Marsupials are distinguished from eutherian mammals in their mode of reproduction and their greater dependence on the teat and mammary gland than on the placenta for their development. They give birth to a highly altricial young which completes its development whilst firmly attached to a teat, usually within the confines of a pouch, drinking a milk that changes its composition dynamically throughout the lengthy period of lactation. At birth, the 400mg marsupial neonate has a precociously developed digestive, respiratory and circulatory system but retains its fetal excretory system with a fully functional mesonephric kidney. Importantly for our studies, the gonads are undifferentiated and there is no sexual dimorphism in the external genitalia. Despite this degree of developmental prematurity, we have evidence that this tiny (400mg) fetus determines the time of its own delivery and negotiates the climb to the pouch without any assistance from the mother.