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Successful reptilian nutritional management requires knowledge of the reptile species and the role of water, nutrients and energy for each individual reptile. These animals present several nutritional challenges for veterinarians. Unknown dietary requirements, lack of balanced commercial diets and owner mismanagement can all cause nutritional problems that affect a reptile’s health. In fact, poor nutrition and failed feeding management are leading causes of disease and death in captive reptiles in the United States!. Other factors, such as habitat, feeding ecology, temperature and humidity can all affect feeding in captivity. Maintaining an animal in a sub-optimal environment is a physiologic stress that can result in poor intake, digestion and utilization of food. Natural history and metabolism also affect each individuals needs and preferences for water, energy and nutrients. Although precise, quantitative nutritional requirements have not been determined for most reptiles, reasonable estimates can be made from limited observations of these species and from practical feeding experiences2. Owners can take several steps to ensure sound nutrition for their reptiles by evaluating current diet, feeding schedule, quality and quantity of food and environment. Water All reptiles must have access to clean, fresh water. It is important to understand the natural history of each individual reptile and strive to simulate its natural source of water and humidity. Acceptance of water depends, in part, on its delivery. Many reptiles will drink from a bowl, however, some geckos, anoles and chameleons only lap water in droplet form that has been sprayed on nearby foliage. Dehydration is a common problem, especially in sick reptiles. It may result from water provided in an inappropriate form or quantity, or from anorexia and disease. Uricotelic species of reptiles require large amounts of water to sustain normal excretion. Dehydration in these species may result in uricemia and visceral gout 2. Rehydration is often a critical factor in the treatment of the sick reptile patient. Daily water requirements for rehydration are 10-25 mL/kg parenterally3. Aquatic species are less likely to suffer from dehydration, but more prone to problems stemming from poor water quality. Routine water analysis may be important for maintaining health in aquatic species. Humidity is also important as inadequate humidity leads to dehydration, stress and dysecdysis. Excessive humidity may contribute to skin infections and hyperkeratinization. Each species has individualized humidity requirements that must be maintained for optimal health. Energy Energy requirements relate to metabolic body size in all species; the smaller the animal, the greater its metabolic rate per unit body weight. Reptiles average about one-eighth to one-quarter the rates for endotherms2. Interestingly, turtles are similar to other reptiles, even though the shell comprises 15-30% of the animals’ bodyweight. In practice, optimal proportions of dietary protein, fat and carbohydrate for each species are estimated from information on natural history, including feeding habits, habitat and gastrointestinal morphology. Carnivorous reptiles (e.g. all snakes, most monitors and aquatic turtles) utilize primarily fat and protein as fuel sources. Healthy carnivorous reptiles consume about 30-60% of their energy from protein and 30-60% from fat2. Herbivorous reptiles (e.g. many tortoises and the green iguana) utilize mainly soluble carbohydrates and fermentable fiber. Healthy herbivores consume at least 28% of their energy from protein, about twice that of herbivorous mammals4, less then 10% from fat and more than 50% from carbohydrate2. Fiber aids in gut motility and serves as a vital source of calories for herbivorous lizards. Omnivores (e.g. box turtles and bearded dragons) usually receive more fat and protein when young, then more carbohydrate and fiber when older. Because dietary fat greatly affects dietary energy density, dietary data for carnivores are reported on an energy basis (milligrams per calorie) and herbivorous data is presented on a dry matter basis (milligrams per gram of DM). Being ectotherms, ambient temperature greatly affects body temperature and thus, calorie requirements, digestion and metabolic rate. Ectotherms require fewer calories than endotherms because energy is not used to maintain body temperature, and energy requirements do not increase with cold temperatures. Management of Feeding The wide variety of foods consumed in the wild is nearly impossible to replicate in captivity. One study researching the gastric contents of wild iguanas found 26 different species of plants in one specimen6. Other studies demonstrate that wild lizards selected prey based on the species of prey, not size of prey7, suggesting lizards possibly select their food based on its nutritional content. Even when great attempts are made to mimic the animal’s natural diet, owners may inadvertently promote anorexia and starvation in their reptiles by offering foods at an inappropriate time or in an inappropriate quantity or form. Certain species, such as the diurnal common green iguana, only feeds during the day. Many geckos only eat in darkness, and, therefore, must be fed very late in the evening to assure the food is alive. Some snakes only accept live food, while other snakes refuse to kill their own prey. The frequency of feeding is also important. Smaller reptiles eat frequently, up to five or six times a day, while the larger carnivores, such as boas and pythons, eat infrequently. Stress induced failure to eat and cold conditions also influence feeding behavior. Ideally, the optimal energy intake should be individualized for each animal, and not only total caloric intake, but also proportions and feeding schedule should be evaluated to avoid malnutrition or obesity. Many species of wild reptiles fast for weeks or months in response to lack of food, cold weather or drought. Fasting may occur in captive reptiles, even if kept in warm temperatures with adequate food and water. Several sources advise to not accept anorexia as a “normal fast” in captive reptiles2,3. Vertebrate Prey Whole vertebrate prey provides high quality proteins, vitamins and trace minerals. Well-nourished adult vertebrates are considered to be nutritionally complete and balanced; obese or underfed prey and very young prey are not. Calories are provided almost entirely by fat and protein, and carbohydrate sources are limited to the digestive contents of the prey. Fresh, humanely killed prey is considered to be the best, as live prey may bite or scratch the reptilian predator. Wild prey often harbor parasites or pathogenic organisms, so commercial or laboratory raised rodents are recommended. Nutritional disorders vary in type and prevalence with species and feeding management. Large snakes consuming whole prey are less likely to acquire specific deficiencies than generalized starvation. Likewise, herbivorous species are more likely to be deficient in specific vitamins and minerals, depending on the diet offered and consumed. Invertebrate Prey Many captive invertebrate-eating reptiles appear to do well on a diet of only one or two species of food. Some animals, however, need a variety of prey species for optimal health and a variety of appropriately sized invertebrates should be offered, including snails, slugs, moths and flies, in addition to the standard mealworms and crickets. Nutrient content of insects vary, but most contain over 30% protein, 40% fat, less then 15% carbohydrate. Most lack a calcium-rich skeleton3. Aside from land snails, which have a calcium-rich shell, calcium content is inadequate in most other invertebrate species, measuring less than 0.5 mg/kcal3. The nutritional content of invertebrates is increased if a balanced diet is offered to the prey prior to feeding it to the reptile . Invertebrates are often dusted with a vitamin/mineral supplement prior to feeding. However, dusting may induce toxicosis if over supplied, or deficiencies if under supplied (see Supplements section). Plants Free-ranging herbivorous reptiles are quite specialized and selective in food choices. When evaluating a herbivore’s diet, it is best to distinguish between folivores and frugivores. The proportions of greens, fruits and vegetables vary between herbivorous species. Tortoises, for example, accept drier foods (e.g, hay) while tropical herbivores tend to prefer moist, sweet vegetables. Color and temperature may also cause reptiles to favor or reject some foods. Protein requirements for herbivorous reptiles range from 14-35% DM2. High protein greens include greens (Romaine), spinach, alfalfa, clover, dandelion, bean sprouts and mushrooms. Because domestic produce rarely provides enough protein, calcium, fiber or trace minerals to support growth or reproduction in reptiles, supplementation is recommended1. Fiber nutrition research has been minimal for reptiles, however, fiber is essential for all herbivores, especially iguanids, which rely on hindgut fermentation. Adult iguanas probably need at least 12% DM as crude fiber, and perhaps 20-30% is optimal5. Fiber is an excellent source of calories for herbivorous reptiles. Fruits are often selected by herbivorous reptiles, however, fruits contain mostly water. The addition of just a small amount of fruit markedly dilutes the calories, nutrients and fiber provided by greens. Oxalates, in spinach, rhubarb, cabbage, peas, potatoes and beet greens, bind calcium and trace minerals in the gastrointestinal tract. Goiterogens occur in cabbage, kale, mustard and other cruciferous plants. Therefore, it is best to avoid feeding these foods in large quantities. Commercial diets Because there are no independent standards for nutritional adequacy of reptile diets, commercial diets for these animals cannot claim to be nutritionally complete. Because of this, no product should be relied upon to supply all essential nutrients unless extensive, long-term feeding trials have demonstrated completeness and balance. Supplements Vitamins and minerals are often added to diets, especially those made up of invertebrates and mixed salads. Supplements should contain the fat and water soluable vitamins, including vitamin D3, and trace minerals known to be essential for other species. One source recommends vitamins A:D:E should be given in a ratio of 100:10:12. Adding supplements to the water is not recommended because it may alter the taste of the water and decrease water consumption. Conclusion Nutritional management is an extremely important part of the care of reptiles. Nutritional disorders are common in reptiles, often due to inappropriate dietary management, poorly formulated diets and inadequate knowledge of the reptile’s natural history, behavior and feeding habits. Knowledge about the clinical significance of water, nutrients and energy for each reptile patient will help ensure good nutrition for reptiles maintained in captivity. References
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