Monotremes (prototherians) diverged from marsupials (metatherians) and eutherians approximately 166 million years ago ( Renfree et al., 2009), and marsupials and eutherians separated around 130 million years ago ( Bininda-Emonds et al., 2007 Luo et al., 2003 Nilsson et al., 2010). Mammals evolved approximately 200 million years ago ( Flight, 2011) and today comprise three groups-prototherians, metatherians and eutherians. Further research is warranted in gaining a better understanding of the role of milk as a source of nutrition, developmental factors and immunity, in a broader range of marsupial species, and monotremes. However, we have much to learn about the role of milk in marsupial and monotreme mother-young bonding. Milk is also essential to the development of the maternal-young bond and is achieved through feedback systems and odor preferences in eutherian mammals. Neonates are unable to mount a specific immune response shortly after birth and therefore rely on immunoglobulins, immunological cells and other immunologically important molecules transferred through milk.

As well as nourishment, marsupial and monotreme milk supplies growth and immune factors. Macronutrient energy composition of marsupial milk varies between species and changes concentration during the course of lactation. We applied nutritional geometry techniques to a limited number of species with values available to analyze changes in macronutrient composition of milk at different stages. Milk is the primary source of sustenance for young marsupials and monotremes and its composition varies at different stages of development. Here we review the role and importance of milk in marsupial and monotreme development. Marsupial (metatherian) and monotreme (prototherian) young are born in a highly altricial state and rely on their mother’s milk for the first part of their life.

All mammals are characterized by the ability of females to produce milk.

Platypus baby drinking milk