Despite many recent advances in the field of zoological companion animals, inappropriate nutrition is still a leading cause of disease in pet birds, reptiles, and many small mammals. Biological factors, such as age, sex, and reproductive status of these species, impact their nutritional needs like those of other domestic species. However, many of these exotic species have additional factors to consider, such as environmental temperature and seasonality (hibernation/brumation). Rabbits (Oryctolagus cuniculus) Wild rabbits are considered concentrate selectors, and feed on the most nutrient-dense portions of vegetation. They also eat approximately 30 times per day, for four to six minutes at each feeding in the wild. Rabbits have a large cecum, consistent with other hind-gut fermenting herbivores, which holds 40 percent of the GI contents, and is the largest organ in their body. Contrary to most larger hind-gut fermenters (i.e. horses), they eliminate fiber from their cecum and GI tract as quickly as possible.1 Despite the extensive fermentation that occurs throughout their GI tract, their total GI tract transit time is only 19 hours. Rabbits produce two types of feces: dry, hard fecal pellets and soft fecal pellets (cecotrophs , also called "night feces"). This second type of feces is ingested, and allows rabbits to absorb more nutrients from their diet due to their relatively fast GI tract transit time for an herbivore. Rabbits also have a unique calcium metabolism—they absorb all the calcium they ingest, independent of vitamin D. The excess calcium is excreted in the urine, which can lead to hypercalciuria and urolithiasis if there is a high amount of calcium in their diet, such as with alfalfa hay or alfalfa-based pellets.1 Rabbits require a high fiber, low calcium diet. The majority of their daily diet should be comprised of a high-quality grass hay (e.g. timothy hay) and fresh greens (Figure 1). Oxalate-containing plants (spinach, Swiss chard, collards, etc.) have been linked to increased risks of calcium oxalate uroliths in people, but this has never been proven in rabbits. In general, it is not recommended to feed these greens in large quantities or exclusively to rabbits, (See: “Green is in”). Offering a variety of greens is key! There are numerous published, and sometimes contradictory, feeding recommendations for pet rabbits. Most sources recommend feeding 1/8 – 1⁄4 cup of pellets once daily per 5 lbs. of body weight. Excessive pellets in diet often lead to obesity and potentially contribute to dental disease. Pellet mixes containing seeds or other concentrates (such as muesli- style mixes) should be avoided due to their high energy density. Juveniles and pregnant/lactating does should always be provided pellets due to their increased energy demands. Green is in Examples of healthy greens for rabbits: Kale Mint Clover Endive Parsley Cilantro Chicory Escarole Raddichio Carrot tops Radish tops Beet greens Swiss chard Watercress Wheat grass Collard greens Broccoli leaves Mustard Greens Romaine lettuce Dandelion Greens Green/Red leaf lettuce It has been debated whether or not pellets are required in a pet rabbit’s diet; however, they do provide an important source of trace minerals, some fat-soluble vitamins, and essential fatty acids.1 Food items high in starch and sugar (including carrots and fruits) can lead to ileus secondary to alterations in cecal flora. Therefore, these items should only be given sparingly to rabbits. Rabbits also require a large amount of fresh water daily (fluid requirement ~100 mL/kg/day), which can be provided in the form of a sipper bottle or bowl. Bowls become soiled more readily than bottles; however, rabbits have been found to drink more water when offered a bowl vs. a bottle.2 Guinea pigs (Cavia porcellus) Similar to rabbits, guinea pigs also possess a large cecum and are hindgut fermenters. Their cecum may hold up to 65 percent of their entire gastrointestinal contents.3 Their total GI transit time is approximately 20 hours. Also, similar to rabbits, guinea pigs practice coprophagy (ingestion of feces), which may provide additional opportunity for digestion and B vitamin absorption. Unlike rabbits, however, guinea pigs do not produce cecotrophs and only have one type of feces. Like all rodents, guinea pigs cannot vomit; therefore, fasting before anesthesia is not necessary or recommended. Guinea pigs, like primates and humans, lack the enzyme L-gluconolactone oxidase, which is responsible for the synthesis of the active form of vitamin C.3 Therefore, they require a source of vitamin C in their diet (10-20 mg/guinea pig/day for non-breeding adults). Pelleted diets for guinea pigs are fortified with ascorbic acid (vitamin C); however, this is a labile vitamin, and is sensitive to oxidation, particularly with exposure to light and heat. It is estimated that half of the initial vitamin C will be oxidized 90 days after initial milling of the pellets. Vitamin C will also oxidize and degrade in the presence of certain metallic ions, many of which are found in household drinking water. Therefore, the best source of vitamin C for the pet guinea pig is a combination of a guinea pig- specific pelleted diet and fresh foods that contain high levels of vitamin C. (See: “Where can guinea pigs get vitamin C?”). Where can guinea pigs get vitamin C? Combine guinea pig-specific pelleted diet with these fresh foods: High vitamin C content Turnip greens Mustard greens Dandelion greens Kale Brussels sprouts Parsley Collards Moderate vitamin C content Broccoli leaves Broccoli florets Beet greens Cauliflower Oranges Most healthy adult guinea pigs should receive ~1/8 cup of pellets or less per day. Feeding excessive amounts of pellets can lead to obesity and dental disease. Approximately 75 percent of the diet should consist of high- quality grass hay, such as timothy hay. Alfalfa is high in calcium and protein compared to other types of hay; therefore, this should only be given in moderation to adult guinea pigs. Dietary grass hay is essential to ensure proper GI tract function and to prevent dental disease. Similar to rabbits, guinea pigs have continuously growing incisors, premolars, and molars. Fresh greens, ideally ones high in Vitamin C, should comprise five to 10 percent of the guinea pig’s diet. The amount of fruit, nuts, and seeds should be kept to a minimum. Many guinea pigs develop dietary preferences when they are young, and acceptance of new food items later in life can be a challenge. Ferrets (Mustela putorius furo) Ferrets are obligate carnivores and lack both a cecum and an ileocolic valve. Their GI tract is extremely short, even for a carnivore, with a corresponding short transit time of approximately three hours. Transit time for kits (juvenile ferrets) may be as short as one hour. Ferrets also have very few brush-border enzymes in their small intestine and are inefficient at carbohydrate absorption compared to other obligate carnivores, like domestic cats. The ideal diet for an adult ferret should be high in protein and fat, and low in fiber. A diet consisting of 15-30 percent fat is recommended for the average healthy adult ferret. Ideally, the protein component of their diet should be composed primarily of high-quality meat sources, and not grains. The total crude protein level is recommended to be between 30-35 percent.4 Some owners elect to feed their ferrets whole prey items (i.e. mice) as a portion of the diet or as their complete diet, as this more closely mimics their natural diet. Ferret kibble is the most common diet fed, and there are several commercial brands to choose from. Canned kitten foods can be used to supplement a kibble-based diet, which will also decrease the amount of carbohydrates ingested from the kibble. It is hypothesized that high carbohydrate concentrations in the diet may contribute to the formation of insulinomas. In addition, increased carbohydrates in the diet have also been associated with urolithiasis in ferrets. Like dogs and cats, grain- free diets for ferrets have become more readily available, and the long-term sequelae to feeding such diets to ferrets are still unknown. A recent case-controlled study found that ferrets with cystine urolithiasis were nearly 58 times more likely to have received a grain-free diet compared with the reference ferret population; however, a definitive causation could not be drawn from that study alone.5 There is ongoing research to evaluate the cause(s) for cystine urolithiasis in ferrets, which may also include genetic predisposition.6 Psittacine (multiple) Birds in the psittaciformes order are classified as florivores (diet consists mainly of the products of flowers). Within this group, there are several subclassifications, including granivores (seed and grain eaters; budgies and cockatiels), frugivores (fruit eaters; macaws), and nectarivores (nectar eaters; lorikeets and lories). Despite extensive research to date on avian nutrition, a majority of this data is based on galliform species (domestic chickens, turkeys, etc.). Therefore, many of the nutritional recommendations for parrots in captivity are based on a combination of scientific study and anecdotal information.7 A recent study evaluated the composition of the diets consumed by wild parrots, and 77 percent of those birds were observed eating insects, which could mean parrots may be more omnivorous than previously thought.8 As with other species, it is important to ask about the bird’s diet during its general history. Specifically, it is also imperative to inquire about not just what foods the bird is offered but also what the bird consumes. Based on a study of African grey parrots in captivity, when the birds were allowed to self- select their diet, it was deficient in 12 vitamins, minerals, and amino acids, as they mainly ingested seeds.9 While many owners think they are feeding a nutritionally complete diet, such as a seed/pellet mixture, in most cases, it is deficient, as parrots rarely consume enough pellets in comparison with the seeds. Another study tested this theory in Amazon parrots, and even though those birds were fed a nutritionally complete mixture of seeds, pellets, and fresh produce, they still had an imbalanced diet due to preferential selection of seeds.10 Seed-only diets are deficient in fat-soluble vitamins and minerals (calcium, zinc, iron) and often high in fat. Wild seed- eating parrots often consume upwards of 60 different types of seeds and expend significantly more energy in their day- to-day life compared with a captive parrot. Hypocalcemia is another significant concern for parrots eating a seed-based diet, especially those that are affected by chronic egg laying (Figure 2). A dorsoventral radiograph of an adult African Grey parrot (Psittacus erithacus) with historic hypocalcemia, which has since been corrected with an appropriate diet. Note the abnormal bowing of multiple long bones despite the normal bone quality. Pelleted diets should comprise 60-70 percent of a parrot’s diet. Vegetables (especially those high in calcium and vitamin A) and fruits should comprise 20 percent, and the remaining small portion could be a combination of nuts (almonds, walnuts, Brazil nuts) and seeds. This feeding scheme is likely not appropriate for all the 350 species of psittacines; however, it is a good starting point for those who are naturally granivores and frugivores. Another significant consideration for captive psittacines is the method of diet presentation. Wild parrots spend many hours of their day foraging for food, and this is the opposite of what occurs in captivity when their diet is presented in a stainless-steel bowl. The lack of foraging behavior and time has been linked to other behavioral problems, such as feather- destructive behavior. Therefore, veterinarians should discuss with owners not only the composition but also the presentation of the diet for companion parrots. Foraging can be as simple as offering food in multiple feeding stations or concealing food in various containers. Conversion from a seed-based diet to a pelleted diet will likely take time and patience, as many parrots will initially refuse to eat pellets. Three pellet conversion strategies (positive reinforcement for eating pellets, gradual replacement, and immediate replacement of pellets) were statistically evaluated in a recent manuscript. There were no statistical differences between the three methods, and overall, they were >90 percent successful in converting parrots from seeds to a pellet-based diet.11 Reptiles (multiple) Almost all reptiles are considered poikilothermic (animals whose body temperature adjusts to the environmental temperature) ectotherms (animals that gain heat from their environment vs. self-generation). This is an extremely important concept, as the environmental temperature greatly affects all aspects of their digestion and overall metabolic rate. Even at their appropriate POTZ (Preferred Optimum Temperature Zone), the average reptile’s metabolism is only 25 percent or less of a comparable-sized mammal. Their metabolism also varies significantly by gender and season. Common pet reptile species include herbivores (most tortoises), omnivores (many turtles), insectivores (many lizard species), and carnivores (snakes and others). In addition, some reptiles change their feeding strategy over time—juvenile bearded dragons are more insectivorous compared to adults of this species, which are more herbivorous.12 Many reptile species also go through normal periods of reduced metabolism and appetite, such as with brumation/hibernation. The frequency of feeding is also quite variable across all reptile species. Smaller, younger animals often require once daily feeding, while larger snakes often only eat once every one to two months. All of these factors make the calculation of their standard metabolic rate (SMR) challenging, to say the least. In addition, there are still large knowledge gaps for the nutrition of many common pet reptile species, so extrapolations are often required. The most common feeder vertebrates fed to reptiles are mice and rats (of varying ages/ sizes), and uncommonly rabbits, other small mammals, fish, or amphibians. In theory, whole prey provides the appropriate balance of amino acids, lipids, vitamins, and minerals, similar to what carnivorous reptiles would obtain in the wild. Frozen-thawed (frozen for less than six months) or fresh- killed prey is recommended over feeding live prey due to the risk of trauma to the predator from the prey. Commercially raised insect species are deficient in calcium and preformed Vitamins A, D3, and E. Therefore, these insects provide an inverse calcium- phosphorus ratio unless supplemented with calcium. A notable exception to this is black soldier fly larvae, which accumulate calcium in their exoskeleton compared with other commercially raised insects and their larvae. The two main forms of calcium supplementation for insect prey include dusting and gut loading, and ideally, both would be performed prior to feeding out to a reptile. Gut-loading insect prey 24- 48 hours prior to feeding is significantly more effective than dusting. Ideally, a gut-loading diet would contain at least eight percent calcium.13 Herbivorous reptiles should be provided with a variety of dark, leafy greens and high-fiber items, such as hay or natural grasses (grazing opportunities). These “salads” are often supplemented with calcium +/- vitamin D to ensure adequate supplementation, as the commercial varieties of most greens, fruits, and vegetables have a very different nutrient profile than their native counterparts. Many of the leafy greens recommended for rabbits would also be very appropriate for herbivorous reptiles. Most herbivorous tropical reptiles also will eat fruits in the wild, but due to the differences in commercial varieties, fruits should be offered sparingly. Diseases secondary to inappropriate diet and husbandry are still quite common in captive reptiles. The most common is nutritional secondary hyperparathyroidism, often called “metabolic bone disease.” This occurs when the diet is low in calcium and/or vitamin D3, a lack of exposure to appropriate amounts of UVB light, and occasionally due to diets high in phosphorus (reverse Ca:P ratio). This nutritional disease is uncommon in carnivorous reptiles being fed whole prey, but extremely common in insectivorous reptiles fed non- supplemented invertebrate prey or herbivores fed a diet lacking calcium supplementation. Diagnosis is often based on physical exam findings and poor bone quality on whole- body radiographs (Figure 3). An adult leopard gecko (Eublepharis macularius) with signs of nutritional secondary hyperparathyroidism including kyphosis and abnormal angulation of multiple limbs, likely secondary to pathologic fractures. Monitoring ionized calcium can be considered if there is evidence of severe hypocalcemia, including muscle fasciculations and seizures. Treatment for this potentially life-threatening condition includes calcium supplementation, provision of appropriate UVB lighting (species variability), analgesics if concurrent pathologic fractures, and most importantly, correction of the dietary and husbandry deficits. Olivia A. Petritz, DVM, DACZM, graduated from Purdue University, and then completed several internships and a residency in the field of zoo and exotic animal medicine. Dr. Petritz became a diplomate in the American College of Zoological Medicine in 2013, specializing in zoological companion animals (exotic pets). Petritz started an exotics service at a specialty hospital in Los Angeles, Calif. following her residency and is currently an Associate professor of Avian and Exotic Animal Medicine at North Carolina State University. References 1. Smith, S. M. “Gastrointestinal physiology and nutrition of rabbits.” Ferrets, Rabbits, and Rodents: Clinical Medicine and Surgery. 4th edn. Elsevier, Inc., New York (2021): 162-173. 2. Tschudin, A., et al. “Water intake in domestic rabbits (Oryctolagus cuniculus) from open dishes and nipple drinkers under different water and feeding regimes.” Journal of animal physiology and animal nutrition 95.4 (2011): 499-511. 3. Pignon, C, et al. “Guinea pigs.” Ferrets, Rabbits, and Rodents: Clinical Medicine and Surgery. 4th edn. Elsevier, Inc., New York (2021): 270-297. 4. Powers, Lauren V., and D. Perpiñán. “Basic anatomy, physiology, and husbandry of ferrets.” Ferrets, Rabbits, and Rodents; Elsevier: Amsterdam, The Netherlands (2020): 1-12. 5. Lamglait, Benjamin, et al. “Retrospective case control study of pet ferrets with cystine urolithiasis in Quebec, Canada: epidemiological and clinical features.” Journal of veterinary medicine and surgery 5.1 (2021). 6. Stockman, Jonathan, et al. “Cystine and amino acid concentrations in the urine of pet ferrets (Mustela putorius furo).” Journal of Exotic Pet Medicine 47 (2023): 59-64. 7. Koutsos, Elizabeth A., Kevin D. Matson, and Kirk C. Klasing. “Nutrition of birds in the order Psittaciformes: a review.” Journal of Avian Medicine and Surgery 15.4 (2001): 257-275. 8. Voltura, Elise V., et al. “Parrot Dietary Habits and Consumption of Alternate Foodstuffs.” Journal of Avian Medicine and Surgery 37.4 (2024): 297-313. 9. Ullrey, Duane E., Mary E. Allen, and David J. Baer. “Formulated diets versus seed mixtures for psittacines.” The Journal of nutrition 121 (1991): S193-S205. 10. Brightsmith, Donald J. “Nutritional levels of diets fed to captive Amazon parrots: does mixing seed, produce, and pellets provide a healthy diet?.” Journal of avian medicine and surgery 26.3 (2012): 149-160. 11. Cummings, Alison M., et al. “An evaluation of three diet conversion methods in psittacine birds converting from seed-based diets to pelleted diets.” Journal of Avian Medicine and Surgery 36.2 (2022): 145-152. 12. Barboza, Trinita, and Marjorie Bercier. “An Update on Companion Inland Bearded Dragon (Pogona vitticeps) Nutrition.” Veterinary Clinics: Exotic Animal Practice 27.1 (2024): 71-84. 13. Boykin, Kimberly, Amy Bitter, and Mark A. Mitchell. “Using a Commercial Gut Loading Diet to Create a Positive Calcium to Phosphorus Ratio in Mealworms (Tenebrio molitor).” Journal of Herpetological Medicine and Surgery 31.4 (2021): 302-306.
