The penis of an echidna has four heads but only two become erect at any one time. Now, Australian researchers have uncovered why.
Scientists discovered the marsupial has unusual reproductive anatomy that causes male echidnas to ejaculate from only two of their four penile heads at one time.
The research, published in the peer-reviewed journal Sexual Development, found that echidna penises have similar features to those of other mammals, including platypuses, but also similarities to reptiles.
The scientists studied the internal anatomy of several echidnas that had been brought into the Currumbin Wildlife Sanctuary in Queensland with severe injuries and had to be euthanised.
Male echidnas have no scrotum and store their penis internally when not using it. Each of the four tips of the echidna penis has a branch of their urethra but only semen passes through.
The researchers found that in echidnas, the major blood vessel of the penis splits into two main branches, which each supply two of the penile heads.
Jane Fenelon of the University of Melbourne, the study’s lead author, said previously it was suspected that male echidnas alternated between two of their four penile heads via a valve mechanism on the urethra, but that they did not find any evidence of this.
Instead, they found the spongy tissue of the penis, which becomes erect when filled with blood, was separated by a septum – meaning the echidna penis has two distinct halves. By directing blood flow down one of the main artery branches, the echidna is able to control which half – and which two heads – becomes erect.
“Their penis is actually acting like two separate penises that just happen to be merged together,” said Fenelon.
“It’s the first time we’ve seen this in mammals. We know that in other species with really elaborate penises, it normally happens because of male–male competition for females.” In that case, a multi-headed penis might decrease the time needed between mating sessions, but the evolutionary reason for the behaviour in echidnas has not been confirmed definitively.
Fenelon said the research was motivated by the behaviour of an echidna at the wildlife sanctuary, who had been observed alternating the use of his penile heads up to 10 times in a row.
“Internally, we think it’s pretty similar to an echidna, but nobody’s ever seen an erect platypus penis, so we’re not sure if they only use one of their two heads at one time,” she said.
Two-pronged penises are common in many other marsupial species, including kangaroos, koalas, bilbies and wombats.
There were also anatomical similarities between the male echidna’s reproductive tracts and those of crocodiles and turtles, particularly in how the sperm enters the penis.
Female echidnas have a cloaca – a single opening that they use to urinate, defaecate, and mate. Internally, the cloaca branches into different systems, including a two-branched reproductive tract.
“We think when the penis is erect that it’s long enough to reach where the uterus branches off,” said Fenelon. Males also use their cloaca to urinate and defaecate.
Unusually, previous studies have shown that echidna sperm cells work cooperatively, swimming in connected bundles of up to 100 sperm to reach the egg.