Certain chemicals called hormones stimulate cells in the mammary glands to produce milk.
Young mammals feed on milk until they are able to get food by themselves.
Milk contains substances necessary for growth, such as carbohydrates, fats, and proteins. When the young stop feeding on milk, the mammary glands stop milk production. For example, the lactation period lasts about 10 months in cows, and about 2 years in walruses. When the mammal becomes pregnant again, the lactation period begins once more. Lawrence C. Wit, Ph.D., Professor of Zoology, Auburn University.