Unfortunately, some of the most common reasons reptiles and amphibians are presented to the veterinarian in private practice are due to underlying nutritional issues. Much of this stems from misinformation from the purveyor of the reptile originally or from outdated books or incorrect websites. Nutrition is a constantly evolving science, and the veterinarian and the owner of the pet need to keep on top of changing information. Some species have very specific requirements, others more opportunistic ones, while for many species best guesses by the experts have to suffice. Generally, most reptiles and amphibians can be classified as insectivores, carnivores, herbivores, and omnivores. Trying to recreate wild diets in captivity, especially when food type variety may be limited by season, cost, availability, or owner laziness; can be extremely challenging. To date, there are no significant studies on the nutritional needs of any reptile or amphibian as with many domestic mammalian pets and poultry. Many owners are initially introduced to manufactured diets, which are often variable in nutritional status from brand to brand and even batch to batch. These diets may oversupply or undersupply important nutrients including vitamins and minerals and may dehydrate the animal due to low moisture content, not to mention raise questions of palatability. At this time, the author does not recommend any manufactured diet for reptiles or amphibians. Nutrition recommendations for many exotic animal pets often revolve around the recommender's experience and opinions versus hard science, more than any other aspect of veterinary medicine. Yet scientific evidence supporting some dietary recommendations does continue to trickle into the literature.1
One of the most frequently asked questions by clients to veterinarians (for which guidance in answering is rarely provided) is "How much and how often?" to feed. There are so many variables that affect the answer to this question that many veterinarians freeze up on the answers. Age, sex, season, size, reproductive status, nutritional value of the food item, ecdysis status, among others, need to be considered. However, one of the most important concepts to remember is that the metabolism of a reptile or amphibian is much lower than for a bird or mammal, about 30% of the latter. The concept of feeding weekly or monthly for certain reptiles and amphibians cannot be easily grasped by many owners, who instead over feed their pet by loving them too much. For all reptiles, variety is important, but may be impractical in many households. So determining food items that provide a balanced diet to the best of one's knowledge is critical for client compliance. With most mammals and birds in zoos and in private collections, food presentation and its use for enrichment are critical. For reptiles and amphibians, enrichment is an often overlooked but important factor in avoiding anorexia or obesity. All reptile and amphibian owners should use a gram scale to weigh their animal at the same time each week to note trends. Post-eating, post-urination, or post-defecation are not necessarily good monitoring times. Weight decreases of 20% should be worked up, while healthy weight gains in a growing reptile can be variable. Likewise, some reptiles like a monitor should be a slim, aggressive, and active reptile; while even a healthy Argentine horned (pacman) frog always looks dumpy. Knowing the correct body condition for each species is important.
Herbivores
For herbivores, one can technically differentiate into folivores (leaf-eaters), grazers (grasses), and opportunistic herbivores. Examples of herbivores are most tortoises, adult bearded dragons, iguanas, and uromastyx. While there are huge lists of different greens and thicker vegetables and their pros and cons out there, one has to keep it simple for the average client. Likewise, owners in Alaska or Montana often have limited access to good plant variety. For most herbivores, the following generic recommendation works as a well-mixed "salad" to minimize preferential eating.
Pre-packaged salad mix--70% of the diet. Rather than wasting a bunch of greens that the owner themselves will be unlikely to eat, focus instead on a Spring Mix, Baby Herbs, or Baby Lettuces mix in a bag or plastic container from the local grocery store. Greens offered should not be the thick portions of the plant, offer the green or red portions of the leaf where the nutrients are. This is why iceberg lettuce and much of the romaine lettuce plant are not recommended. These greens are rich in calcium versus phosphorus and high in vitamin A. They also provide good fiber with limited calories for slow steady growth. A high water content helps to provide an additional source of moisture. Most of the above green mixes are not strongly flavored, increasing the probability of palatability.
Colored (red, orange, yellow, and purple) vegetables--20% of the diet. These are food items often preferred by the reptile, but should provide a minority portion of the diet. They are often high in vitamin A, but also high in phosphorus versus calcium. These should be chopped up finely enough to avoid eating around the greens. These include small squash, sweet potatoes/yams, yellow/red peppers, beets, eggplant, and carrots.
Fruits--10% of the diet. These should initially focus on items that provide some nutritional adequacy and also are attractive to the animal eating it. As with the colored vegetables, cutting these up finely enough to mix into the salad to preclude selective eating is important. Berries, especially blueberries and papaya are good starting points. Bananas and melons are not.
Free choice grazing of high quality grasses or hay is extremely important for most large tortoises if available.
Uromastyx species are specialized herbivores requiring legumes (beans, peas, lentils, alfalfa) and even moistened bird seed as part of the diet. For this species, my ratio is 30% greens, 40% legumes and seeds, 20% colored vegetables, and 10% fruit.
Frequency of feeding varies, with immature herbivores being offered fresh "salad" twice daily, older herbivores once daily, and many adult herbivores may only want to eat every other day. The amount to feed should only be the amount that each individual animal will finish within one hour of feeding. Each herbivore should have its own "feeding station", though visual evidence of others in the herd eating seems to inspire a competitive spirit to eat more robustly in some reptiles. Decreasing or increasing the volume offered can vary dependent on size, age, and season among other factors.
For supplementation, I recommend cherry or fruit-flavored Tums® (calcium carbonate, GlaxoSmithKline, Brentford, Middlesex, United Kingdom) crushed into a powder and added to the salad daily. One full tablet for reptiles greater than one kilogram, a half tablet at 500 grams, and a small pinch added to the salad of very small reptiles. A small pinch of spirulina (blue green algae) can be added to the salad once weekly when ingestion of vitamin A may be in question. None of this is based on hard science, instead focusing on more help than harm for clients and their pets. I avoid general vitamin supplements and only use human products that I know are more closely regulated than animal products.
Insectivores
Most amphibians and a fair number of reptiles (chameleons, young monitors, young bearded dragons, and small snakes) fit into this category. Here, the adage, "You are what you eat" comes into play. Insects themselves are generally nutrient deficient, with the exception of their stomach contents. "Gut-loading" invertebrates after purchasing them on-line or from a pet store is imperative. Wild-caught insects do not have the same requirements and are generally "ready to eat" meals. "Variety is the spice of life" is true for all reptiles and amphibians, but especially so for the insectivore. Unfortunately, the option to purchase invertebrate variety is often limited. Many reptile and amphibian owners are encouraged to develop their own "farm" to raise invertebrates. I recommend high quality soil/mulch with added Tums® for earthworms, veggies as for herbivores above for roaches, dry dog or cat food for crickets, and replace the sawdust with meal/super/wax-worms with oatmeal and Tums®. This environmental/diet change for invertebrates should take place for at least four days before feeding to the reptile or amphibian. Evidence of the benefits of "shake and bake" application of vitamins and minerals before feeding the invertebrate is not strong and often causes palatability issues. Live invertebrates are always recommended versus some of the canned or dried versions available at this time. Invertebrates also vary in their nutritional value, with earthworms, snails, and cockroaches being the best; while mealworms, wax-worms, and super-worms are fat, vitamin E, selenium, chitin and not much else, though they sure seem to taste better to the insectivore. Fat has more bang for the buck nutritionally, so it is understandable that these animals are drawn to it. Allowing these "worms" to meta-morph into beetles does allow for a better nutritional product but requires more work. Crickets are in the middle. Obesity is a huge problem for some insectivores (especially amphibians), as they are usually opportunistic feeders--when food is available, they will eat all they can. Be especially careful to avoid offering invertebrate gel diets or water gel, as I have seen several cases on reptile and amphibian ingestion of the product and subsequent gastrointestinal rupture or blockage leading to death. While expected with carnivore prey, invertebrates also can do a lot of external damage feeding on their predator, and the stress of the constant presence of food items in a confined space can also be detrimental to the reptile or amphibian. Feed only the number of invertebrates that can be eaten within 15-30 minutes and remove any left over to be offered at a later date.
As with herbivores, feed immature animals more frequently and adults less frequently. Feed smaller items that will be easily ingested more frequently. Those reptiles and amphibians that sit most of the time waiting for prey only need to be feed a batch of invertebrates weekly to even monthly, while those constantly chasing invertebrates and requiring high amounts of energy based on their metabolism (monitors), should be fed daily, until supplementing with vertebrate prey, when feeding frequency and amount can be diminished. Fast days can also be beneficial.
Carnivores
These include Asian water dragons, monitors, most snakes, and tegus. Items available include mice, rats, anoles, day old chicks, fish, gerbils, guinea pigs, and rabbits. Hamsters do not seem to be very palatable. Most owners of these types of reptiles seem to be drawn to the live feeding spectacle... and need to be broken of it when rodents are involved. Mice and especially rats can do severe damage to their predators, eating down to the bone and internal organs leading to expensive, slow healing wounds. With the exception of fish, it is safer and more humane to feed either freshly killed/stunned prey or better yet--frozen prey thawed in warm water to mammal body temperature. Soak in cup/bowl of hot water until no more abdominal ice crystals felt and body feels warm, then put in hot water for several minutes pre-feeding. Feed in a separate container than normal enclosure or soaking bin. As with invertebrates, feed smaller items more frequently, with many of these reptiles eating weekly to every six months depending on their size and amount of prey offered. Rats have a lot more fat in them than mice do, with rabbits and guinea pigs often exceeding rats in fat content. "Pinkies" which are newborn mice, are calcium deficient, so be careful in over feeding. Specialized reptiles that prefer birds, reptiles, or amphibians can be converted to mice by rubbing a normal prey item onto the mice to scent it. For fish, try to avoid goldfish and try getting baitfish minnows and feed in a bowl or tank--remember their water needs to be dechlorinated or they may die before the predator gets to them. To stimulate a carnivore to eat a dead prey item, make sure it is warm, try moving it with long tweezers or tongs as if alive, or the skull can be opened to expose brain (fat). Pull any prey not eaten within two to three hours and dispose of it rather than refreezing it. Obesity is a huge problem for carnivores too, as they are usually opportunistic feeders--when food is available, they will eat all they can. Most vertebrate prey is a well-packaged meal, requiring no supplementation. Variety is not of great concern here.
Omnivores
Being an omnivore is the best (or worst) of all worlds for box turtles, many semi-aquatic turtles, transitioning bearded dragons, blue-tongued skinks. The balance here does not differ greatly from ourselves: lots of the herbivore diet, less of the insectivore diet, very little of the carnivore diet (ok, so most humans do not eat insects). So while salads may be offered daily, insects should be once or twice weekly in small quantities, and a mouse should be a weekly to monthly treat. Many omnivores become persnickety and may only eat single items (box turtles with earthworms for example). Variety needs to be continually tried and modified, though food waste will occur.
Critical Patients
Obviously, palatability is of concern here when self-feeding is still an option, and some of the above rules need to be tossed aside in these cases until the illness is resolved. Some reptiles going several weeks without eating may not be a concern, dependent on species, and may actually be a normal physiologic response such as pre-ecdysis. However, when an animal has gastrointestinal or respiratory compromise, they may refuse to or be unable to eat on their own. At these times, I will gavage feed the reptile Oxbow Hay® with an (Oxbow Animal Health, Murdock, NE 68407 USA) product: Herbivore Critical Care, Carnivore Critical Care (also use in insectivores), or a combination of the two for omnivores.
Having tried other veterinary and human products, I have found these items to be simple and to meet almost all the needs nutritionally of most commonly kept reptiles and amphibians. The amount and frequency to feed depends on the state of the patient. Mixing is simple and can be sold to the clients in individual bags or large tubs (also for in-hospital use). It is better to start with small quantities rather than to try to overfeed. For an adult iguana, 20 mL daily should be plenty. An adult ball python may receive 60mL once weekly. Use the guidelines for normal feeding of a reptile or amphibian to decide how often and how much to feed the critical patient. Tortoises do well with anesthesia to place esophageal feeding tubes for home care and the critical care diet daily works extremely well with most size tubes, though very small tubes may need to feed with human baby food products.
Reference
1. Donoghue S. 2006. Nutrition. IN: Mader DR (ed). Reptile Medicine and Surgery, 2nd Edition. Elsevier: St. Louis, MO. p 251-298.