The monotremes contain some of the animal kingdom’s greatest oddities. From platypuses with their 10 sex chromosomes and milk-sweating skin, to the echidna that looks like a hedgehog and a niffler had a baby. Both lay eggs and produce milk, making them the only mammals equipped to make their own custard. Now, new research is shining a light on another spectacular adaptation born of the monotremes: the echidna’s four-tipped penis.
Published in the journal Sexual Development, the tell-all paper unravels the “Unique Penile Morphology of the Short-Beaked Echidna, Tachyglossus aculeatus,” to quote its title. It appears beneath these prickly and portly animals (which can swim, by the way) sits quite the phallus, poetically described as having “four rosette glans”, each of which contains a urethra. That really is more than just a tip.
Each of the four tips is capable of delivering sperm as well as urine, but remarkably only two of them are used per reproductive attempt. Lead author and research fellow Dr Jane Fenelon believes this is due to the corpora spongiosa, the mass of spongy tissue that surrounds the males’ urethras, being entirely separate from the rest of the echidnas' penis.
“In most other species these start separate but then they merge,” Fenelon told IFLScience. “Together with the split of the major blood vessel and urethra it gives the impression that the end of the echidna penis is acting like two separate glans penises, which explains how they could do the unilateral ejaculation. Before the study we predicted there was going to be some sort of valve mechanism that controlled this, but we found nothing like that.”
This combined with the penile artery which is split into two branches in the echidna penis explains why only two tips turn up to the party, and how this unilateral ejaculation adaptation is made possible. Four tips in a penis that can essentially switch sides per mating event is all well and good, but how does it help these prickly bachelors?
“It's possible it's to do with male-male competition for females,” explained Fenelon. “In the one male echidna we have erection details for, he alternates the use of each side each time which suggests the potential for a quick turnaround. They may also be able to alternate which epididymis the sperm come from each time. But this is the first time this has been seen in a mammal, so we really don't know.”
Monotremes diverged around 184 million years ago, which could explain why they share some of their traits with unrelated animals (let’s be honest, the platypus is like a beaver with a beak). While the similarities between egg-laying female echidnas and reptiles are obvious, the penile paper notes there’s potential to prove that the males have something of a reptilian edge to their side of things, too.
“We'd like to look into how this unusual structure develops and how similar it is to the crocodiles and turtles, which the adult form looks most like,” Fenelon said. “There's some evidence that the penis in all amniotes (reptiles, birds, and mammals) has the same evolutionary origin and the monotremes are a missing piece of that puzzle.”