AUSSIE SAVANTS UNRAVEL MYSTERY OF THE 4-HEADED ECHIDNA PENIS

Scientists in Australia have uncovered the mystery behind the bizarre four-headed echidna penis by creating an advanced 3D model of the peculiar organ.

There are four species of echidnas that, along with platypuses, make up a unique group known as monotremes — the smallest of the three mammal groups — whose members lay eggs like birds and fish, but also produce milk like other mammals, Harry Baker reported for Live Science.

Much about this group remains a mystery, the study researchers said. One of the biggest monotreme mysteries is the echidna penis, which has four separate heads, or glans, at the end of the shaft. Only two of the heads are used during each erection, and echidnas can alternate between which two they use.

Moreover, the penis is used only for sexual reproduction and not for urination, for which they use the cloaca.

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. Researchers took the euthanized echidnas and created 3D models of their penises using specialized CT scans. 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.

"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.

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 makes sense, but the finding did not explain why only two of the heads are used during sex.

"Initially, we thought we'd find some sort of valve mechanism" that would "control the one-sided action seen in echidna," the researchers wrote. But instead of a valve, they found that it was actually the type of tissue within the penis that was responsible.

Mammalian penises consist of two main types of erectile tissue — the corpus cavernosum and the corpus spongiosum. Both tissues fill with blood during an erection, but the role of the corpus cavernosum is predominantly to provide a rigid structure to the penis, whereas the corpus spongiosum keeps the urethral tube open to allow sperm to pass through.

Each tissue starts off as two different structures at the base of the penis. In most mammals, the two corpus spongiosum structures merge into one. while the corpus cavernosum remains separated. But in short-beaked echidnas, the cavernosum was merged while the spongiosum remained separate. This separate spongiosum tissue is what allows echidnas to erect each half, or pair of heads, independently from the other, the researchers said.

During a separate experiment on a living but anesthetized echidna, the researchers found that by alternating pairs of heads the individual could ejaculate 10 times in a row without significant pause. This may allow some males to gain an advantage over others, but more experiments are needed to confirm this idea. The study was published online April 29 in the journal Sexual Development.

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