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molting,periodical shedding and renewal of the outer skin, exoskeleton, fur, or feathers of an animal. In most animals the process is triggered by secretions of the thyroid and pituitary glands. Nearly all birds molt annually in the late summer, losing and replacing their feathers gradually over a period of several weeks. Except among ducks, rails, and diving birds the ability to fly is not lost. Some birds undergo a second or prenuptial molt in the spring, changing from dull to bright plumage. The development of the young bird is marked by successive molts: first, from the down of the very young to the juvenal plumage, which resembles that of the female in species showing color differences between the sexes; then to the first winter plumage, when the bird is called an immature; and finally to the first nuptial plumage, the adult stage. Arthropods (e.g., insects and crustaceans) must molt their exoskeletons periodically in order to grow; in this process the inner layers of the old cuticle are digested by a molting fluid secreted by the epidermal cells, the animal emerges from the old covering, and the new cuticle hardens. In insects the stages between molts are called instars. Amphibians and snakes usually shed their skins several times a year. Mammals change from heavy winter to light summer pelage. Protective coloration is exhibited in the color changes of such mammals as the ermine and the varying hare and, more dramatically, among such birds as the ptarmigan.
the periodic shedding of external coverings in animals. In invertebrates, such as crustaceans, myriapods, insects, other arthropods, and some worms, molting consists in shedding the chitinous covering and replacing it with a new one. This process is necessary for the growth and development of the organism. In arthropods and other invertebrates, molting is confined to specific stages of individual development and is a complex process, involving the desquamation and partial dissolution of the old cuticle, proliferation of the cells of the epidermis, and the secretion and hardening of a new cuticle. In insects, molting is effected predominantly by the action of the molting hormone ecdysone, which acts on the chromosomal apparatus of the cells by changing the permeability of the cellular and nuclear membranes. The insect larva has glands in its head or thorax that produce and discharge a molting hormone under the influence of an activating hormone manufactured by the neurosecretory cells of the brain.
In vertebrates that is, amphibians, reptiles (except crocodiles and most turtles—which do not molt), birds, and mammals, molting is caused by the need for restoring worn-out coverings and is associated not with stages of development but with seasonal changes. In amphibians and reptiles, moltings occur one after another throughout the summer; their frequency depends on the temperature regime. At the onset of winter frosts, molting ceases. In birds and mammals each molting is confined to a specific time of the year. Its onset is associated with the change in the length of the solar day, which regulates the activity of the pituitary gland. The thyrotropic hormone produced by the pituitary gland affects the activity of the thyroid gland, whose hormone promotes molting.
As a result of molting, plumage and hairy coverings become thicker; bird plumage and the hairy covering of some mammals change in color. Molting does not always include the entire covering; there are additional moltings that involve only part of the covering. During molting the animals’ metabolism changes: protein metabolism increases and the use of oxygen increases. The rate of molting in birds and mammals can be regulated by artificially changing the light regime.
REFERENCESBeliaev, D. K. “Rol’ sveta ν upravlenii biologicheskimi ritmami mlekopitaiushchikh.” Zhurnal obshchei biologii, 1950, vol. 11, no. 1.
Naumov, N. P. Ekologiia zhivotnykh, 2nd ed. Moscow, 1963.
Wigglesworth, V. B. Insect Hormones. Edinburgh, 1970.