The bizarre sex life of the spiny anteater has been exposed by researchers - the male ejaculates using only one half of its penis. New findings about the creature's sex life may seem salacious but they could help shed light on an evolutionary mystery.
It seems that the way the mammal ejaculates is similar to the way reptiles do - by shutting down one side of its penis before secreting semen from the other side. Reptiles have a pair of male members called hemipenes for sex, and they use only one of the two during each act of copulation.
The spiny anteater (Tachyglossus aculeatus), also known as the short-beaked echidna, is a primitive mammal found in Australia and New Guinea. Like the platypus, it is a monotreme, laying eggs instead of bearing live young.
Monotremes have many features in common with reptiles, and the hope is that by studying them, scientists may find clues as to how mammals evolved.
The spiny anteater, however, is notoriously difficult to observe in the wild and shows little enthusiasm for breeding in captivity, so nobody had managed to observe them ejaculate before.
Then Steve Johnston of the University of Queensland in Gatton, Australia, and his colleagues inherited a male spiny anteater that was not so shy. The creature had been retired' from a zoo as it produced an erection when being handled at public viewing sessions.
By filming this animal, the researchers have been able to describe the unique spiny anteater erection and ejaculation behaviour for the first time.
The spiny anteater's four-headed phallus had been puzzling scientists. "When we tried to collect semen by [electrically-stimulated ejaculation] before, not only did we not get a single drop, but the whole penis swelled up to a four-headed monster that wouldn't fit the female reproductive tract, which has only two branches," says Johnston.
"Now we know that during a normal erection, two heads get shut down and the other two fit," he told New Scientist. The heads used are swapped each time the mammal has sex.
The evolutionary significance of one-sided ejaculation is unknown, but may play a role in sperm competition - where sperm from many males may compete to fertilise an egg. Indeed in the spiny anteater, up to 11 males may form a queue behind one female to copulate with her.
The researchers have also observed that hundreds of sperm team up to form bundles that swim much faster than individual sperm in the spiny anteater's semen - another possible adaptation for sperm competition.
"We can now study echidna sperm much better, which should offer fascinating insights into the evolution of mammals", says Russell Jones from the University of Newcastle in New South Wales, who first dissected sperm bundles from dead echidna in the 1980s.
Journal reference: The American Naturalist (DOI: 10.1086/522847)
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