Unlike other cetaceans that continuously replace skin, beluga whales undergo an annual epidermal molt and rub in estuaries to remove old skin and become more hydrodynamic.
Beluga whales are unique among cetaceans as they undergo an annual molt. Typically, growth and replacement of the epidermis, or outer layer of skin, of cetaceans is a continuous process. In belugas, though, it is a cyclical process that may be driven by their seasonal migrations between frigid arctic oceans and relatively warm estuarine waters.
Given the dramatic change in habitat a beluga undergoes when entering an estuary, the molting process may be controlled by environmental cues such as temperature and salinity. By molting, belugas remove the thick surface layer of the skin that may increase the resistance to the smooth flow of water over the whale. After the molt, the flow of water over the beluga’s skin would be smoother, which would make them more hydrodynamic.
Beluga rubbing, tied to the seasonal epidermal molt, is frequently observed in estuaries. The completion of the molting process may be the most important reason for the belugas’ migration into estuaries.