1. The seasonal loss of hair, fur, or feathers that occurs in mammals and birds. In mammals the hair growth cycle can be described by four main stages: active hair proliferation (anagen), regression (catagen), rest (telogen), and shedding (exogen). The mechanisms controlling hair loss and renewal are still poorly understood but are regulated by changes in photoperiod linked to the animal’s internal biological clock and variations in the levels of hormones such as melatonin. So, for example, as days become shorter in the autumn, the summer coat is replaced by a winter coat, which is often denser and longer than the summer coat. In some species, such as the snowshoe hare (Lepus americanus), the winter coat is a different colour to provide camouflage against a snowy landscape. Many bird species undergo a ‘pre-basic’ moult following the breeding season or laying season, and some may have a ‘pre-alternate’ moult to produce breeding plumage.

2. The periodic loss of the integument of arthropods and reptiles. See ecdysis.