Wattle (anatomy)

A wattle is a fleshy caruncle hanging from the head or neck of certain birds and mammals, and it is usually a paired structure.

In birds the term covers several kinds of caruncle, including facial patches, throat wattles, dewlaps, snoods and earlobes.

Wattles are often sexually dimorphic, and their size and colour can signal sex, health and genetic quality.

Structure

A wattle is made of skin and subcutaneous connective tissue over a core of vessels, fat and sometimes erectile tissue.
The surface may be bare or covered by small feathers, and the covering differs between species and between sexes.
Wattles are generally paired, but a single midline structure is sometimes called a dewlap.
In some birds the wattle is erectile, and it can be inflated during courtship or alarm.
The caruncle of the turkey and the comb of the chicken are related structures, but they differ in position and in vascular anatomy.
The wattle of the cassowary hangs from the throat and is supported by a cartilaginous or fibrous core.
In mammals, wattles occur as throat tassels in some goats and as fleshy appendages in some pig breeds.

In birds, wattles are mainly ornaments used in courtship and mate choice, and large wattles are often preferred by females.
Large wattles correlate with high testosterone, good nutrition and the ability to evade predators, and thus may advertise mate quality.
Ornamental organs such as wattles may be associated with genes for disease resistance, including MHC variants.^[1]
In umbrellabirds the wattle amplifies the bird's call, and it is inflated during vocal display.
Some wattles may also help with thermoregulation, because their large surface area can dissipate heat.
In mammals, throat wattles are usually non-erectile and are thought to be vestigial or selected for breed appearance.
Wattles can signal social status, and in some species their colour changes with breeding condition.

The wattle is supplied by branches of the facial artery and by the external carotid system, and its venous drainage follows the same routes.
In birds, the wattle receives branches of the external carotid artery and is drained by the jugular veins.
Sensory innervation comes from branches of the trigeminal nerve, and motor control of erection involves smooth muscle and autonomic fibres.
The erectile tissue of some wattles resembles cavernous tissue, and it fills with blood during display.
The vascularity of the wattle makes it prone to bleeding and to haematoma after trauma.
Because the wattle is thin and vascular, it is used in some species as a site for blood sampling or for assessing hydration.

Wattles develop from neural crest mesenchyme and from the skin covering the head and neck, and their growth is often hormone-dependent.
In many birds, wattle growth accelerates at sexual maturity under the influence of testosterone and other sex steroids.
Sexual dimorphism in wattle size arises from differential growth rather than from a different developmental origin.
Some domestic breeds have been selected for exaggerated wattles, which can impair feeding or vision.
The genetic basis of wattle colour and size is being studied in poultry and in wild birds.

In poultry, large wattles can be injured by fighting or by cold, and frostbite is a common welfare problem.
Dubbing, the surgical removal of wattles and combs, is practiced in some breeds but is restricted on welfare grounds.
Wattles are a site for fowlpox lesions and for bacterial dermatitis, and they can bleed heavily when injured.
In mammals, throat wattles may be removed for cosmetic reasons, and the procedure raises similar welfare questions.
Wattle colour is used in some species as an indicator of health, and pale wattles may signal anaemia or infection.
Because wattles are highly vascular, surgery requires careful haemostasis and attention to the airway in birds with throat wattles.

Wattles have evolved independently in many bird lineages, which makes them an example of convergent ornamentation.
Cassowaries, turkeys, pheasants, mynas and bellbirds all have wattles, but their positions and tissue composition differ.
Among mammals, goats and pigs are the best-known examples, and their wattles are smaller and less vascular than avian ones.
The presence of wattles in domestic breeds shows how artificial selection can exaggerate a naturally variable trait.
Wattles are absent in most reptiles and amphibians, although some lizards have throat fans that serve similar display functions.

Charles Darwin discussed wattles as examples of sexual selection, and he argued that ornaments could evolve through female choice.
John James Audubon and nineteenth-century ornithologists described wattles in their species accounts and plates.
The word wattle comes from an Old English term for a woven branch, and it was later applied to the fleshy neck folds of birds.
Dubbing was practiced for centuries in poultry breeding before welfare regulations restricted it.
Modern studies of wattle colour use spectrophotometry to quantify sexual signals that were once described only qualitatively.

Whether wattles honestly signal genetic quality or are arbitrary ornaments is still debated in sexual selection research.
The link between wattle size and MHC genotype has been reported in pheasants, but the mechanism remains unclear.
The erectile mechanism of avian wattles is poorly studied, and its homology with mammalian erectile tissue is uncertain.
Welfare standards for dubbing vary between countries, and the practice remains controversial among breeders and veterinarians.
Climate change may affect wattle development if temperature alters hormone levels or food availability during growth.

The chicken comb and wattles share a vascular plexus that responds to testosterone, which is why caponisation reduces both.
Turkey wattles are highly vascular and can change colour with excitation, which is used in display and in thermoregulation.
The white bellbird has a long wattle that is inflated during its very loud call, amplifying sound in the canopy.
Goat wattles are inherited as a simple dominant trait in some breeds, and their presence is used in breed descriptions.
The Red Wattle pig is named for its throat wattles, which are fleshy appendages covered by the same skin as the neck.

Ornithologists distinguish wattles from dewlaps, snoods and combs by position and tissue composition, although popular usage often merges them.
The same word is used for unrelated fleshy structures in birds and mammals, which is an example of descriptive rather than homologous terminology.

References

^M Baratti, et al.. Are large wattles related to particular MHC genotypes in the male pheasant?. Genetica. 2010, 138, 657-65. DOI: 10.1007/s10709-010-9440-5