The year was 2003, and Mark Siddall was in South Africa a-hunting the elusive hippo butt leech. This was a good place to look, after all, on account of all the hippos, and on account of all those hippos having rectums—the flesh this particular leech, Placobdelloides jaegerskioeldi, fancies. Happily, a local game officer had gotten wind of the American’s quest, and was more than willing to help.

“He knew that these crazy Americans were in the area trying to get a hippo ass leech," says Siddall, the curator of invertebrates at the American Museum of Natural History. “And they had to cull [a hippo] because it was hanging out in a community of people. So he had the presence of mind to cut its butt out.” You can see that very butt below (it's not as bad as you think). That big leech is an adult, the others juveniles.

Now, I tend to write about animals that are saddled with unfortunate living situations. A fish, for instance, that lives in water so cold it has antifreeze for blood. And people tend to ask me, “Why on Earth do they even bother? Why not move to warmer waters, or out of a hippo’s rectum?” Well, in the animal kingdom, if a creature can exploit a niche others cannot, it has a huge advantage—a monopoly really. Far from getting a bum deal, it’s safe to say that this leech has cornered the market on hippo rectum flesh.

And anyway, that’s the only good bit of the hippo to feed on. “We believe that that is so because hippos have very, very thick skin and a layer of blubber,” says Siddall. “The only part on the hippo that's vascularized enough to get a good blood meal would actually be from the rectal region.” And getting there is no real problem at all. “They're crawling up the hippos hind legs and crawling their way right up.” So these leeches are literally a pain in the ass.

Mark Siddall

Once inside, the leech latches on with two suckers. At the back of its body, the more bulbous end, is a big sucker, while at the front is a smaller sucker. And it’s got to be real sure it can keep a tight grip, for hippo BMs can be rather ... explosive: They flap their tails back and forth to fling dung all over the place.

So is that a strategy to evict leeches? Nah, Siddall says. “Hippos, especially male hippos, engage in so much combat and they experience so much scraping and ripping and tearing from the teeth of other hippos, the amount of stimulus, negative or otherwise, that a leech in their butthole is going to cause has got to be fairly minimal.”

These leeches are, after all, nice and flat. “Let's just say they're very aerodynamic with respect to explosive hippo poo,” Siddall says. “I can't imagine that explosive pooing has much to do with getting rid of leeches, especially because I don't think it would be terribly effective at it anyway.” Instead, male hippos fling poo to front on their rivals in confrontations over territory and mates.

Food for Thought

Now, while some leeches have strong jaws that serrate the flesh to get the blood flowing, the hippo-rectum variety just has a proboscis that it snakes into the vascular tissue. But it’s not hard like a mosquito’s mouthparts. Instead, it’s muscular—good and soft and flexible.

You may have heard that leeches, like mosquitoes, produce anticoagulants to keep the blood from clotting in the wound. And leeches do indeed keep the blood from clotting, but not until it’s actually in their bodies. These critters can take in up to 10 times their weight in blood, then drop out of the host and avoid feeding for up to a month. “And over the course of that time they're digesting the liquid blood meal inside them,” says Siddall. “If that blood meal isn't liquid they turn into a brick, and they can't swim, they can't hide from predators, they can't go have sex and do all the things that leeches like to do.”

It would seem that a liquid like blood would be an inefficient way to get nutrition, as opposed to, say, just eating the rectum instead. But in fact blood is packed with good stuff—all kinds of proteins and fatty acids. The problem, though, is that it’s missing some key amino acids and vitamins. To solve that problem, leeches have partnered with friendly bacteria. The microbes live inside the leech’s cells and help churn out the nutrition that’s in short supply in the blood.

Mark Siddall

As for sex, it tends to get interesting in hippo bum leeches, and all leeches for that matter. They’re hermaphrodites, so it’s a lot easier for them to find a mate, since they all have the requisite parts. When they do, it’s far from a romantic encounter. “What they do is when they mate, one leech will stab the other with a spermatophore, which is a harpoon full of sperm, something called traumatic insemination,” says Siddall. (This is not uncommon in the animal kingdom. Bed bugs, too, mate this way, with the male stabbing through the female’s exoskeleton with his genitals. It’s horrific, but it works.)

When the fertilized leech eventually lays its eggs, it sandwiches them between itself and a rock or stick or some such. The eggs hatch, and the baby leeches crawl onto the underside of their parent and attach. When the parent bellies up to the next hippo rectum, the youngsters find themselves with a convenient first meal. “It's a form of parental care,” says Siddall. “They're very caring leeches.”

Well, caring toward anything that isn’t a hippo rectum. So the next time you see a hippo acting all pissy-pants, keep in mind that it could be hosting a family of leeches in its bum. So think about giving it a hug, maybe.*

*Please do not actually give a hippo a hug. They prefer kisses.

Big thanks to Luke Groskin at Science Friday for suggesting this week’s critter. Browse the full Absurd Creature of the Week archive here. Know of an animal you want me to write about? Are you a scientist studying a bizarre creature? Email matthew_simon@wired.com or ping me on Twitter at @mrMattSimon.