"Initially, we thought we'd find some sort of valve mechanism" that would "control the one-sided action seen in echidna," the researchers wrote. This makes sense, but the finding did not explain why only two of the heads are used during sex. The 3D computer model revealed that the urethral tube, which the sperm moves through, splits below the heads into two separate tubes, which each split again to allow for sperm to be delivered to each of the four heads. "This meant we could create a 3D model of the whole echidna penis and its important internal structures in order to see how it operates," the researchers wrote. Normal CT scans only detect hard tissue like bones, so the researchers stained the penises with iodine to enable the soft tissues to be mapped out. Researchers took the euthanized echidnas and created 3D models of their penises using specialized CT scans. But luckily for the researchers, the euthanized echidnas' penises are still in good enough shape to study, Fenelon said. Unfortunately, rescued echidnas tend to have injuries, usually inflicted by road collisions, that are so severe the animals are often euthanized. To understand more about how echidna penises work, Fenelon's team turned to short-beaked echidnas ( Tachyglossus aculeatus) at a wildlife sanctuary in Australia. Very few animals' sperm are known to do this, and the reason behind it is unknown, Fenelon said. "These bundles have been observed to swim progressively forward in a vigorous and coordinated pattern, and bigger bundles seem to swim better than individual sperm or smaller bundles." "Ejaculated semen samples contained bundles of up to 100 sperm that are joined at the tip of their heads so they form a sphere-like shape," Jane Fenelon, lead author of the study and a reproductive biologist at the University of Melbourne, told Live Science. In addition to their unconventional genitalia, echidnas' sperm are also unconventional and have the astonishing ability of being able to work as a team. When not in use, echidnas' penises are retracted inside their bodies and emerge through the cloacal opening when erect their testes, which unusually have no scrotum, remain inside their bodies all the time. Instead, echidnas use a cloaca - a multipurpose opening for urinating, defecating and, in females, egg laying. In addition to their distinctive shape, echidna penises are also unusual because, unlike those of most other mammals, they are used only for sexual reproduction and not urination. (Image credit: Jane Fenelon) Weirdness abounds A close up of the distinctive four-headed penis emerging from the cloaca of a short beaked echidna.
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