Urodeles

Salamanders are a group of amphibians typically characterized by their lizard-like appearance, with slender bodies, blunt snouts, short limbs projecting at right angles to the body, and the presence of a tail in both larvae and adults. All ten extant salamander families are grouped together under the order Urodela. Salamander diversity is highest in eastern North America, especially in the Appalachian Mountains; most species are found in the Holarctic realm, with some species present in the Neotropical realm.

Salamanders rarely have more than four toes on their front legs and five on their rear legs, but some species have fewer digits and others lack hind limbs. Their permeable skin usually makes them reliant on habitats in or near water or other cool, damp places. Some salamander species are fully aquatic throughout their lives, some take to the water intermittently, and others are entirely terrestrial as adults.

This group of amphibians is capable of regenerating lost limbs as well as other damaged parts of their bodies. Researchers hope to reverse engineer the regenerative processes for potential human medical applications, such as brain and spinal cord injury treatment or preventing harmful scarring during heart surgery recovery.

Members of the family Salamandridae are mostly known as newts and lack the costal grooves along the sides of their bodies typical of other groups. The skin of some species contains the powerful poison tetrodotoxin; these salamanders tend to be slow-moving and have bright warning coloration to advertise their toxicity. Salamanders typically lay eggs in water and have aquatic larvae, but great variation occurs in their lifecycles. Some species in harsh environments reproduce while still in the larval state.

Description

The skin lacks scales and is moist and smooth to the touch, except in newts of the Salamandridae, which may have velvety or warty skin, wet to the touch. The skin may be drab or brightly colored, exhibiting various patterns of stripes, bars, spots, blotches, or dots. Male newts become dramatically colored during the breeding season. Cave species dwelling in darkness lack pigmentation and have a translucent pink or pearlescent appearance.

Salamanders range in size from the minute salamanders, with a total length of , including the tail, to the Chinese giant salamander body form with a cylindrical trunk, four limbs, and a long tail. Except in the family Salamandridae, the head, body, and tail have a number of vertical depressions in the surface which run from the mid-dorsal region to the ventral area and are known as costal grooves. Their function seems to be to help keep the skin moist by channeling water over the surface of the body.

Some aquatic species, such as sirens and amphiumas, have reduced or absent hind limbs, giving them an eel-like appearance, but in most species, the front and rear limbs are about the same length and project sidewards, barely raising the trunk off the ground. The feet are broad with short digits, usually four on the front feet and five on the rear. Salamanders do not have claws, and the shape of the foot varies according to the animal's habitat. Climbing species have elongated, square-tipped toes, while rock-dwellers have larger feet with short, blunt toes. The tree-climbing salamander (''Bolitoglossa'' sp.) has plate-like webbed feet which adhere to smooth surfaces by suction, while the rock-climbing ''Hydromantes'' species from California have feet with fleshy webs and short digits and use their tails as an extra limb. When ascending, the tail props up the rear of the body, while one hind foot moves forward and then swings to the other side to provide support as the other hind foot advances.Stebbins & Cohen (1995) pp. 26–30

In larvae and aquatic salamanders, the tail is laterally flattened, has dorsal and ventral fins, and undulates from side to side to propel the animal through the water. In the families Ambystomatidae and Salamandridae, the male's tail, which is larger than that of the female, is used during the amplexus embrace to propel the mating couple to a secluded location. In terrestrial species, the tail moves to counterbalance the animal as it runs, while in the arboreal salamander and other tree-climbing species, it is prehensile. The tail is also used by certain plethodontid salamanders that can jump, to help launch themselves into the air. The tail is used in courtship and as a storage organ for proteins and lipids. It also functions as a defense against predation, when it may be lashed at the attacker or autotomised when grabbed. Unlike frogs, an adult salamander is able to regenerate limbs and its tail when these are lost.

Skin

The skin of salamanders, in common with other amphibians, is thin, permeable to water, serves as a respiratory membrane, and is well-supplied with glands. It has highly cornified outer layers, renewed periodically through a skin shedding process controlled by hormones from the pituitary and thyroid glands. During moulting, the skin initially breaks around the mouth, and the animal moves forwards through the gap to shed the skin. When the front limbs have been worked clear, a series of body ripples pushes the skin towards the rear. The hind limbs are extracted and push the skin farther back, before it is eventually freed by friction as the salamander moves forward with the tail pressed against the ground.Stebbins & Cohen (1995) pp. 10–16 The animal often then eats the resulting sloughed skin.

Glands in the skin discharge mucus which keeps the skin moist, an important factor in skin respiration and thermoregulation. The sticky layer helps protect against bacterial infections and molds, reduces friction when swimming, and makes the animal slippery and more difficult for predators to catch. Granular glands scattered on the upper surface, particularly the head, back, and tail, produce repellent or toxic secretions. Some salamander toxins are particularly potent. The rough-skinned newt (''Taricha granulosa'') produces the neurotoxin tetrodotoxin, the most toxic nonprotein substance known. Handling the newts does no harm, but ingestion of even a minute fragment of skin is deadly. In feeding trials, fish, frogs, reptiles, birds, and mammals were all found to be susceptible.

Mature adults of some salamander species have "nuptial" glandular tissue in their cloacae, at the base of their tails, on their heads or under their chins. Some females release chemical substances, possibly from the ventral cloacal gland, to attract males, but males do not seem to use pheromones for this purpose. In some plethodonts, males have conspicuous mental glands on the chin which are pressed against the females' nostrils during the courtship ritual. They may function to speed up the mating process, reducing the risk of its being disrupted by a predator or rival male. The gland at the base of the tail in ''Plethodon cinereus'' is used to mark fecal pellets to proclaim territorial ownership.

Senses

Olfaction in salamanders plays a role in territory maintenance, the recognition of predators, and courtship rituals, but is probably secondary to sight during prey selection and feeding. Salamanders have two types of sensory areas that respond to the chemistry of the environment. Olfactory epithelium in the nasal cavity picks up airborne and aquatic odors, while adjoining vomeronasal organs detect nonvolatile chemical cues, such as tastes in the mouth. In plethodonts, the sensory epithelium of the vomeronasal organs extends to the nasolabial grooves, which stretch from the nostrils to the corners of the mouth. These extended areas seem to be associated with the identification of prey items, the recognition of conspecifics, and the identification of individuals.Stebbins & Cohen (1995) pp. 37–40

The eyes of most salamanders are adapted primarily for vision at night. In some permanently aquatic species, they are reduced in size and have a simplified retinal structure, and in cave dwellers such as the Georgia blind salamander, they are absent or covered with a layer of skin. In amphibious species, the eyes are a compromise and are nearsighted in air and farsighted in water. Fully terrestrial species such as the fire salamander have a flatter lens which can focus over a much wider range of distances.Stebbins & Cohen (1995) pp. 42–44
To find their prey, salamanders use trichromatic color vision extending into the ultraviolet range, based on three photoreceptor types that are maximally sensitive around 450, 500, and 570 nm. The larvae, and the adults of some highly aquatic species, also have a lateral line organ, similar to that of fish, which can detect changes in water pressure.

All salamanders lack middle ear cavity, eardrum and eustachian tube, but have an opercularis system like frogs, and are still able to detect airborne sound. The opercularis system consists of two ossicles: the columella (equivalent to the stapes of higher vertebrates) which is fused to the skull, and the operculum. An opercularis muscle connects the latter to the pectoral girdle, and is kept under tension when the animal is alert. The system seems able to detect low-frequency vibrations (500–600 Hz), which may be picked up from the ground by the fore limbs and transmitted to the inner ear. These may serve to warn the animal of an approaching predator.Stebbins & Cohen (1995) pp. 69–72

Salamanders are usually considered to have no voice and do not use sound for communication in the way that frogs do; however, in mating system they communicate by pheromone signaling; some species can make quiet ticking or popping noises, perhaps by the opening and closing of valves in the nose. The California giant salamander can produce a bark or rattle, and a few species can squeak by contracting muscles in the throat. The arboreal salamander can squeak using a different mechanism; it retracts its eyes into its head, forcing air out of its mouth. The ensatina salamander occasionally makes a hissing sound, while the sirens sometimes produce quiet clicks, and can resort to faint shrieks if attacked. Similar clicking behaviour was observed in two European newts '' Lissotriton vulgaris'' and '' Ichthyosaura alpestris'' in their aquatic phase. Vocalization in salamanders has been little studied and the purpose of these sounds is presumed to be the startling of predators.Stebbins & Cohen (1995) pp. 76–77

Respiration

Respiration differs among the different species of salamanders, and can involve gills, lungs, skin, and the membranes of mouth and throat. Larval salamanders breathe primarily by means of gills, which are usually external and feathery in appearance. Water is drawn in through the mouth and flows out through the gill slits. Some neotenic species such as the mudpuppy (''Necturus maculosus'') retain their gills throughout their lives, but most species lose them at metamorphosis. The embryos of some terrestrial lungless salamanders, such as ''Ensatina'', that undergo direct development, have large gills that lie close to the egg's surface.Stebbins & Cohen (1995) pp. 17–25

When present in adult salamanders, lungs vary greatly among different species in size and structure. In aquatic, cold-water species like the southern torrent salamander (''Rhyacotriton variegatus''), the lungs are very small with smooth walls, while species living in warm water with little dissolved oxygen, such as the lesser siren (''Siren intermedia''), have large lungs with convoluted surfaces. In the terrestrial lungless salamanders (family Plethodontidae), no lungs or gills are present, and gas exchange mostly takes place through the skin, supplemented by the tissues lining the mouth. To facilitate this, these salamanders have a dense network of blood vessels just under the skin and in the mouth.

In the amphiumas, metamorphosis is incomplete, and they retain one pair of gill slits as adults, with fully functioning internal lungs. Some species that lack lungs respire through gills. In most cases, these are external gills, visible as tufts on either side of the head. Some terrestrial salamanders have lungs used in respiration, although these are simple and sac-like, unlike the more complex organs found in mammals. Many species, such as the olm, have both lungs and gills as adults.

In the ''Necturus'', external gills begin to form as a means of combating hypoxia in the egg as egg yolk is converted into metabolically active tissue. However, molecular changes in the mudpuppy during post-embryonic development primarily due to the thyroid gland prevent the internalization of the external gills as seen in most salamanders that undergo metamorphosis. The external gills seen in salamanders differs greatly from that of amphibians with internalized gills. Unlike amphibians with internalized gills which typically rely on the changing of pressures within the buccal and pharyngeal cavities to ensure diffusion of oxygen onto the gill curtain, neotenic salamanders such as Necturus use specified musculature, such as the levatores arcuum, to move external gills to keep the respiratory surfaces constantly in contact with new oxygenated water.

Feeding and diet

Salamanders are opportunistic predators. They are generally not restricted to specific foods, but feed on almost any organism of a reasonable size. Large species such as the Japanese giant salamander