Nutrition in Reptiles
Also see Reptiles.
Appropriate husbandry of reptiles is as important as providing adequate nutrients. Photoperiod, temperature, humidity, substrate, and cage “furniture” can affect feeding behavior and, thus, nutrient intake. Temperature and humidity gradients within a reptile enclosure allow the animal to select warm, dry spots or cooler, moist areas. Competition for preferred sites and for food pans in an enclosure with multiple animals should also be assessed. Sufficient numbers of warm spots, UVB exposure spots, and food pans should be available for all animals within an enclosure. Visual barriers may be useful to reduce competition for preferred sites or food dishes.
Prey such as rabbits, rats, or mice should come from commercial breeding centers and be offered dead to prevent injury to the reptile and for welfare reasons of the offered prey. Although it is not common, prey have been known to attack predators and can inflict serious bites. Offering dead prey can also reduce the chance of injury to the predator caused by striking the walls of the enclosure. However, some reptiles may initially need the stimulation of live prey, particularly if they are not adapted to captivity. The possibility of disease or parasite transmission from prey to predator should be considered.
Vertebrate prey should be fed nutritionally complete diets appropriate for the species (eg, mouse diet, rabbit diet, rat diet, etc). The nutrient content of the prey depends on what it is fed (eg, mice raised on a diet deficient in vitamin A have decreased liver storage of this essential nutrient). Additionally, if frozen mice or rats are routinely used to feed carnivorous reptiles, freezer storage conditions should be optimal (eg, ≤6 mo and in thick, plastic bags to retard deterioration). Methods of thawing that minimize water loss are also important. Because many carnivorous reptiles rely on their prey not only as sources of nutrients but also as sources of water, the state of hydration of the prey can be very important.
Familiarity with a species’ food habits in the wild is essential if appropriate foods and nutrient levels are to be offered. Common practice has been to offer two or more different prey species, because differences in nutrient content exist among vertebrate and invertebrate prey. Reduced dependence on a single food or prey species is also desirable, because some prey items may be periodically difficult to obtain. Dependence on a single prey item is frequently seen in snakes and may be unavoidable.
Many commercial diets for reptiles are marketed. Products for carnivorous, herbivorous, and omnivorous reptiles are now available in frozen, freeze-dried, canned, extruded, pelleted, or sausage forms. Acceptability may be better when the commercial diets are offered to reptiles when they are young. Appropriately formulated, manufactured diets for reptiles are a potentially simpler and more economical alternative to feeding fresh produce or live prey. However, some of these diets may not be formulated rationally, and frequently little information concerning micronutrient concentrations is provided by the manufacturers. When selecting a commercial product, the buyer should obtain accurate information about product formulation and specific nutrient concentrations. Unfortunately, little controlled research has been conducted on nutrient requirements of reptiles, and claims of product superiority may not have a scientific justification.
Herbivorous reptile pellets should make up 25%–50% of the diet of herbivorous reptiles. Animals should be fed 1%–4% of their body weight on a dry-matter basis. Vegetables with a low amount of oxalate should be fed to prevent kidney stones. A good quality grass hay or a so-called herbs-hay should be fed. No more than 50% of the diet should consist of fresh greens, fruits, and vegetables. The amount of fruit should be no more than 5%. In Europe, often herbs and dandelions are fed to herbivorous reptiles. Fresh, clean water must be available at all times.
See table: Recommended Nutrient Concentrations for Reptiles for recommended nutrient concentrations for reptiles.
Recommended Nutrient Concentrations for Reptiles
Vitamin C synthesis has been reported in many reptile species. It has been suggested that ulcerative stomatitis seen in snakes and lizards may be associated with a vitamin C deficiency, although there is no supportive evidence. In controlled studies with garter snakes (Thamnophis sp) fed supplemental vitamin C, tissue levels and body stores remained stable, although synthesis by the snakes was reduced.
Although most reptiles excrete nitrogen primarily as uric acid, aquatic reptiles typically excrete excess nitrogen as urea or ammonia. The relative proportions of various nitrogenous wastes may depend on the amount and composition of feed, frequency of feeding, and state of hydration. The excessive precipitation of urate crystals in joints, kidneys, or other organs (gout) can be a common condition in some species of captive reptiles. The etiology is not clear, but it is commonly thought that diets high in protein may predispose reptiles to gout. Impaired renal function and dehydration have also been suggested as possible causes.
If poor-quality protein is fed (unbalanced amino acids) or when tissue is catabolized for energy, uric acid excretion increases. Although gout in some reptiles is associated with increased circulating levels, postprandial transient increases in circulating uric acid may be seen in some species and confound the diagnosis. Assuring an adequate state of hydration in a susceptible animal may help prevent uric acid precipitation in joints and organs. Feeding diets low in protein to carnivorous reptiles is unwise, because they are adapted to feeding on high-protein prey.
Most vertebrates can either absorb vitamin D from the diet or synthesize it in the skin from 7-dehydrocholesterol using energy from ultraviolet (UVB) light of certain wavelengths (290–315 nm) in a temperature-dependent reaction. Thus, vitamin D is required in the diet only when endogenous synthesis is inadequate, as develops when animals are not exposed to UV light of appropriate wavelengths.
Many captive basking species appear susceptible to rickets or osteomalacia. Bone fractures, soft-tissue mineralization, renal complications, and tetany can develop. Reptiles frequently show few premonitory signs, although lethargy, inappetence, and reluctance to move are commonly reported. Serum calcium concentrations may not be diagnostically useful. Although blood levels of vitamin D can be measured, normal values for most species are not known. Supplementation with injectable calcium and vitamin D may provide some short-term relief. However, exposure to UV light, or lack of it, may be an important, yet often overlooked, factor in the differential diagnosis. Complicating the diagnosis may be soft-tissue mineralization, seen radiographically or at necropsy.
In green iguanas, metastatic calcification may not result from vitamin D toxicity. Iguanas with both fractured bones and extremely low or undetectable levels of circulating 25-hydroxycholecalciferol also had calcified soft tissues. The etiology of the metastatic calcification is not understood and is contrary to conventional understanding of the signs of vitamin D deficiency and toxicity in domestic species. Dietary sources of vitamin D may not be sufficient to prevent rickets and osteomalacia. Diets with as much as 3,000 IU vitamin D3/kg did not prevent bone fractures and cortical thinning in green iguanas. Bulbs emitting UVB placed over the lizards at ~12–18 in. for 12 hr/day appeared to reverse the signs in the least severely affected lizards.
Because some lizards seek a warm spot to increase body temperature, placement of a warming bulb, usually incandescent, adjacent to a UVB bulb helps ensure adequate exposure to UVB light. Exposure to unfiltered natural sunlight, depending on latitude, during warmer months and use of UVB bulbs during the rest of the year usually eliminate the risk of bone disease caused by insufficient absorption of calcium (due to a vitamin D deficiency).
Some lizard species may be unable to absorb sufficient dietary vitamin D3, although the reason is poorly understood. New World primates are believed to have exceptionally high dietary requirements for vitamin D, which may be related to lower numbers of vitamin D cellular receptors than are present in Old World primates. Similar metabolic differences may exist in some basking lizard species, although this has not been established. UVB bulbs are sold in pet stores, but label claims may not be reliable.
Three types of UVB lighting are on the market: fluorescent tubes, compact fluorescent lamps, and mercury lamps. Fluorescent tubes supply a diffuse light with a low amount of visible light. Heat radiation is low, and the UVB gradient is fairly uniform. The light from fluorescent tubes resembles more or less the natural UVB in the shade of a sunny day spread over a relatively large area. Compact fluorescent lamps provide a more intensive UVB gradient focused on a small area. These lamps are characterized by fairly low intensity visible light and little heat. Mercury lamps (vapor spot and narrow spots) produce an intensive UVB gradient on a smaller area, producing heat and an intense light.
Mercury lamps can become very hot. Reptiles must be prevented from getting burned during UVB basking. It is important to recognize that when a UVB lamp is added to a terrarium, the emission of UVB drops with the square of the distance; this explains the low exposure level of UVB at the level of the reptile when the lamp has been hung too high.
The radiation of UVB declines during the lighting time. In general, UVB lamps should be replaced once a year. However, it is best to regularly measure the amount of UVB with meters used in the artificial sunbath industry. A “D3 Yield Index” that compares the vitamin D3–producing ability of the lamp with the sun is under development.
How long and how much exposure to UVB is needed in reptiles is not exactly known. In general, reptiles that need UVB must be exposed to 30 min to 2 hr of UVB each day. However, there can be exceptions. For example, bearded dragons do not develop metabolic bone diseases if they are exposed only a few times a week to UVB for a limited time.
Enlisting the assistance of a specialist is advised, because there is no ideal UV bulb (see Environmental Lighting).