Mammary Gland - Other Mammals

Other Mammals

The constantly protruding breasts of the adult human female, unusually large relative to body size, are a unique evolutionary development whose purpose is not yet fully known (see breasts); other mammals tend to have less conspicuous mammary glands that protrude only while actually filling with milk. The number and positioning of complex and simple mammary glands varies widely in different mammals. The nipples and glands can occur anywhere along the two milk lines, two roughly-parallel lines along the ventral aspect of the body. In general most mammals develop mammary glands in pairs along these lines, with a number approximating the number of young typically birthed at a time. The number of nipples varies from 2 (in most primates) to 18 (in pigs). The Virginia Opossum has 13, one of the few mammals with an odd number. The following table lists the number and position of glands normally found in a range of mammals:

Male mammals typically have rudimentary mammary glands and nipples, with a few exceptions: male mice don't have nipples, and male horses lack nipples and mammary glands. The male Dayak fruit bat has lactating mammary glands; male lactation occurs infrequently in some species, including humans.

Mammary glands are true protein factories, and several labs have constructed transgenic animals, mainly goats and cows, in order to produce proteins for pharmaceutical use. Complex glycoproteins such as monoclonal antibodies or antithrombin cannot be produced by genetically engineered bacteria, and the production in live mammals is much cheaper than the use of mammalian cell cultures.