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Solve the mystery of the four-headed echidna penis

by Dr. Jane Fenelon, Professor Marilyn Renfrey and Associate Professor Stephen Johnston, University of Melbourne


Credit: CC0 Public Domain

Monotremes are among the strangest animals in the world, mixing the characteristics of mammals and reptiles into one creature.

When British scientists first saw a platypus in the 1

8th century, they dismissed it as a hoax. Eventually, a new group of mammals had to be created to account for the platypus and its monotrem brethren, the four different species of echidnas.

Even today, they remain the least understood group of living mammals.

Monotremes are the only mammals that lay eggs, but they also have a number of other unique reproductive characteristics.

In men, their testicles never go down, they have no scrotum when not in use, their penis is stored inside, and their ejaculate contains bundles of up to 100 sperm that swim together until they reach the egg.

In most other species, the sperm swim individually and these are all sperm for themselves.

Unlike other mammals, the monotremic penis is used only for mating and never carries urine.

Solve the mystery of the four-headed echidna penis

The echidna penis is stored internally when not in use. Credit: Jane Fenelon

Among female echidnas, in addition to laying an egg, the sac in which they raise their young is only a temporary structure and develops by thickening the lateral ends around the abdominal area that surrounds the mammary glands.

But perhaps the strangest thing about the echidna’s penis is that it has four heads that actually look like rosette heads at the end. Only two of these four heads ever become functional during an erection, and which heads are functional seem to alternate between subsequent erections.

How exactly the vicious do this has always been a mystery. But for the first time we unraveled what is happening anatomically, with the results now published in the journal Sexual development.

Our research is a joint project involving scientists from the University of Melbourne, the University of Queensland and Monash University, but the most important for the work is the Currambin Sanctuary on the Gold Coast, which has created a small colony for breeding echidnas.

Each year, about 50 injured echidnas are delivered to Currumbin Wildlife Hospital, most of them in road accidents. Unfortunately, many of these echidnas are injured after recovery and need to be euthanized. It was these animals that we used for the study, but we were also able to observe a domesticated echidna.

To understand the mechanisms of operation, we used microCT (computed tomography) scanning in combination with microscopy techniques. A normal CT scan, which uses computer technology to make 3D images from X-rays, captures only mineralized (hard) tissue, but by staining the penis with iodine, we can take the details of the soft tissues.

This meant that we could create a 3D model of the entire echidna penis and its important internal structures to see how it worked.

Most mammals have a urethral tube that carries sperm to the tip of the penis. The echidna urethra begins as a single tube, but towards the end of the penis it splits in two and each of them then splits again – as a result of which each of the four branches ends in one of the four heads.

Solve the mystery of the four-headed echidna penis

The penis has four heads or heads. Credit: Jane Fenelon

We initially thought we would find some sort of valve mechanism in the urethra when it first began to branch to control the unilateral action seen in our meek echidna. Instead, we found that the erectile tissues that make up the echidna’s penis are very unusual.

All mammalian penises consist of two erectile tissues, the corpus cavernosum and the spongy body. The main role of the corpus cavernosum is to fill with blood and maintain an erection. The spongy body is also filled with blood, but its main role is to ensure that the urethral tube remains open during an erection so that semen can pass.

In most other mammals, both the corpus cavernosum and the spongy body begin as two separate tissues at the base of the penis, but then the spongy bodies merge into one.

In the echidna, the cavernous bodies merge into one structure and the spongy bodies remain as two separate structures.

In addition, we found that the main blood vessel of the penis also divides into four branches after the branch of the urethra.

In fact, this means that the end of the echidna’s penis acts as two separate penises on the head. Blood flow can be directed down one side of the spongy body or the other to control which half becomes upright and which branch of the urethra remains open.

We’re not sure why they use just two heads at a time. This may be related to male competition for women. By alternating the use of each side, our meek echidna can ejaculate 10 times without a significant pause, potentially allowing it to outlive less efficient men.

At first glance, the platypus penis looks very different – not only does it have only two heads (or hubs), but the entire penis is covered with various keratinized thorns. However, the internal structures look very similar to those in the echidna.

We currently have no data on what an erect penis looks like on a platypus, so we don’t know if they use both at the same time.

The echidna penis is very unusual among mammals.

Some marsupials, such as bilbi, also have a divided urethra, but they divide into only two branches, and in these species the corpora cavernosa divide into two structures when the urethra divides.

Previous studies have suggested that the echidna resembles some snakes and lizards that have hemipenes (split penises). However, we found that the echidna penis has some similarities to those of crocodiles and turtles. For example, some turtles have a five-headed penis that appears to have a similar internal anatomy.

There is some evidence that the penis in all amniotes (reptiles, birds and mammals) has the same evolutionary origin. Our study shows that although the echidna penis is a mammal in origin, it has some evolutionary innovations.

This is probably because they do not need to use their penis for urine, so they did not have the evolutionary limitations of other mammals to adhere to the standard design of the penis.

Nevertheless, the echidna penis functions effectively to transfer sperm directly into the female reproductive tract.

Breeding research can save the endangered echidna with a long beak

More information:
Jane C. Fenelon et al, The unique morphology of the penis of the short-beaked echidna, Tachyglossus aculeatus,, Sexual development (2021). DOI: 10.1159 / 000515145

SD Johnston et al, Unilateral ejaculation of bundles of echidna sperm, The American naturalist (2007). DOI: 10.1086 / 522847

Provided by the University of Melbourne

Quote: Solving the secret of the four-headed echidna penis (2021, June 9) downloaded on June 10, 2021 from https://phys.org/news/2021-06-mystery-four-headed-echidna-penis.html

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