Milk: milk of monotremes and marsupials

Lactation has evolved from an ancient reproductive strategy which appears to have been present long before the evolution of extant mammals. The ability to lactate is a feature only found among mammals and involves a facet of maternal care where mothers secrete a nutrient-rich milk which is delivered to the young by the mammary gland. Evolutionary studies indicate that lactation was established prior to divergence of extant mammalian lineages. It also seems that lactation evolved long before gestation in utero. Secretions of ancestral mammary glands may have had antimicrobial properties that protected either eggs or hatchlings and organic components that supplemented offspring nutrition. Over the course of evolution, lactation became a highly efficient, effective and adaptable means of providing maternal care for neonates. The evolution of a placenta in eutherian mammals resulted in more extensive intrauterine development of an embryo and the ability to lactate after birth became a critical part of the reproductive strategy of mammalian species. Following development of highly nutritious milks, evolution produced diversity in milk composition and function, quantity of milk output, length of lactation, length of intervals between nursing and contributions of lactation to offspring nutrition.