Frog
Skin

A frog's skin is protective, has a respiratory function, can absorb water, and helps control body temperature. It has many glands, particularly on the head and back, which often exude distasteful and toxic substances (granular glands). The secretion is often sticky and helps keep the skin moist, protects against the entry of moulds and bacteria, and makes the animal slippery and more able to escape from predators. The skin is shed every few weeks. It usually splits down the middle of the back and across the belly, and the frog pulls its arms and legs free. The sloughed skin is then worked towards the head where it is quickly eaten.

Being cold-blooded, frogs have to adopt suitable behaviour patterns to regulate their temperature. To warm up, they can move into the sun or onto a warm surface; if they overheat, they can move into the shade or adopt a stance that exposes the minimum area of skin to the air. This posture is also used to prevent water loss and involves the frog squatting close to the substrate with its hands and feet tucked under its chin and body. The colour of a frog's skin is used for thermoregulation. In cool damp conditions, the colour will be darker than on a hot dry day. The grey foam-nest tree frog (Chiromantis xerampelina) is even able to turn white to minimize the chance of overheating.

Many frogs are able to absorb water and oxygen directly through the skin, especially around the pelvic area, but the permeability of a frog's skin can also result in water loss. Glands located all over the body exude mucus which helps keep the skin moist and reduces evaporation. Some glands on the hands and chest of males are specialized to produce sticky secretions to aid in amplexus. Similar glands in tree frogs produce a glue-like substance on the adhesive discs of the feet. Some arboreal frogs reduce water loss by having a waterproof layer of skin, and several South American species coat their skin with a waxy secretion. Other frogs have adopted behaviours to conserve water, including becoming nocturnal and resting in a water-conserving position. Some frogs may also rest in large groups with each frog pressed against its neighbours. This reduces the amount of skin exposed to the air or a dry surface, and thus reduces water loss. Woodhouse's toad (Bufo woodhousii), if given access to water after confinement in a dry location, sits in the shallows to rehydrate. The male hairy frog (Trichobatrachus robustus) has dermal papillae projecting from its lower back and thighs, giving it a bristly appearance. These contain blood vessels and are thought to increase the area of the skin available for respiration.

Some species have bony plates embedded in the skin, a trait that appears to have evolved independently several times. In certain other species, the skin at the top of the head is compacted and the connective tissue of the dermis is co-ossified with the bones of the skull (exostosis).

Camouflage is a common defensive mechanism in frogs. Features such as warts and skin folds are usually on ground-dwelling frogs, for whom smooth skin would not provide such effective camouflage. Certain frogs change colour between night and day, as light and moisture stimulate the pigment cells and cause them to expand or contract. Some are even able to control their skin texture. The Pacific tree frog (Pseudacris regilla) has green and brown morphs, plain or spotted, and changes colour depending on the time of year and general background colour. The Wood frog (Lithobates sylvaticus) uses disruptive coloration including black eye markings similar to voids between leaves, bands of the dorsal skin (dorsolateral dermal plica) similar to a leaf midrib as well as stains, spots and leg stripes similar to fallen leaf features.
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