a UVB and heat gradient with the UVB bulb covering one‐third to three‐quarters of the cage length/depth.3 Turn UVB light on for 10–12 hours daily.
Note: Depending on the type of mesh, there may be a 30–50% reduction in UVB, which must be taken into consideration when selecting the bulb. The use of a UVB radiometer is more critical when the bulb is placed above a mesh to determine the actual UVB exposure.
Suggested Reading
1 Acierno MJ, Mitchell MA, Zachariah TT, et al. Effects of ultraviolet radiation on plasma 25‐hydroxyvitamin D3 concentrations in corn snakes (Elaphe guttata). Am J Vet Res 2008; 69(2):294–297.
2 Burger RM, Gehrmann WH, Ferguson GW. Evaluation of UVB reduction by materials commonly used in reptile husbandry. Zoo Biol. 2007; 26:417–423.
Anatomy and Physiology of Amphibians
Amphibians are a group of vertebrates with significant conservation challenges. They have a worldwide distribution, but most species occur in tropical climates. There are over 8,000 amphibian species distributed among three orders: Anura, Cuadata, and Gymnophiona.
The order Anura includes frogs and toads and comprises the majority of amphibians with over 7,100 known species. In general terms, frogs live in or near water while toads are more terrestrial. As such, frogs have thinner more permeable skin while toads have thicker skin to protect against dehydration. The vast majority of anurans are oviparous but a few ovoviviparous to viviparous species exist.
The order Caudata includes salamanders and newts, with over 700 species. Salamanders are primarily terrestrial while newts are aquatic to semiaquatic. Sirens and amphiumas are primarily aquatic. Amphibians in this order are mostly oviparous and are capable of neoteny, which is the retention of juvenile characteristics in fully mature animal. As such, neotenic amphibians can reproduce despite the retention of juvenile features like gills. There are three types of neoteny described—obligate, inducible, and facultative. Obligate neotenic species are not able to metamorphose either in nature or through experimental stimulation. The families Amphiumidae (amphiumas), Sirenidae (sirens), Proteidae (mudpuppies, waterdogs, European blind cave salamander), and Cryptobranchidae (giant salamanders) are all known to express obligate neoteny. Inducible obligate neotenic species do not metamorphose in nature but can be induced to metamorphose through the administration of thyroxine or thyroid‐stimulating hormone. Perhaps the best‐known neotenic species, the axolotl (Ambystoma mexicanum) is an inducible obligate neotenic species, although many specimens are reported to have a shorter lifespan after metamorphosis. Some members of the family Plethodontidae (lungless salamanders) are also inducible obligate neotenic species. Metamorphosis in the facultative neotenic species occurs according to environmental conditions, most commonly associated with the lack of development of the thyroid gland. Facultative neotenic species can be found in the families Ambystomatidae, Salamandridae (true salamanders, newts), and Hynobiidae (Asiatic salamanders). The tiger salamander (Ambystoma tigrinum) is the best‐known facultative neotenic species.
The order Gymnophiona includes caecilians, which are a lesser known and studied group with over 200 species. Caecilians resemble earthworms and have reduced or absent eyes and limbs. There are both aquatic and terrestrial species that are mostly found in tropical lowlands. New world species are primarily viviparous while those from the old‐world are oviparous.
METABOLISM
When housed in captivity, providing an appropriate temperature and thermal gradient is critical for amphibian health. Amphibians may show a variety of clinical signs at extreme temperatures (Figure 2– see web image supplementary content for section I).
Amphibians are sensitive to desiccation and have developed techniques for regulating body temperature and retaining moisture. Changes in posture, locomotion, and skin color can be used to regulate temperature. Cooling can be accomplished by standing away from the ground and changing to a pale body color to decrease heat absorption. Peripheral vasodilation or constriction can also regulate the rate at which they dissipate or absorb heat. Some amphibians, anurans in particular, can produce glandular secretions that allow them cover part of their bodies in a waxy substance to protect against desiccation. These secretions are usually limited to the dorsal skin only, as ventral skin must remain thin to allow water uptake via the pelvic drink patch. Up to 80% of the total water uptake may occur through this pelvic patch. Dried mucus may also be used to cover the skin. Finally, they have iridophores that provide high skin reflectivity near the infrared spectrum (700–900 μm) to decrease solar heat exposure.
Some amphibians have developed specialized methods for surviving cold temperatures through a technique known as super cooling. Super cooling lowers the freezing point of tissues through a combination of physiological adaptations: Higher amount of fibrinogen, glucose‐transporting proteins and ice‐nucleating proteins in blood, low molecular weight carbohydrates (glycerol or glucose) in blood and tissues, and increasing plasma osmolarity by dehydration. These adaptations also promote ice growth in extracellular compartments while sparing the cells. In addition, they have good anoxia tolerance for freeze‐induced ischemia.
ANATOMY
Anurans can range from 1 cm to 30 cm in length and have glandular skin that may be smooth or have protuberances. Adults lack gills and claws are absent except in Xenopus sp. While anurans lack tails, frogs in the genus Ascaphus are referred to as tailed frogs due to the presence of a cloacal protrusion resembling a tail, which is actually an organ used by the males for copulation and internal fertilization.
Salamanders range from 4 cm to 1.5 m in length, have glandular skin and lack claws. Most have four legs except sirens, which lack pelvic limbs.
Caecilians range from 7.5 cm to 75 cm in length and lack limbs. A short tail is present in some species. A unique feature is the presence of small olfactory and sensory tentacles in the nasolabial groove.
Skeleton
| Order | Features |
|---|---|
| Anurans | 4 legs with 4 toes on the front and 5 toes on the back |
| Fused vertebrae—presacral, sacral, postsacral | |
| Lack sacrum | |
| Pelvic girdle is fused | |
| Forelimb—scapula, humerus, radioulnar joint, carpals, metacarpals, phalanges | |
| Hind limb—femur, tibiofibular joints, tarsals, metatarsal, phalanges | |
| Urostyle—fused caudal vertebrae | |
| Caudata | 4 legs but hind reduced in mud eels and missing in sirens |
| 4 toes on the front and 5 toes on the back | |
| Capable of limb and toe regeneration | |
| Tail autotomy | |
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