Moulting

Cicada molting

In biology, moulting or molting^[1] (both pronounced /ˈmoʊltɪŋ/), also known as sloughing, shedding, or for some species, ecdysis, is the manner in which an animal routinely casts off a part of its body (often but not always an outer layer or covering), either at specific times of year, or at specific points in its life cycle.

Moulting can involve the epidermis (skin), pelage (hair, fur, wool), or other external layer. In some species, other body parts may be shed, for example, wings in some insects. Examples include old feathers in birds, old hairs in mammals (especially dogs and other canidae), old skin in reptiles, and the entire exoskeleton in arthropods.

Examples

Species	Item shed	Timing	Known as	Notes
Chickens	Feathers	Usually autumn	Moulting	Chickens generally stop laying eggs before their moulting begins until their new set of feathers has grown back.[2][3]
Dogs and other canids	Hair (Fur)	Semi-annually, spring and fall (autumn)	Shedding Moulting	Moulting in canids, as in all mammals,[4] is thought to be due to fluctuations in the amount of melatonin secreted by their pineal gland in response to seasonal sunlight variations. It has nothing to do with seasonal temperature variations. This seasonality in shed pattern is most preserved in Arctic breeds of dogs that shed twice a year,[5] other breeds now shed year round.
Snakes	Skin	Regularly, when old skin is outgrown	Moulting	Snakes will rub against rough surfaces to shed their skin.
Lizards	Skin	Regularly, when old skin is outgrown	Moulting	Lizards consume their shed skin for calcium, grooming, and other nutrients
Hermit crabs	Exoskeleton	Regularly, when the carapace is outgrown	Moulting	Land hermit crabs bury themselves for many weeks while they moult and consume their exoskeleton.
Amphibians	Skin	Regularly	Moulting	Salamanders and frogs shed their skins regularly, and often eat it.

Specific order notes

Birds

See also: Plumage

A Loggerhead Shrike in mid-moult (left) compared to one with regular plumage (right).

A moulting yellow-eyed penguin.

In birds, moulting is the periodic replacement of feathers by shedding old feathers while producing new ones. Feathers are dead structures at maturity, and they become gradually worn down and need to be replaced. Adult birds moult at least once a year, though many moult twice, and a few three times.^[6] It is a comparatively slow process, as a bird never sheds all its feathers at once; it must keep enough of its feathers to regulate its body temperature and repel moisture. The amount of shed feathers varies. In some moulting periods, a bird may renew only the feathers on the head and body, shedding the wing and tail feathers during a later moulting period.^[6] Some species of wild bird become flightless during an annual "wing moult" and must seek protected habitat with a reliable food supply during that time. While plumage may look thin or uneven during moult, the general shape is maintained despite the loss of apparently many feathers; genuine bald spots are typically signs of unrelated illnesses, such as gross injuries, parasites, feather pecking (in commercial poultry), or (in pet birds) feather plucking.

The process of moulting in birds is as follows: First, the bird begins to shed some old feathers, then pin feathers grow in to replace the old feathers. As the pin feathers become full feathers, other feathers are shed. This is a cyclical process that occurs in many phases. It is usually symmetrical, with feather loss equal on each side of the body.^[6] Because feathers make up 4-12 percent of a bird's body weight, it takes a large amount of energy to replace them. For this reason, moults are frequently timed to occur right after the breeding season, but while food is still abundant. The plumage produced during this time is called postnuptial plumage.^[6]

Forced moulting

Main article: Forced molting

In some countries, flocks of layer hens are force moulted to reinvigorate egg-laying. This usually involves complete withdrawal of their food (and sometimes water) for 7-14 days or up to 28 days under experimental conditions^[7] which presumably reflect standard farming practice in some countries. This causes a body weight loss of 25 to 35%,^[8] which stimulates the hen to lose her feathers, but also reinvigorates egg-production. Some flocks may be force moulted several times. In 2003, more than 75% of all flocks were moulted in the US.^[9] Other methods of inducing a moult include low-density diets (e.g. grape pomace, cotton seed meal, alfalfa meal)^[10] or dietary manipulation to create an imbalance of a particular nutrient(s). The most important among these include manipulation of minerals including sodium (Na), calcium (Ca), iodine (I) and zinc (Zn), with full or partially reduced dietary intakes.^[11]

Dogs

Main article: Coat (dog)

Dogs shed their fur twice a year, in the spring and fall(or more often), depending on breed, living environment, and temperature. The shedding of dogs is never called "moulting". As dogs live with humans, the topic of dog shedding is of more immediate interest to the average pet owner than the shedding cycles of wild canidae. Wikipedia articles on individual dog breeds may contain more information on the shedding cycle of a particular breed.

The following is a list of breeds that are alleged by a pet website to have little or no shedding, but no proof or research is cited. Such breeds are often claimed to be hypoallergenic, but relying on such information can prove dangerous to an allergic person.^[12] The same pet website page from which the following list was obtained clearly states "All dogs shed".^[13]

Breeds alleged to shed very little

Affenpinscher	Basenji	Bedlington Terrier	Bergamasco	Bichon Frise	Bolognese
Brussels Griffon	Chinese Crested	Chinese Shar-Pei	Coton De Tulear	Dandie Dinmont Terrier	Giant Schnauzer and Standard Schnauzer
Havanese	Irish Water Spaniel	Lakeland Terrier	Lhasa Apso	Maltese	Miniature Schnauzer
Polish Lowland Sheepdog	Poodle	Portuguese Water Dog	Puli	Pumi	Shih Tzu
Soft-Coated Wheaten Terrier	Komondor	Welsh Terrier	West Highland White Terrier	Wirehaired Pointing Griffon	Yorkshire Terrier

Reptiles

Moulted snake skin

Close up view of snake's moulted skin

The most familiar example of moulting in reptiles is when snakes "shed their skin". This is usually achieved by the snake rubbing its head against a hard object, such as a rock (or between two rocks) or piece of wood, causing the already stretched skin to split. At this point, the snake continues to rub its skin on objects, causing the end nearest the head to peel back on itself, until the snake is able to crawl out of its skin, effectively turning the moulted skin inside-out. This is similar to how one might remove a sock from one's foot by grabbing the open end and pulling it over itself. The snake's skin is often left in one piece after the moulting process, including the discarded brille, so that the moult is vital for maintaining the animal's quality of vision. Conversely, the skins of lizards generally fall off in pieces.

Amphibians

A Northern Leopard frog moulting and eating the skin.

Both frogs and salamanders moult regularly and consume the skin, with some species moulting in pieces and others in one piece.^[14]

Arthropods

Main article: Ecdysis

In arthropods, such as insects, arachnids and crustaceans, moulting is the shedding of the exoskeleton (which is often called its shell), typically to let the organism grow. This process is called ecdysis. It is commonly said that ecdysis is necessary because the exoskeleton is rigid and cannot grow like skin, but this is simplistic, ignoring the fact that Chelonia do not shed even the rigid external keratinous shell, and that most Arthropoda with soft, flexible skins also undergo ecdysis. The subject is far more complex than a matter of skin rigidity; it includes considerations such as nature of metamorphosis, the differences between the morphology of successive instars, and the fact that a new skin includes new external lenses for eyes etc (compare this with the replacement of a snake's brille in sloughing). The new exoskeleton is initially soft but hardens after the moulting of the old exoskeleton. The old exoskeleton is referred to as "exuviae".

Moulting phase of a Southern Hawker

Humans

While humans do not strictly molt, an important part of the skin's defense against fungi is stratum corneum turnover that removes the microbes by shedding the old skin surface. This is increased by the immune system detection of dermatophytes (skin living fungi) such as Trichophyton rubrum.^[15]

References

1. ^ "Molting" vs. "moulting" – see spelling differences.
2. ^ Harold Alvah Nourse. A Book Of Complete And Reliable Information On The More Profitable Production Of Eggs On The City Lot, The Village Acre And The Farm. ed. 11. Webb publishing company, 1908. pp. 95-97.
3. ^ Nicole Faires. Deliberate Life: The Ultimate Homesteading Guide. p. 116.
4. ^ Characterizing a Mammalian Circannual Pacemaker
5. ^ Hair Loss in Dogs - Why Is My Dog Loosing It’s Hair? Alopecia X [sic]
6. ^ ^{a b c d} Terres, J. K. (1980). The Audubon Society Encyclopedia of North American Birds. New York, NY: Knopf. pp. 616–617. ISBN 0-394-46651-9. 
7. ^ Molino, A.B., Garcia, E.A., Berto, D.A., Pelícia, K., Silva, A.P. and Vercese F., (2009). The effects of alternative forced-molting methods on the performance and egg quality of commercial layers. Brazilian Journal of Poultry Science, 11: 109-113
8. ^ Webster, A.B., (2003). Physiology and behavior of the hen during induced moult. Poultry Science, 82: 992-1002
9. ^ Yousaf, M. and Chaudhry, A.S., (2008). History, changing scenarios and future strategies to induce moulting in laying hens. World's Poultry Science Journal, 64: 65-75
10. ^ Patwardhan, D. and King, A., (2011). Review: feed withdrawal and non feed withdrawal moult. World's Poultry Science Journal, 67: 253-268
11. ^ Khan, R.U., Nikousefat, Z., Javdani, M., Tufarelli, V. and Laudadio, V., (2011). Zinc-induced moulting: production and physiology. World's Poultry Science Journal, 67:497-506. doi:10.1017/S0043933911000547
12. ^ Nonallergenic Dog? Not Really by Denise Grady, New York Times, February 5, 1997
13. ^ Go Pets America: Dogs that do not shed - Retrieved September 7, 2008
14. ^ Frost, S. W. (1932). "Notes on Feeding and Molting in Frogs". The American Naturalist 66 (707): 530–540. doi:10.1086/280458. JSTOR 2456779. 
15. ^ Dahl MV. (1993).Suppression of immunity and inflammation by products produced by dermatophytes. J Am Acad Dermatol. 28(5 Pt 1):S19-S23. PMID 8496406

External links

• Crayfish Moulting Video on YouTube