The short-beaked echidna (Tachyglossus aculeatus) is one of only four extant species of egg-laying mammals (Monotremata: three echidnas and one platypus), but despite its common and ubiquitous distribution throughout Australia, information on its reproductive biology is limited. What is known is that the reproductive biology of the echidna is anatomically and behaviourally distinct in comparison with both marsupial and eutherian mammals. After a short gestation, the egg is incubated in the pouch and once hatched, the young puggle continues its development while residing in the pouch, sucking milk form the mammary patches.
In conjunction with Currumbin Wildlife Sanctuary (Queensland) we have been investigating echidna reproduction and development in detail for the first time. The newly hatched pouch young is developmentally remarkably similar to a marsupial neonate, but has a number of distinct differences, particularly with regards to their sexual differentiation. The developing gonads have a typically mammalian embryonic appearance, but their differentiation is delayed compared to marsupials. Together with the platypus, the echidna has a unique sex chromosome make-up (echidna 5Xs:4Ys; platypus 5Xs:5Ys), but neither have the sex determining gene SRY. However, many of the other common mammalian sexual differentiation genes are present in the developing echidna gonads. The adult male echidna has a unique phallus with four glans penises, but at ejaculation only two of these become erect. This is due to the presence of a split urethra and two separate corpora spongiosa for the entirety of its length. Whilst the phallus is present at hatching as a distinct protuberant structure in the young, urethra patency is delayed since they excrete both gut contents and urine from a single hole: the cloaca. Together, these results provide the first insights into the evolution of the sexual development pathways of the monotremes and how they differ from the marsupials and eutherians.