The unique penile morphology of the short-beaked echidna

Just accepted by the journal Sexual Development is our paper on echina penile structure with out collaborators at Monash Uni, Currumbin Wildlife Sanctuary and the University of Queensland.

The unique penile morphology of the short-beaked echidna, Tachyglossus aculeatus, by Jane C Fenelon, Caleb McElrea, Geoff Shaw, Alistair Evans, Michael Pyne^, Stephen Johnston and Marilyn B Renfree

Monotremes diverged from other mammals approximately 184 million years ago (MYA) and have a number of novel reproductive characteristics. One, in particular, is the unique penile morphology which differs between the echidna and the platypus but which has been suggested to be similar to the structure of the squamate hemipenes. The echidna penis consists of four rosette-glans, each of which contain a termination of the quadfurcate urethra, but it appears that only two of the four glans become erect at any one time. Despite this, there are only a few historical references that examine the structure of the echidna penis, and none that provide an explanation for the mechanisms of unilateral ejaculation.  This study confirmed that the echidna penis contains many of the same overall structures and morphology as other mammalian penises and a number of features homologous with reptiles. However, the echidna possesses two distinct corpora spongiosa each of which only surrounds the urethra once the initial urethral bifurcation occurs in the glans penis. Together with the distinct bifurcation of the main penile artery, this provides a mechanism by which directed blood flow to only one corpus spongiosum at a time could dictate unilateral erection.