Milk of monotremes and marsupials

Lactation is a defining feature of mammals. The production of milk by the mammary gland during lactation is generally regarded as a mechanism required to supply nutrition to the neonates before they are able to digest other types of food. Increasing evidence suggests that components of milk have other functions in addition to nutrition. Monotremes, marsupials, and eutherians are all members of the class mammalia, but monotremes and marsupials have evolved a lactation pattern very different from that of eutherians. In eutherians, the gestation period is longer relative to the lactation period. However, in marsupials and monotremes the gestation period is relatively short. As a result, the newborn is relatively small and undeveloped, and most of the early development occurs postnatally. Eutherians produce milk of an approximately constant composition after the expression of the initial colostrum, whereas marsupials, such as the tammar wallaby (Macropus eugenii), have a short gestation, give birth to a highly immature young, and then commence a long lactation during which the milk production and composition change progressively to suit the needs of the developing young. The comparative study of monotreme, marsupial, and eutherian milk is of significant interest and may reveal the bioactives required for developmental processes. These factors may have been lost, downregulated, or altered/modified in eutherians, as most part of the development of the eutherian young occurs in utero with nutrition and developmental signaling provided by the amniotic fluid and supplemented from the mother via the placenta.