Difference between revisions of "Molt" - New World Encyclopedia

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[[Image:Lyristes plebejus.jpg|240px|right|thumb|A [[cicada]] molting]]
 
[[Image:Lyristes plebejus.jpg|240px|right|thumb|A [[cicada]] molting]]
'''Molting''' (American English) or '''moulting''' (British English) is the routine shedding of old [[feather]]s in [[bird]]s, old [[hair]]s in [[mammal]]s, old [[skin]] in [[reptile]]s, and the entire exoskeleton in [[arthropod]]s.  
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'''Molting''' (American English) or '''moulting''' (British English) is the routine shedding of the outer covering of an animal, including old [[feather]]s in [[bird]]s, old [[hair]]s in [[mammal]]s, old [[skin]] in [[reptile]]s, and the entire exoskeleton in [[arthropod]]s.  
  
 
In the arthropods, such as [[insect]]s and [[crab]]s, molting also is known as '''ecdysis'''.
 
In the arthropods, such as [[insect]]s and [[crab]]s, molting also is known as '''ecdysis'''.
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==Molting in mammals==
 
==Molting in mammals==
Mammals replace old hair, a process called shedding. This is even true for marine mammals, such as the pinnipeds: walruses, sea lions, fur seals, and true seals.  
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In [[mammal]]s, the hair, fur, or wool that covers the animal is called a ''pelage''. The pelage is provides insulation, concealment on land, byoyancy and streamlining in water, and may be modified for defense or display (Ling 1970). Occasionally replacement or "shedding" of the pleage is essential for survival.  
  
The pattern of molting varies between species. Some mammals shed their hair year-around, replacing a few hairs at a time, while some may molts may be annual or semiannually, such as more strongly in the spring or summer months, or even more frequently. Elephant seals shed hair all at once, called a catastrophic mold.
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This process of molting in mammals, also called shedding, is true even for marine mammals, such as the pinnipeds (walruses, sea lions, fur seals, and true seals). Molting in mammals includes both shedding of hair and the outer layer of skin, with whales shedding their old skin.
  
A juvenal pelage is the first coat of hair of a mammal, and it is commonly of fine texture. Teh postjuvenile molt replaces this fur and gives way to the adult or subadult pelage.  
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Different pelages occur at different stages in the life history  and can relate to varying seasonal requirements dictated by the environment, such as climate, and life processes such as reproduction. (Ling 1970). A juvenal pelage is the first coat of hair of a mammal, and it is commonly of fine texture. The postjuvenile molt replaces this fur and gives way to the adult or subadult pelage. Molting is established before sexual maturity and even prenatally, and are inherent features of mmamals (Ling 1970).
  
 +
The pattern of molting varies between species. Some mammals shed their hair year-around, replacing a few hairs at a time, while some molts may be annual or semiannually, such as more strongly in the spring or summer months, or even more regularly. Elephant seals shed hair all at once, called a catastrophic mold. Beluga whales (''Delphinapterus leucas'') molt each summer, shedding their old yellow skin (Lowry 1994).
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Beluga whales tend to rub on coarse gravel to facilitate the removal of their skin, and the skin grows about 100 times faster than normal during the molting period (Lowry 1994).
 
   
 
   
 
 
==Molting in reptiles==
 
==Molting in reptiles==
 
The most familiar example of molting in [[reptile]]s is when [[snake]]s "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 a 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 molted skin inside-out. This is similar to how you might remove a sock from your foot by grabbing the open end and pulling it over itself. The snake's skin is often left in one piece after the molting process.
 
The most familiar example of molting in [[reptile]]s is when [[snake]]s "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 a 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 molted skin inside-out. This is similar to how you might remove a sock from your foot by grabbing the open end and pulling it over itself. The snake's skin is often left in one piece after the molting process.
  
 
==Molting in arthropods (ecdysis)==
 
==Molting in arthropods (ecdysis)==
{{main|Ecdysis}}
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In [[arthropod]]s, such as [[insect]]s, [[arachnid]]s and [[crustacean]]s, moulting is the shedding of the [[exoskeleton]] (which is often called its [[Animal shell|shell]]), typically to let the organism grow. This process is called [[ecdysis]]. Ecdysis is necessary because the exoskeleton is rigid and cannot grow (like [[skin]]). The new exoskeleton is initially soft but hardens after the moulting of the old exoskeleton.
+
In [[arthropod]]s, such as [[insect]]s, [[arachnid]]s and [[crustacean]]s, molting is the shedding of the exoskeleton (which is often called its shell), typically to let the organism grow. This process is called [[ecdysis]]. Ecdysis is necessary because the exoskeleton is rigid and cannot grow (like skin). The new exoskeleton is initially soft, but hardens after the molting of the old exoskeleton.
  
 
[[Image:Callinectes sapidus.jpg|thumb|right|240px|Series of pictures showing the progression of ecdysis in ''[[Blue crab|Callinectes sapidus]]'', the blue crab]]
 
[[Image:Callinectes sapidus.jpg|thumb|right|240px|Series of pictures showing the progression of ecdysis in ''[[Blue crab|Callinectes sapidus]]'', the blue crab]]

Revision as of 17:30, 23 December 2006

A cicada molting

Molting (American English) or moulting (British English) is the routine shedding of the outer covering of an animal, including old feathers in birds, old hairs in mammals, old skin in reptiles, and the entire exoskeleton in arthropods.

In the arthropods, such as insects and crabs, molting also is known as ecdysis.

Molting in birds

Molting in birds is a comparatively slow process, as a bird never sheds all its feathers at once; it must keep enough feathers to regulate body temperature and repel moisture. However, some species of birds become flightless during an annual "wing molt" and must seek protected habitat with a reliable food supply during that time.

The process of molting in birds is as follows:

  1. The bird begins to shed some old feathers
  2. Pin feathers grow in to replace the old feathers
  3. As the pin feathers become full feathers, other feathers are shed

This is a cyclical process that happens in many phases. Commonly, a molt begins at a bird's head, progresses down the body to its wings and torso, and finishes with the tail feathers.

A molting bird should never have any bald spots. If a pet bird such bald spots, the bird should be taken to an avian veterinarian to search for possible causes for the baldness, which may include giardia, mites, or feather-plucking.

Molting in mammals

In mammals, the hair, fur, or wool that covers the animal is called a pelage. The pelage is provides insulation, concealment on land, byoyancy and streamlining in water, and may be modified for defense or display (Ling 1970). Occasionally replacement or "shedding" of the pleage is essential for survival.

This process of molting in mammals, also called shedding, is true even for marine mammals, such as the pinnipeds (walruses, sea lions, fur seals, and true seals). Molting in mammals includes both shedding of hair and the outer layer of skin, with whales shedding their old skin.

Different pelages occur at different stages in the life history and can relate to varying seasonal requirements dictated by the environment, such as climate, and life processes such as reproduction. (Ling 1970). A juvenal pelage is the first coat of hair of a mammal, and it is commonly of fine texture. The postjuvenile molt replaces this fur and gives way to the adult or subadult pelage. Molting is established before sexual maturity and even prenatally, and are inherent features of mmamals (Ling 1970).

The pattern of molting varies between species. Some mammals shed their hair year-around, replacing a few hairs at a time, while some molts may be annual or semiannually, such as more strongly in the spring or summer months, or even more regularly. Elephant seals shed hair all at once, called a catastrophic mold. Beluga whales (Delphinapterus leucas) molt each summer, shedding their old yellow skin (Lowry 1994).

Beluga whales tend to rub on coarse gravel to facilitate the removal of their skin, and the skin grows about 100 times faster than normal during the molting period (Lowry 1994).

Molting in reptiles

The most familiar example of molting 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 a 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 molted skin inside-out. This is similar to how you might remove a sock from your foot by grabbing the open end and pulling it over itself. The snake's skin is often left in one piece after the molting process.

Molting in arthropods (ecdysis)

In arthropods, such as insects, arachnids and crustaceans, molting is the shedding of the exoskeleton (which is often called its shell), typically to let the organism grow. This process is called ecdysis. Ecdysis is necessary because the exoskeleton is rigid and cannot grow (like skin). The new exoskeleton is initially soft, but hardens after the molting of the old exoskeleton.

Series of pictures showing the progression of ecdysis in Callinectes sapidus, the blue crab

Ecdysis is the moulting of the cuticula in arthropods and related groups (Ecdysozoa). Since the cuticula of these animals is also the skeletal support of the body and is inelastic, it is shed during growth and a new, larger covering is formed.

After moulting, an arthropod is described as teneral; it is "fresh", pale and soft-bodied. Within one or two hours, the cuticle hardens and darkens following a tanning process similar to that of the tanning of leather. It is during this short phase that the animal grows, since growth is otherwise constrained by the rigidity of the exoskeleton.

Ecdysis may also enable damaged tissue and missing limbs to be regenerated or substantially re-formed, although this may only be complete over a series of moults, the stump being a little larger with each moult until it is of normal, or near normal size again.

Process

The-arthropods-moulting-1.gif

In preparation for ecdysis, the arthropod becomes inactive for a period of time, undergoing apolysis (separation of the old exoskeleton from the underlying epidermal cells). For most organisms, the resting period is a stage of preparation during which the secretion of fluid from the moulting glands of the epidermal layer and the loosening of the underpart of the cuticula occur.

The-arthropods-moulting-2.gif

Once the old cuticle has separated from the epidermis, the digesting fluid is secreted into the space in between them. However, this fluid remains inactive until the upper part of the new cuticula has been formed.

The-arthropods-moulting-3.gif

While the old cuticula is being digested, the new layer is secreted. All cuticular structures are shed at ecdysis, including the inner parts of the exoskeleton, which includes terminal linings of the alimentary tract and of the tracheae if they are present.

Then, by crawling movements, the animal pushes forward in the old integumentary shell, which splits down the back allowing the animal to emerge. Often, this initial crack is caused by an increase in blood pressure within the body (in combination with movement), forcing an expansion across its exoskeleton, leading to an eventual crack that allows for certain organisms such as spiders to extricate themselves.

Moulting in insects

A Luna moth caterpillar just after ecdysis. The old exoskeleton is visible behind it.

Each stage in the development of an insect between moults is called an instar, or stadium. Higher insects tend to have fewer instars (4-5) than lower insects (anywhere up to ~15). Higher insects have more alternatives to moulting, such as expansion of the cuticle and collapse of air sacs to allow growth of internal organs.

The process of moulting in insects begins with the separation of the cuticle from the underlying epidermal cells (apolysis) and ends with the shedding of the old cuticle (ecdysis). In many of them it is initiated by an increase in the hormone ecdysone. This hormone causes:

  • apolysis - the separation of the cuticle from the epidermis
  • excretion of new cuticle beneath the old
  • degradation of the old cuticle

After apolysis, moulting fluid is secreted into the space between the old cuticle and the epidermis (the exuvial space), this contains inactive enzymes which are activated only after the new epicuticle is secreted. This prevents them from digesting the new procuticle as it is laid down. The lower regions of the old cuticle - the endocuticle and mesocuticle - are then digested by the enzymes and subsequently absorbed. The exocuticle and epicuticle resist digestion and are hence shed at ecdysis.

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