Members of three mammalian orders, Carnivora [carnivores, including the pinnipeds (seals and sea lions) and otters], Cetacea (dolphins and whales), and Sirenia (manatees and dugongs), live and feed at sea. In all groups, lactation entails production of a fat- and energy-rich milk, either to promote deposition in the young of an insulating layer of subcutaneous fat, or to cover high metabolic costs of life at sea. Yet there are remarkable differences in all other aspects of lactation, including duration, pattern of nursing, growth of the young, and milk and energy outputs. In this article, the unusual lactation patterns seen among the three orders of marine mammals are briefly reviewed.

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