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.[2]

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

X-ray image of salamander

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.[3]
Salamanders range in size from the minute salamanders, with a total length of 27 mm (1+1⁄8 in),
including the tail, to the Chinese giant salamander which reaches 1.8 m (6 ft) and weighs up to 65 kg
(145 lb). Most, however, are between 10 and 20 cm (4 and 8 in) in length.[4]

Trunk, limbs and tail

An adult salamander generally resembles a small lizard, having a basal tetrapod 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.[5]

Sirens have an eel-like appearance.

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.[6]

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.[6] 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.[6]
Skin

Rough-skinned newt

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.
[7] The animal often then eats the resulting sloughed skin.[3]

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.[7] 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.[8]

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.[9] 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.[10] The gland at the base of
the tail in Plethodon cinereus is used to mark fecal pellets to proclaim territorial ownership.[9]

Senses

The front part of the olm's head carries sensitive chemo-, mechano-, and electroreceptors.
Biofluorescence can be observed across various salamander species

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.[11]

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.[12] 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.[13] 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.[3]

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.[14][15] 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.[16] 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.[17]

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.[18] Vocalization in salamanders has been little studied and the purpose
of these sounds is presumed to be the startling of predators.[19]

Salamanders need moist environments to respire through their skin.

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.[20]

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.[20][21]

In the amphiumas, metamorphosis is incomplete, and they retain one pair of gill slits as adults, with fully
functioning internal lungs.[22] 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.[3]

A dissected view of the levatores arcuum muscles in a Necturus maculosus specimen. These (shown in
the purple circles) move the external gills, as a means of respiration.

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.[23] 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.[24] 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.[25][26]

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.[27] Large species such as the Japanese giant salamander
(Andrias japonicus) eat crabs, fish, small mammals, amphibians, and aquatic insects.[28] In a study of
smaller dusky salamanders (Desmognathus) in the Appalachian Mountains, their diet includes
earthworms, flies, beetles, beetle larvae, leafhoppers, springtails, moths, spiders, grasshoppers, and
mites.[27] Cannibalism sometimes takes place, especially when resources are short or time is limited.
Tiger salamander tadpoles in ephemeral pools sometimes resort to eating each other, and are seemingly
able to target unrelated individuals.[29] Adult blackbelly salamanders (Desmognathus quadramaculatus)
prey on adults and young of other species of salamanders, while their larvae sometimes cannibalise
smaller larvae.[30]

The head of a tiger salamander

Most species of salamander have small teeth in both their upper and lower jaws. Unlike frogs, even the
larvae of salamanders possess these teeth.[3] Although larval teeth are shaped like pointed cones, the
teeth of adults are adapted to enable them to readily grasp prey. The crown, which has two cusps
(bicuspid), is attached to a pedicel by collagenous fibers. The joint formed between the bicuspid and the
pedicel is partially flexible, as it can bend inward, but not outward. When struggling prey is advanced
into the salamander's mouth, the teeth tips relax and bend in the same direction, encouraging
movement toward the throat, and resisting the prey's escape.[31] Many salamanders have patches of
teeth attached to the vomer and the palatine bones in the roof of the mouth, and these help to retain
prey. All types of teeth are resorbed and replaced at intervals throughout the animal's life.[32]

A terrestrial salamander catches its prey by flicking out its sticky tongue in an action that takes less than
half a second. In some species, the tongue is attached anteriorly to the floor of the mouth, while in
others, it is mounted on a pedicel. It is rendered sticky by secretions of mucus from glands in its tip and
on the roof of the mouth.[33] High-speed cinematography shows how the tiger salamander (Ambystoma
tigrinum) positions itself with its snout close to its prey. Its mouth then gapes widely, the lower jaw
remains stationary, and the tongue bulges and changes shape as it shoots forward. The protruded
tongue has a central depression, and the rim of this collapses inward as the target is struck, trapping the
prey in a mucus-laden trough. Here it is held while the animal's neck is flexed, the tongue retracted and
jaws closed. Large or resistant prey is retained by the teeth while repeated protrusions and retractions
of the tongue draw it in. Swallowing involves alternate contraction and relaxation of muscles in the
throat, assisted by depression of the eyeballs into the roof of the mouth.[34] Many lungless salamanders
of the family Plethodontidae have more elaborate feeding methods. Muscles surrounding the hyoid
bone contract to store elastic energy in springy connective tissue, and actually "shoot" the hyoid bone
out of the mouth, thus elongating the tongue.[35][36] Muscles that originate in the pelvic region and
insert in the tongue are used to reel the tongue and the hyoid back to their original positions.[37]

An aquatic salamander lacks muscles in the tongue, and captures its prey in an entirely different
manner. It grabs the food item, grasps it with its teeth, and adopts a kind of inertial feeding. This
involves tossing its head about, drawing water sharply in and out of its mouth, and snapping its jaws, all
of which tend to tear and macerate the prey, which is then swallowed.[37]

Though frequently feeding on slow-moving animals like snails, shrimps and worms, sirenids are unique
among salamanders for having developed speciations towards herbivory, such as beak-like jaw ends and
extensive intestines. They feed on algae and other soft-plants in the wild, and easily eat offered lettuce.
[38]

Defense

Further information: Antipredator adaptations

Salamanders have thin skins and soft bodies, and move rather slowly, and at first sight might appear to
be vulnerable to opportunistic predation. However, they have several effective lines of defense. Mucus
coating on damp skin makes them difficult to grasp, and the slimy coating may have an offensive taste or
be toxic. When attacked by a predator, a salamander may position itself to make the main poison glands
face the aggressor. Often, these are on the tail, which may be waggled or turned up and arched over the
animal's back. The sacrifice of the tail may be a worthwhile strategy, if the salamander escapes with its
life and the predator learns to avoid that species of salamander in the future.[39]

Aposematism

Further information: aposematism

Skin secretions of the tiger salamander (Ambystoma tigrinum) fed to rats have been shown to produce
aversion to the flavor, and the rats avoided the presentational medium when it was offered to them
again.[40] The fire salamander (Salamandra salamandra) has a ridge of large granular glands down its
spine which are able to squirt a fine jet of toxic fluid at its attacker. By angling its body appropriately, it
can accurately direct the spray for a distance of up to 80 cm (30 in).[41]

The Iberian ribbed newt (Pleurodeles waltl) has another method of deterring aggressors. Its skin exudes
a poisonous, viscous fluid and at the same time, the newt rotates its sharply pointed ribs through an
angle between 27 and 92°, and adopts an inflated posture. This action causes the ribs to puncture the
body wall, each rib protruding through an orange wart arranged in a lateral row. This may provide an
aposematic signal that makes the spines more visible. When the danger has passed, the ribs retract and
the skin heals.[42]