Reproductive biology and behavior of monotremes Monotremes are oviparous or egg-laying mammals. There are only 3 extant species of monotremes: the playtpus and two species of echidna. Their reproductive systems are highly specialized to facilitate both egg and milk production. The masculine trait is quite simple. The feminine trait has similar qualities to those of birds, although female echidnas also possess sacs. The monotreme egg is also very specialized and somewhat similar to a reptile egg. Platypuses and echidnas have very different behaviors when it comes to mating, but their genetics are quite similar. Monotremes possess some large chromosomes and several unpaired microchromosomes. Descendants of the first radiation of mammals, monotremes have characteristics of both placentals and marsupials, while retaining characteristics of reptiles and birds in a combination all their own. Introduction At first glance, platypuses appear to be a fusion of mammals, birds, and everything in between. In fact, even their reproductive biology has a lot in common with a variety of animals: reptiles, birds, placentals and marsupial mammals. Platypuses belong to a group of animals called monotremes. These egg-laying mammals have been a mystery to researchers since their discovery by Westerners over 200 years ago. The first platypus reported to England were thought to be mermaid-like pranksters (Moyal, 2001). Found only in and around Australia, there are only three living species of monotremes: the platypus (Ornithorhynchus anatinus) and two species of echidna (Tachyglossus aculeatus and Zaglossus bruijni). The word “monotreme” is Greek for “a hole,” referring to the cloaca which is the exit for the urinary, reproductive, and excretory systems (Dawson, 1983). The creatures are oviparous: the females lay eggs that develop outside her body. This article will explain the animals' background, tract and egg anatomy, reproductive behavior, and the genetics behind this unique reproductive system. We will pay particular attention to the similarities of the reproductive system of monotremes with that of animals with which we are more familiar. BackgroundMonotremes are crepuscular animals found only in Australia and New Guinea.