19 Odd Facts About Animal Sex

Penguins mate with a "cloacal kiss."
Penguins mate with a "cloacal kiss." / Paul Souders/Stone/Getty Images

Do you know which animal has a corkscrew-shaped penis? Or what type of animal engages in “selfing”? From critter-killing copulation to beautiful “bladder songs,” here are some of the strangest facts about sex in the animal kingdom (non-human division), adapted from an episode of The List Show on YouTube.

1. Some male antechinuses mate to death.

An antechinus in a tree.
Mating can be a deadly activity for male antechinuses. / tracielouise/E+/Getty Images

Almost every animal species on the planet has sex. But perhaps no creature shows as much commitment to their reproductive craft as antechinuses. The small marsupials are indigenous to Australia, and every summer they engage in a short period of nonstop sexual couplings. The physical toll of this weeks-long mate-a-thon is a 100 percent mortality rate among several species of male antechinuses, leading some sources to dub the phenomenon “mating to death.”

During mating season, each male antechinus seeks out as many female partners as possible in an effort to pass on his genes. It can be extremely exhausting work—a single mating session can last up to 14 hours. The dramatic influx of hormones and the stress of the grueling schedule wreak havoc on the animals’ bodies. Internal bleeding, hair loss, and blindness are just some of the common consequences. Still, these single-minded marsupials doggedly pursue their reproductive goals. As the University of Queensland’s Diana Fisher told National Geographic, “By the end of the mating season, physically disintegrating males may run around frantically searching for last mating opportunities.” 

Experts differ on why these animals have evolved to be semelparous, breeding just once in their lifetime. It’s a fairly common trait among certain types of animals, but that usually doesn’t  include mammals. Dr. Fisher thinks it could be related to antechinuses’ diet. They subsist on insects, and unfortunately they can’t go to the Insect-Mart to stock up every week (although it would be objectively adorable if they could). Instead, they may have evolved to make the best use of annual swells in food availability, giving birth in a narrow range of time when mothers would be best-fed and able to support their young. That tight timeframe creates a lot of pressure on males to make hay while the sun shines. 

2. Mangrove killifish couple up (or not) in a variety of ways.

For mangrove killifish, mating is less dangerous, but maybe a bit lonelier. Most of them are hermaphrodites, and can actually sexually reproduce with themselves—a process which, as Sarah Jane Alger wrote for Nature’s Scitable, is, “amusingly enough, called selfing.” That’s kinda cool, but it’s not particularly strange: Hundreds of fish species exhibit some form of hermaphroditism; it actually occurs in about one-third of non-insect animal species

But not all killifish reproduce through selfing. A small but significant percentage of the species has male-only sex organs, while there seem to be no reproductive females. Even though one hermaphroditic killifish could theoretically mate with another, in practice that doesn’t happen. It all leads to a situation reminiscent of a logic problem: Hermaphroditic killifish can reproduce with themselves or with males, but seemingly not with other hermaphrodites; males can only reproduce with hermaphrodites.

The impact of this sexual arrangement and killifish demographics is that hermaphroditic individuals will mate with basically any male, regardless of whether or not they are closely related to that particular male. The males, in contrast, can afford to be more picky—tending to avoid closely-related hermaphrodites they encounter in an apparent attempt to maximize genetic diversity. Suddenly selfing is starting to seem a lot simpler. 

3. Sea slugs engage in reciprocal mating.

Sea slugs also commonly exhibit hermaphroditism, and they get even more creative with their sexual couplings. In some species, about half of sexual encounters are considered “reciprocal matings,” where both individuals simultaneously act as male and female. 

Each slug basically harpoons the other with a “penile appendage terminating in a syringe-shaped stylet to extragenitally inject prostate fluids into their partner,” in the not-exactly-Harlequin-Romance-level language of a team of German researchers. Each penis has hook-shaped spines on it “which are spread like an anchor in the female genital tract during mating,” or, in this case, in both partners simultaneously. It can all make for some contentious encounters, but the reciprocal matings can theoretically increase the chance of reproductive success.

4. Some flatworms participate in “penis fencing.”

Two 'Pseudobiceros bedfordi' flatworms preparing to engage in penis fencing.
Two 'Pseudobiceros bedfordi' flatworms preparing to engage in penis fencing. / Photo by Nico Michiels, from John Whitfield's Everything You Always Wanted to Know about Sexes, PLoS Biol 2/6/2004; Wikimedia Commons, CC by 2.5

One other hermaphroditic species, the flatworm, sometimes decides who gives or receives sperm a bit more aggressively. Some species inseminate their partners by injecting sperm directly into the surface of their skin. This process, called “hypodermic insemination,” is decided with an epic battle known as “penis fencing.” This is exactly what it sounds like—the two flatworms compete in a phallic jousting match lasting as long as an hour. Eventually, one lands the first hit, fertilizing the other’s eggs. The consolation prize for the loser? Motherhood.

5. Male haddock use a bladder mating call to attract partners.

Sometimes the lead-up to sex can be as fascinating as the actual act of reproduction. For male haddock, this involves a unique mating call. 

Apparently some species of fish engage in what amounts to underwater shouting matches. Fish communities off the coast of Australia were documented singing “seven distinct choruses, which rise up at both dawn and at dusk,” according to Science Alert. And as The New Yorker noted, “more than eight hundred fish species are known to hoot, moan, grunt, groan, thump, bark, or otherwise vocalize.”

In the case of haddock, a percussive tremolo is produced by muscles attached to their swim bladders. The sounds become “longer and faster as the level of arousal of the male increases,” according to a paper published in Environmental Biology of Fishes

6. Male ducks have corkscrew-shaped penises.

A male duck in a lake.
Not all birds have penises, but ducks do. And they're corkscrew shaped. / Yawar Nazir/GettyImages

Male ducks have corkscrew-shaped penises that, when not mating, rest in a sack on their bodies. They don’t become erect until after mounting. When the duck finds a mate, it mounts the female and turns its penis inside out to enter her body. The penis enlarges to a quarter of the duck’s total body length—about 20 centimeters, on average. The record for penis length in ducks belongs to an Argentine lake duck, clocking in at 42.5 centimeters, or nearly 17 inches. Just in case you were wondering.

7. Female ducks have “organic chastity belts.”

A female mallard (Anas platyrhynchos) moving its wings in a...
Female ducks have what some have called "organic chastity belts." / Marcos del Mazo/GettyImages

Unfortunately, many instances of duck sex are non-consensual on the female’s end. Luckily, female ducks have a built-in self defense mechanism. Duck vaginas are also corkscrew shaped, twisting in the opposite direction of their phallic counterparts. To prevent itself from getting cork-screwed over into an unwanted pregnancy, the hen uses its vagina to limit how far the male’s penis can travel, and subsequently how far up the vaginal cavity ejaculate can travel. Discover magazine dubbed this unique bit of duck anatomy “organic chastity belts.”

8. Most birds reproduce through “cloacal kisses.”

Most birds, male or female, don’t have penises at all—over 95 percent of all bird species lack the appendages, in fact, including eagles and penguins. Instead, males and females briefly press their genital openings, known as cloacas, against one another in a move that’s been called a “cloacal kiss.”

9. Male argonaut octopuses give partners an arm and a leg dose of sperm.

Illustration of an argonaut octopus.
The male argonaut octopus is totally fine with losing an arm during mating. / mikroman6/Moment/Getty Images

Female argonaut octopuses can weigh as much as 600 times their male counterparts, but those little guys display some big-time creativity when it comes to mating. During copulation, males insert a special arm called a hectocotylus into the females. “[T]hat reproductive arm is used to transfer the sperm inside the female,” marine biologist James Wood told WIRED in 2015. Despite that important function, males aren’t very attached to their hectocotyli. They actually leave the arms behind when they swim away.

10. Giant Pandas could use some help with flirting.

Two giant pandas on a branch.
Pandas aren't great at flirting. / Buena Vista Images/Stone/Getty Images

Giant pandas are known for their low breeding levels. It’s not that they’re not naturally down to clown—in fact, wild pandas show no evidence of trouble breeding in their natural habitats. Since pandas in reserves are usually separated by sex until it’s time to mate, they simply don’t know how to flirt. It doesn’t help that male pandas have among the smallest penises in the animal kingdom in proportion to body size. An adult male panda’s penis is only 3 centimeters long, and in this case, size does matter—it makes successful reproduction less likely. To make matters worse, mating occurs only a handful of days per year due to the female’s ovulation cycle. 

11. An unlikely animal has the largest penis (proportional to its body size).

Burrowing barnacles, in contrast, have the (proportionally) largest penises, which can be up to nine times the length of their bodies. Since burrowing barnacles can’t move around easily, they whip their penises out in hopes that it bumps up against a nearby female.

12. Mouse sperm is bigger than elephant sperm.

And on the subject of size, you may be surprised to learn that mouse sperm is actually longer than elephant sperm—not just proportionally to the animals, but in absolute terms. Even then, the male deer mouse’s relatively large swimmers will group together to improve the likelihood of fertilizing an egg. Apparently a group of about seven sperm is the sweet spot to limit competition but enhance the ability to reach their destination. File that away for your next game of bar trivia. 

13. Alligators have permanently erect penises.

Alligator in the grass at the PGA TOUR - 2004 The Players Championship - Third Round
You'll never look at alligators the same way again. / Al Messerschmidt/GettyImages

Alligators have permanently erect penises. That’s not to suggest they’re constantly filled with blood—they’re actually made from collagen, a stiff protein. And though they don’t exactly “whip” them around like a barnacle, the best guess is that they “[shoot] out like toothpaste from a tube,” in the evocative words of Ed Yong, writing for National Geographic, before bouncing back as if it were attached to a rubber band.

14. Cat penises have sandpaper-like bumps.

If your pet cat has ever licked you, you probably experienced a rough, sandpaper-y sensation. This is due to cats’ tongues having hard, tiny spines called papillae, which also happen to be found on the male’s penis. These spines are thought to help trigger ovulation. After mating, a cat’s penis, spines and all, retracts back into the foreskin. Yeah, they have those too.

15. Porcupines use urine for foreplay.

North America's Highest Peak Mt. McKinley, Denali National Park
Pee is part of foreplay for the North American porcupine. / Lance King/GettyImages

North American porcupines are known to put the “pee” in foreplay. The female expels urine, along with other odorous vaginal secretions, to bring all the boys to the yard. The male porcupines that show up fight for her honor. The winner sits at a low tree branch and uses his pee to gauge whether or not the female is interested. Porcupine expert and author of The North American Porcupine Uldis Roze describes this victorious ejaculation as “a high-speed projectile that launches drops of urine from one tree branch to another.” If the female isn’t into it for some reason, she’ll bite or yell at the male until he takes the hint.

16. Giraffes also work urine into mating. 

Regional Visitors Return To Taronga Western Plains Zoo Following Easing Of NSW Travel Restrictions
Pee plays a part in the giraffe mating. / Jenny Evans/GettyImages

Urine also plays a role in giraffe mating. Males will smell and even taste a female’s urine in order to determine if she’s in heat. 

17. Hippos don’t limit themselves to urine. 

Hippopotamuses, on the other hand, don’t limit themselves to one form of seduction secretion. Males will poop, pee, and fart near potential mates to attract attention. They even spin their tails around to make sure the irresistible aromas waft out in all directions. To show that the feeling is reciprocated, an interested female will “[shower] him in dung.”

18. Horseshoe crabs engage in group sex. 

Horseshoe Crabs Mating, Captiva Island, USA
Just a couple of horseshoe crabs going at it. / Tim Graham/GettyImages

During their mating season, some horseshoe crabs flock to beach shorelines by the tens of thousands for a month-long orgy. According to marine biologist Martin Schreibman, group sex is typical among the horseshoe crab population: “The more males there are attempting to fertilize a female that is laying 80,000 eggs in a season, the better the odds the eggs will be fertilized.” And the odds have proven effective. With such successful group sex, it’s no surprise that horseshoe crabs have been around for over 445 million years.

19. Bonobos have sex for pleasure. 

Bonobos also engage in group sex, although their orgies are likely more for pleasure than for procreation. According to National Geographic, “Studies suggest 75 percent of bonobo sex is nonreproductive and that nearly all bonobos are bisexual.” Seeing as humans share 98 percent or more of our DNA with bonobos, it probably shouldn’t be too surprising that we have some overlapping sexual habits. In addition to developing casual sexual relationships, bonobos frequently kiss during intercourse, and even engage in foreplay and oral sex. 

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