Molting and shedding

As a normal part of growth and development, some species of invertebrate and vertebrate animals undergo a process commonly called molting or shedding. Scientists term this process ecdysis, which is derived from a Greek word meaning “to escape or slip out.” It is common for animals to molt more frequently as larvae or juveniles and less often as they mature and become adults. Molting is also an integral part of metamorphosis. Immediately after a molt, animals may not be fully protected from the environment, and they may be more vulnerable to predators. For example, the new exoskeleton of arthropods may not be fully hardened, or some birds may lose flight feathers.

During a molt, animals may shed and replace their entire body covering or structures associated with the body surface. Molting may involve the replacement of the skin, an exoskeleton, or a cuticle in its entirety. In temperate regions, it is not unusual in the summertime to see the old exoskeleton of a cicada still clinging to a tree. Feathers, fur, or hair, which are derived from skin cells, are often shed in a cyclic fashion rather than all at once. Animals may also lose body structures during a molt. Adult male deer shed their antlers at the end of the mating season, and lemmings replace their claws. In most animals, the interval between molts is called the intermolt, except in insects, where it is referred to as an instar.

The Molting Process

Environmental factors, such as stress, temperature, and light cycle, can serve as stimuli to molting in different species of animals, but the actual sequence of events leading up to shedding and replacement is strongly influenced by hormones and by specific interactions between the nervous system and the endocrine system. Hormone production is the primary function of the endocrine system. However, regions of the brain also produce hormones, which in this case are called neurohormones. One class of hormones, in particular, steroid hormones, is actively involved in regulating molting.

Before an animal molts, a new skin or exoskeleton will have formed beneath the old one. Often, the old exoskeleton splits along the midline of the posterior surface, and the animal then crawls out. Different animals have specific ways to stretch the new exoskeleton before it hardens. The best way to stretch the skeleton is by increasing body size. This might be accomplished by a growth stage or by artificially increasing size. A lobster, for example, might absorb enough water to increase body size by 20 percent while other animals take up air.

Molting in Amphibians and Reptiles

Molting of an entire body surface, such as the skin or exoskeleton, requires separating the layer being shed from the underlying tissue that will become the new outer surface. The chemical and physical processes leading up to the separation will vary slightly between animal groups. Amphibians such as frogs, toads, and salamanders periodically shed the outermost surface layer of their skin. Before molting, mucus is secreted beneath this layer of cells. Since the mucus is secreted between the old and new surface layers, it may assist in the separation by creating a space between them. The mucus may also act as a lubricant and aid the animal in the removal of the old skin. Frogs are also known to bloat themselves and increase movement to break out of the old skin. Animals will often eat the skin they have shed as a way to recycle nutrients.

Reptiles show a good deal of variability in the replacement of their scales. In fact, snakes may shed their skin several times a year. In the time immediately preceding ecdysis in snakes, a fragile zone develops. The old skin will now separate from the underlying, newly developed scales. During the molt, the snake’s eyes will appear cloudy because the outer part of the eye, which is a part of the skin, is also detaching. The rattlesnake’s rattle is formed from the part of the skin that remains attached to the tail at the end of each shedding cycle. Snakes shed their skin in one piece, but lizards do not. Turtles are different from both snakes and lizards. First, their scales do not overlap to form sheets, and second, most turtles add new growth to existing scales and do not molt. The molting pattern in crocodiles and alligators, which shed and replace individual scales, more closely reflects events in birds than other reptiles.

Molting in Birds

Birds shed their feathers each year, and it is not unusual for some species to molt several times during the year. During the molting season, a bird’s behavior may be affected in several ways, including events associated with reproduction. Molting will be influenced by the time of year and the mating season. Many female chickens often stop laying eggs when they are molting. New feathers will develop in the same skin area as the old ones. As the new feathers grow from the follicle they may push the old feathers out of the skin, or they may pull them out. The primary purpose of molting is to replace worn feathers with new ones, and often, a pattern of feather loss will be noted. Wing feathers closest to the body are lost first, and the molt progresses outward along the wings.

It is common for male birds to molt so that they can replace their duller plumage with more colorful feathers associated with attracting a mate. This is called a prenuptial molt and generally occurs in late winter or early spring. A postnuptial molt is common in both males and females. The chicks will lose their original feathers (down) after a postnatal molt to juvenile feathers. If it is a species of bird where the males have a coloration pattern different from the females, the early juvenile coloration more closely resembles the female. As the juveniles mature, they will undergo successive molts to adult plumage. The time frame for maturation can vary considerably. Eagles can take up to five years before a final molt into full adult feathers. Mature birds that depend upon flight as a method of escaping predation will not molt all their feathers at one time. A complete molt would result in the loss of flight feathers on the wings and tail feathers, which assist in stability and guidance during flight. Flight is less adversely affected if there is a symmetrical loss of feathers on the wings and if tail feathers are shed in groups. Birds such as ducks and geese, which can spend extended periods of time in the water, can avoid most predators by swimming or hiding in tall grass. These birds molt all their flight feathers at one time. Male ducks are also somewhat unusual because, during the summer months, they molt from mating colors to plumage similar in color to females. This change is called an eclipse and makes it more difficult to tell males from females.

Molting in Mammals

When they lose hair, mammals, including humans, are molting. Like feathers, hair grows outward from a follicle in the skin, and as new hairs grow, old hairs are lost. Under normal conditions in humans, hair loss will be a gradual process over an individual’s lifetime, and it does not occur all at once. Molting in many mammals is directly influenced by the length of day and interactions between the endocrine and nervous systems. Fluctuations in melatonin have a particularly important role in regulating molting in mammals. The number of molts per year varies, and many mammals molt twice a year, once in the spring and once in the fall. Foxes, however, molt once a year in the summer, and the snowshoe hare molts three times: summer, autumn, and winter. Similarly, seals molt once each year after they give birth, which depends on the species' location. The elephant seal experiences a catastrophic molt each year, which means they shed the top layer of skin and their fur in large clumps.

Mammals' summer coats are commonly thinner than their winter coats, and sometimes, they may be different colors. Changing hair color provides one method of camouflage. White fur lacks pigment and blends well with snow and light surroundings. In contrast, darker shades blend better in the summer and fall. Arctic foxes use these changes in fur color to acquire the appropriate camouflage for the season. In mammals, like other animals, the transition from juvenile into adult can be associated with a molt. Deer fawns, for example, have very pronounced white spots, but when they become further developed, they molt into the solid coloration of adults.

Some domesticated animals, like sheep, llamas, and alpacas, have lost their shedding ability and must be sheared regularly. If one of these animals is not sheared, they can become weighed down by the matted fur, which eventually limits their movements.

Principal Terms

Arthropods: Animals with jointed exoskeletons, including insects, crayfish, and spiders

Exoskeleton: A skeleton made up of proteins and minerals found on the outer surface of an animal

Hormones: Chemical messengers produced by specialized organs and cells either in the endocrine system or the nervous system; they have specific effects on different cells of the body

Invertebrate: An animal without an internal skeleton made up of individual bones or vertebrae

Metamorphosis: A change in the physical state of an animal, such as the transformation of a tadpole into a frog or the development of a moth from a caterpillar

Steroid Hormone: A hormone that is made from cholesterol

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