Platypus are, however, true mammals because they elaborate a complex milk under
the control of the same hormones that control lactation in both other groups of
mammals (Griffiths 1978). The 210 million years of separate evolution of
monotremes from other extant mammalian groups has led to many striking
differences in their anatomy and physiology, some of which are highly specia lized to
the group and reflect their early divergence from other mammals. Fundamental
differences in modes of reproduction and development provide further unique
opportunities to understand the genes and hormones that control mammalian
reproduction and development. Despite this potential, and although they are relatively
common animals in the wild, monotremes do not lend themselves readily to captive
breeding for this reason. Only recently have researchers in Melbourne and Sydney
been able to successfully breed these animals.