Echidnas appear on our 5-cent coin. Baby echidnas are so small after they hatch from their egg that it would take 8 baby echidnas to weigh as much as a single 5-cent coin.Male echidnas have a four-headed penis.Echidnas on Kangaroo Island are now listed as endangered.Echidnas only breed during July-September every year, so that is the best time to spot them in the wild as they are most active.The egg is only the size of a grape and the shell is very thin, fragile, and has a leathery texture. Monotremes are the only egg-laying mammals.The word monotreme literally means ‘one hole’. Echidnas and platypuses are called monotremes.The echidna and their cousin the platypus are the world’s oldest mammals – they are almost 200 million years old!.“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. “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. 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. 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). But this is the first time this has been seen in a mammal, so we really don't know.” Most of the time the four-tipped wonder is tucked away. They may also be able to alternate which epididymis the sperm come from each time. “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. “It's possible it's to do with male-male competition for females,” explained Fenelon. 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? 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. Before the study we predicted there was going to be some sort of valve mechanism that controlled this, but we found nothing like that.” The males can flit from one side to the next quite quickly, potentially increasing their reproductive opportunities. “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. “In most other species these start separate but then they merge,” Fenelon told IFLScience. 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. 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.

A platypus penis