sherleys) of only 1.4 1. Nutrobal (Vetark) and Arkvits (Vetark) with ratios of 46:1 and 30:1 respectively are more suitable. A sensible approach when relying on grocery greens is to try and feed a variety of types every day, each with a calcium: phosphorus ratio of at least 1: 1 and to use a calcium supplement such as Nutrobal regularly. Greens with a calcium: phosphorus ratio of at least 1: 1 include Chinese cabbage, turnip greens, mustard greens, parsley, collard, dandelions, romaine lettuce, kale, Swiss chard, spinach, endive and watercress (Innis, 1994). This is complicated further by the fact that oxalates found in some greens such as spinach, Swiss chard, cabbage and beet greens bind calcium reducing its absorption by the digestive tract. However, Innis (1994) suggests that tortoises may have evolved an ability to deal with high levels of oxalates and as long as these items are fed sparingly as part of a varied diet they are unlikely to cause problems. Rhubarb, which contains very high levels of oxalates however, should never be fed to tortoises.
It is difficult to specify the amount of any particular supplement and frequency with which it should be given as this will depend on the age, sex and species of the tortoise, as well as the nutritional composition of the diet; however, some generalisations can be made. Daily supplementation may be necessary in reproductively active females or actively growing juveniles, as well as in those animals with nutritional disease. In other tortoises supplements should not be necessary more than every other day and a healthy tortoise grazing natural forage in the summer, or on a balanced and varied diet composed of grocery greens with the correct calcium: phosphorus ratio, may not need to be supplemented more than once weekly.
The recommended daily requirements for vitamins and minerals for chelonians are not known and recommendations vary widely. Kirkwood worked out the calcium and vitamin D requirements of reptiles using the concept of metabolic scaling, which calculates dosages based on an animal's metabolic rate rather than its body weight. He then calculated the daily amount of various nutritional supplements needed and concluded that many of the supplements available did not provide these requirements at the recommended doses. However, the recommended dose of Nutrobal is 0.1g/Kg per day or every other day (Scott 1996) and in practice this seems to be sufficient. With the possible exception of vitamin D3, it is difficult to overdose a tortoise by using oral vitamin and mineral supplements as they have the ability to excrete the excess in faeces or urine. Over- supplementation of some reptiles with vitamin D3 can result in abnormal mineralisation of soft tissues. A dose of vitamin D3 of 100iu/Kg has been reported to have caused problems in iguanas (Zwart 1980); however, African spurred tortoises (Geochelone sulcata) fed diets containing 2000iu/Kg vitamin D3 showed no signs of disease (Bernard J 1995). Nevertheless it is recommended that tortoises are given vitamin D3 in the form of a multi-vitamin and mineral supplement such as Nutrobal (Vetark) rather than the pure vitamin D3 products available such as Solar drops (T-Rex).
Whichever supplement is used, it is important to note the expiry date, as vitamin content will reduce with time. Also, trace minerals in contact with vitamins can be oxidised in the presence of air and therefore supplements containing trace minerals should be added just before feeding (Donoghue 1996).
Insects such as crickets, mealworms and wax worms which may be fed to omnivorous species such as American box turtles also have poor calcium: phosphorus ratios and should be gut-loaded with a product such as Bug-Grub (Vetark) for 48 hours before feeding. They should also be regularly dusted with a high calcium product such as Nutrobal (Vetark) before feeding.
The Inappetant Chelonian:
If you are faced with an inappetant chelonian, the first thing to do is to check the environmental conditions and correct them if necessary temperature, humidity and photoperiod should all be suitable for the species concerned. Correction may involve setting up an indoor enclosure with basking lamps and suitable lighting. Some species, e.g. Cuora amboinensis, may prefer feeding in water rather than on land. Offer preferred food items, which may include brightly coloured flowers or fruits for herbivorous species, or live worms, other invertebrates or pinkies (baby mice which can be bought frozen from pet shops) for omnivores and carnivores. Give small amounts at a time and place the animal directly in front of the food. If necessary hand-feed items. If the above procedures do not result in your chelonian regaining its appetite, then a veterinary check-up is essential.
A deficiency of vitamin A is common in red-eared terrapins and American box turtles, but can also occur in terrestrial tortoises, where it is seen most often in hatchlings at about 6 months of age when their supply of vitamin A derived from the yolk and stored in the liver is depleted. Clinical signs include swollen eyes, conjunctivitis and impaired vision, skin changes and an increased susceptibility to respiratory and ear infections. Diagnosis may involve blood tests, or in larger animals liver biopsy to measure vitamin A levels, but is often possible on clinical signs and response to treatment alone. Treatment involves vitamin A supplementation either orally (e.g. ACE High, Vetark) or occasionally by injection. Dark green leafy vegetables and orange or yellow vegetables such as carrots, squashes and sweetcorn are good sources of vitamin A for herbivores. Vitamin A deficiency in red-eared terrapins can be avoided by basing the diet on a good quality terrapin food such as ReptoMin (Tetra) and providing smaller amounts of a variety of other items such as green vegetables and pondweed, pond fish pellets, blood and tubifex worms, raw whole fish, prawns, pinkies and liver, which is a good source of vitamin A for omnivores and carnivores. Supplements such as ACE High can be provided by dipping preferred items like prawns in the powder and feeding directly from a pair of tweezers, by rehydrating low fat dog or cat pellets in a small amount of water containing the supplement or by incorporating the supplement into feeding cubes containing various items set with gelatine. Feeding meat, fish or prawns exclusively will lead to nutritional imbalances if a supplement is not used.
Disease due to overdose of vitamin A is also unfortunately seen in chelonians. It results from excessive amounts of vitamin A given by injection, either in a single dose or through repeated injections. Clinical signs are often similar to those of hypovitaminosis and can include skin changes such as blisters, ulcers and skin sloughing.
Hypothyroidism has also been reported in captive tortoises, especially giant tortoises, and is thought to be due to a diet containing excessive amounts of goitrogenic greens such as cabbage and kale (Frye 1974). Clinical signs include decreased appetite, lethargy and tissue swelling.
Metabolic Bone Disease / Accelerated Growth
Hatchlings and juvenile tortoises are commonly kept inside and often maintained all year round under conditions of optimal heating, lighting and food availability. They may be given unsuitable high protein diets. This does not mimic the situation in the wild where food availability and quality may vary significantly throughout the year. In addition, young wild tortoises like adults may hibernate in winter and/or aestivate during hot spells. As a consequence many captive juveniles achieve abnormally fast rates of growth, which may be too fast for the supply of calcium available. Metabolic bone disease is therefore common in these tortoises and is usually due to inadequate calcium or vitamin D provision, an inappropriate dietary calcium: phosphorus ratio, or to lack of exposure to UVB light. It can result in soft shell, shell deformity, muscular weakness, egg retention, and cloacal organ prolapse. Correct nutritional and environmental management will avoid metabolic bone disease, accelerated growth and early maturity.
Bernard, J. (1995). Spectral irradiance of fluorescent lamps and their efficacy for promoting vitamin D synthesis in herbivorous reptiles. PhD dissertation. Michigan State University.
Donoghue, S. (1995). Clinical Nutrition of Reptiles and Amphibians. ARAV Proceedings 1995.
Donoghue, S. (1996). Nutrition of the Tortoise. ARAV Proceedings 1996.
Frye, F. & Dutra, F. (1974). Hypothyroidism in Turtles and Tortoises. Veterinary Medicine / Small Animal Clinician. August 1974.
Highfield, A. C. (1994). Tortoise Trust Guide to Tortoises and Turtles. Carapace Press 1994.
Innis, C. (1994). Considerations in Formulating Captive Tortoise Diets. ARAV Vol. 4 No. 1.
Scott, P. W. ( 1996). Nutritional Diseases in Reptile Medicine and Surgery. BVZS Proceedings 1996.
Zwart. P. (1980). Nutrition and Nutritional Disturbances in Reptiles. Proceedings of European Herpetological Symposium, Cotswold Wildlife Park.
Testudo Volume Five Number Three 2000