Pelage and Molting in Wild Mammals with Special Reference to Aquatic Forms

Hairs are epidermal derivatives, the type, structure, and arrangement of which are related to the role of the mammalian pelage. Hair follicles probably evolved from amphibian apidermal excrescences which became modified into sensory pits of the reptilelike ancestors of mammals. Pelage affords insulation, is sensitive to external stimuli, may be modified for defense or display, assists in buoyancy and streamlining in water, and aids concealment on land. The basic grouping of hair follicles by threes (trio group) appears almost to have been lost in the Pinnipedia, in which other anatomical adaptations to an aquatic habit include flattened guard hairs, loss of erector (arrector pili) muscles and modified cutaneous glands. Periodic growth and replacement of the pelage is essential to survival. The pelage cycle is closely related to the annual cycle with respect to seasonal requirements dictated by the environment, such as climate and life processes, particularly reproduction. Different pelages occur at different stages of the life history and also according to varying seasonal demands, especially in arctic and subarctic terrestrial species. Hair growth and molting are established before sexual maturity,or even prenatally, and are inherent features in the biology of mammals. Nevertheless, pelage cycles are coordinated with respect to season (and hence the annual cycle) by the direct proximate stimulus of light acting through neuro-endocrine pathways. Pelage growth may also be regulated directly by nutritional status and, to some extent, indirectly by temperature and behavior. The pelage cycle must be regarded from the point of view of the whole process of follicular activity and fiber production, rather than simply the externally visible molt, if that unique mammalian characteristic, hair, is to be understood in its true biological perspective.