Enigmas in reproductive biology

Monotremes and marsupials are defined by their respective structural forms and modes of reproduction. Possessing a bizarre mixture of sauropsidan and mammalian characteristics, monotreme reproduction has always been the subject of intense fascination and intrigue; for what self-respecting mammals lay eggs and ejaculate sperm in bundles? Equally, some biologists regard the marsupial way of conducting reproductive business, a "half-baked" evolutionary effort that has failed to reach the same level of anatomical and physiological sophistication as that found in eutherian mammals. Nevertheless, the birth of altricial young and an extended period of lactation has allowed marsupials a degree of plasticity in their reproductive strategy not available to eutherians, and which has endowed them with the opportunity to adapt to extreme and unpredictable environments. This chapter will introduce you to the sex life and early development of monotremes and marsupials. Information on the monotreme section has been gathered from a relatively small but recently expanding literature and is therefore a thorough representation of current knowledge. In the marsupial section, we have drawn from a much deeper well of scientific publications, so we have been more selective in what is presented; consequently, we have only attempted to highlight the more enigmatic elements of marsupial reproductive biology