Milk: 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 neonate before it is able to digest other types of food. Increasing evidence suggests that components of milk have additional 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 that is very different from eutherians. In eutherians, the gestation period is long relative to its 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 a constant composition after the expression of the initial colostrum, whereas marsupials such as the tammar wallaby ( Macropus eugenii ), have a short gestation, and give birth to a highly immature young and then commence a long lactation during which the milk production and composition progressively change in composition to suit the needs of the developing young. The comparative study of monotreme, marsupial and eutherian milk is of significant interest and may reveal bioactives required for developmental processes. These factors may have been either lost, down-regulated or altered/modified in eutherians since most development of eutherian young occurs in utero with nutrition and developmental signaling provided by the amniotic fluid and supplemented from the mother via the placenta.