Posted: 2024-08-23 21:00:00

A lot of reproduction in the natural world happens via "internal insemination", and most is done with a penis (or a penis-like organ).

But there's no one shape or size when it comes to a penis in the animal world.  

A heads up: This article contains images of weird and wonderful animal penises, so you might get a few looks and questions if you're reading this on your commute or at work.

Animal penises can be coiled and spiky, feature jaws or nails and are often made of collagen.

This staggering array of genitalia can tell scientists a surprising amount about the evolution of animal biology and behaviour. 

They're even used to help distinguish some species of insects and bats.

Ann Jones, presenter of the Sex is Weird series for ABC RN's What the Duck?!, has been ruining her pristine internet search history by researching the science behind some of the animal world's most bizarre penises.

Tiny spike 

A microscope image of an incredibly spiky penis head.

This looks like a medieval weapon, but it's the penis of a seed beetle (Callosobruchus maculatus).

A small brown beetle.
A male seed beetle.()

"They have penises that are armed with spikes and structures that probably evolved originally as ways of anchoring inside the female and keeping hold of the female during sperm transfer," Bruno Buzatto, a lecturer in terrestrial ecology at Flinders University, says.

Studies have found that seed beetles with particularly long spikes father more children. 

But it comes at a cost for female beetles.

"Often those things really hurt the female badly, and mating can diminish their life span because it injures them so badly inside."

A microscope image of an incredibly spiky beetle penis.
Seed beetles are also known as cowpea weevils or cowpea seed beetles.()

He says studying how seed beetles behave, where the interests of the male and female do not align, can help answer broader questions about sexual conflict.

"The priority here for males is 'Let's maximise how many eggs I can fertilise', even if that comes at a cost of female life span."

Oh, and when we're talking insect genitalia — "penis" isn't really the correct term.

"We call the mating bits of the male 'aedeagus', which is just a fancy name for this structure," Dr Buzatto says.

An aedeagus is a modified body part that was not originally related to the genital or reproductive system.

In the seed beetle's case, its aedeagus is a modified plate from the end of their abdomen.

Forked tail

Two small pink nubs protruding from scaled skin.

This is an Indian python's (Python molurus) penis.

A brown and pale brown patterned snake in a tree.
Indian pythons are native to the Indian subcontinent and South-East Asia.()

Well, penises. Snakes have two, called hemipenes, usually found in the tail.

"[Hemipenes] are quite diverse, and they are fairly spiny … but the spines are soft," Andrew Durso, a wildlife biologist at the Florida Gulf Coast University, says.

"Sometimes they seem like they get dragged around in the dirt, so they must be pretty resilient to punishment."

Two small pink nubs on the underside of a snake.
Indian pythons are classified as a near-threatened snake species by the International Union for the Conservation of Nature and Natural Resources.()

Dr Durso says snakes are under-studied but probably only pop their hemipenes out when they are about to mate.

"They have this inversion mechanism, so when they're not in use, it's like a pocket that's inside the tail."

"And when they use them they evert it ...  it's like a sock when you take it off your foot. 

So the pocket becomes the exterior of the hemipenis and the inside is filled with lymphatic fluid."

Some hemipenes are forked and look like a pair of Ys.

Regardless of penis forks, snakes only use one hemipenis at a time.

So why do they have more than one? Having a second penis, complete with an undepleted batch of sperm, could be handy should they get two mating opportunities in rapid succession.

It's thought that the sperm produced in the left testis come out through the left hemipenis, and those in the right testis through the right, Dr Durso says.

Experiments with lizards, which also have hemipenes, found they tend to alternate sides when mating multiple times in quick succession.

Extreme monotreme

A bright pink, four-headed penis.

This fantastic phallus belongs to a short-beaked echidna (Tachyglossus aculeatus).

A front-on photograph of an echidna.
Short-beaked echidnas are one of Australia's most iconic but least-researched species.()

A 2021 study detailed the anatomy and workings of the short-beaked echidna's penis, which happens to be bright red … and has four heads.

Lead researcher Jane Fenelon told ABC Hobart that the echidna penis was "very distinctive".

A four-headed bright pink echidna penis.
During courtship, male short-beaked echidnas can form a "train" of up to 10 individuals, to pursue a female.()

"It looks like one single tube, and at the end it has … four rosette-like glands on the end," she says.

"What's even more unusual, particularly for mammals, is that they only seem to use two of them at any one time, and then they alternate between the two sides."

The echidna's blood supply, which courses down one side of the penis or the other, controls which pair of heads is used during mating.

"In essence, the end of their penis is acting like two completely separate penises," Dr Fenelon says.

Echidna sperm cells cooperate with each other. They form torpedo-like bundles of up to 100 sperm which lets them swim faster than if they were on their own, she adds.

"Then when they get to the egg, it's every sperm for himself."

Final buzz

A white bent organ with two orange horn-like projections either side.
A honey bee's endophallus is designed to shoot semen with great speed and force.()

This is the genitalia of a European honey bee (Apis mellifera) drone.

A male bee flying.
A drone's main job is to provide sperm to a queen bee.()

This "endophallus" is usually tucked up inside the body of a male honey bee or drones.

When it's time to mate with a queen bee, which takes a few seconds and is done mid-flight, the drone contracts abdominal muscles that turns their endophallus inside out and "inflates" it inside the queen.

A white bent organ with two orange horn-like projections either side.
A honey bee's endophallus is designed to shoot semen with great speed and force.()

His ejaculation is so explosive that it blasts his endophallus off, leaving it inside the queen, and the drone dies shortly afterwards.

Ballistic bird

An orange, corkscrew-like penis

This corkscrew penis belongs to a mallard or wild duck (Anas platyrhynchos).

A male duck scratching his face with his foot with his corkscrew penis hanging out.
Duck vaginas are also corkscrew-shaped. They twist in the opposite direction to the duck penis.()

Scientists have actually filmed the corkscrew shooting out at 121km per hour.

"The swirling duck penis is like a ballistic sperm delivery system that inverts itself at high speed out of the mallard in question," says biologist Diane Kelly of  the University of Massachusetts Amherst.

It is unusual, not just for its shape, but its rarity. Only around 3 per cent of bird species have a penis.

The rest haven't exactly lost their penis, but rather their penis fails to develop.

"[These] birds start to make it ... but the urethra eats the entire penis," Dr Kelly explains.

They rely on a "cloacal kiss" for mating, or the meeting of two orifices called cloacas, between which sperm is transferred.

Scientists don't know why most birds have evolved without a penis.

"There are a lot of ideas … that maybe it's weight saving," Dr Kelly says.

But this doesn't explain why ducks have penises, says Patty Brennan, an evolutionary biologist at Mount Holyoke College in the US.

"Ducks are some of the longest migrating species and they have huge penises, so that doesn't really work," Professor Brennan says.

Another hypothesis is to reduce the chances of getting a disease, because most animals haven't separated out their digestive outlets from their sexual inlets.

"Birds have all sorts of stuff in their cloaca — they get organisms that can cause disease, and it it possible that by eliminating the penis they are reducing their incidences of sexually transmitted diseases."

Bungee croc

A smooth, pale penis with a hooked end.
This one has been dissected from a dead specimen.()

This is the penis of an American alligator (Alligator mississippiensis).

An alligator walking on a path.
Male American alligators make low frequency sounds to attract females.()

When crocodilians, such as alligators and crocodiles, are not in mating mode, their penis sits inside an orifice called a cloaca.

And the penis is always stiff, according to Diane Kelly, who studies copulatory systems at the University of Massachusetts Amherst.

A smooth, pale penis with a hooked end.
This one has been dissected from a dead specimen.()

"Where we [humans] would have that central fluid space [in a penis], a crocodilian has that space packed with so much collagen that it becomes effectively permanently erect," Dr Kelly says.

"They've got this little erect penis sitting there, ready to go 24/7."

So how does a crocodilian pop his penis out when it's time to mate, and back in again once the deed is done?

There are no muscles attached to the penis itself. Instead, muscles in the cloaca, which form a sling around the penis's base, cause it to shoot out.

Then a pair of tendons fling it back inside.

"Those two tendons are like big elastic bungee cords, and they get put into tension as the penis [emerges from the cloaca]," Dr Kelly says.

"When the muscles that are holding it all in place relax, the whole system flips back into its resting position by elastic recoil."

Pumped papillon 

A microscope image of a long, curly, tube-like penis.

This is the inflated penis of a Helicoverpa moth.

A small brown moth on a yellow flower.
The larva stage of Helicoverpa moths feed on many important cultivated crops.()

This image and video were made possible thanks to an invention called the "phalloblaster", developed by the CSIRO.

Many insects look the same under a microscope and tiny differences in genitalia are often the best way to tell two species apart.

According to a CSIRO scientist, "the phalloblaster inflates the penis with a stream of pressurised alcohol to create the same shape as when the insect was alive.

"The alcohol dehydrates and hardens the structure, so that once the process is over, the genitalia remain inflated rather like miniature balloons. It makes the genitalia much easier to study."

A small thread-like tube is inflated by a fine needle.
The phalloblaster in action.()

Some butterflies can, in a way, "see" with their genitalia, thanks to light-sensing receptors.

Such "hindsight" is thought to help with mating, guiding the male to successfully dock with a female.

In 1980, insect vision researcher Kentaro Arikawa, then a graduate student, accidentally discovered the light-sensing-genitals of the Chinese yellow swallowtail butterfly (Papilio xuthus).

If the genitalia were misaligned, Professor Arikawa writes, "some space would be available through which light could enter", and the male would try to reposition himself.

Hanging out

A tiny black set of testicles and penis.

You're looking at Lyle's flying fox (Pteropus lylei).

A flying fox hanging upside down in a tree with its genitals exposed.
Flying fox penises can be up to a quarter the length of their body.()

As is the case in insects, bat genitals are often used by scientists to distinguish between smaller, similar-looking species.

The southern free-tailed bat (Ozimops planiceps) and inland free-tailed bat (Ozimops petersi) are closely related, and sometimes called the "long-penis" and "short-penis" forms of free-tailed bat respectively.

And some bats hang with their penis out … literally.

Diane Kelly from the University of Massachusetts Amherst says most male mammals don't keep their penis out in the open the way that humans do.

"Most mammals keep their penises tucked under their belly in a specialised sheath called a prepuce, but for some reason some bats and primates have lost [most of their prepuce]."

The flipper 

A pale pink penis poking out of pale pink skin.

This is a penis on a common bottlenose dolphin (Tursiops truncatus).

common bottlenosed water swimming through water
Male common bottlenosed dolphins (Turisops truncatus) pursue females for weeks.()

Like crocodilian penises, dolphin penises contain collagen-rich erectile tissue, which means they don't ever fully soften.

A pointy pink penis poking out of pale pink skin.
Dolphins have what's known as a "fibroelastic" penis. ()

Diane Kelly from the University of Massachusetts Amherst says male bottlenose dolphins can force females into mating, but females have a trick up their, er, sleeve.

Bottlenose dolphins have a "complex vagina", full of internal folds and flaps.

"If she rotates her body a few degrees, he can be shunted off into the blind alley of her vagina and not be able to inseminate her," Dr Kelly says.

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