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Down in the forested streams of South America lives a curious little mammal called the yapok. Also known as the “water opossum,” it’s the only marsupial adapted for an aquatic lifestyle.

And to be so at home in the water, it’s developed a curious trait: the males have pouches, too. And what’s a male yapok do with his pouch? Tuck his junk, of course.

Yapoks are nocturnal, spending their nights diving in streams in search of small fish, crustaceans, insects, and frogs to eat. They’re sort of like an otter or muskrat, but cousin to the good ‘ole Virginia opossum. And they have a lot of similar aquatic adaptations—webbed feet, water-repellent fur, and tails that they use as rudders.

But one of their water-adapted traits stands out. They’re the only living marsupial species in which both sexes have pouches. The females use their pouch like other marsupial ladies do—as a nice protective case for their developing young.

But because they enjoy such a wet lifestyle, a female yapok’s pouch is watertight. That way, it keeps the babies safe and dry while mom’s swimming. The male’s pouch isn’t watertight, but it’s definitely there, which is more than you can say for any other adult male possum—or any other adult male marsupial, for that matter.

Some young male marsupials have pouches, but they lose them by the time they mature—except for yapoks. And like a female’s pouch, the male yapok’s pouch keeps something safe and warm while the animal swims: his scrotum. And that’s probably in part to help keep his sensitive sac safe, as dangly bits are thought to be a bit inconvenient when swimming because they create drag and could snag on an unseen object.

Youch. Perhaps more importantly, a nice pouch ensures the sperm-makers stay at just the right temperature. One of the reasons mammals have an external sac for the testes is actually to keep them cool.

Overheating damages sperm, so hot testicles can hamper a male’s fertility. But it’s not good for sperm to get too cold, either, and getting wet is a sure way to catch a chill. That’s because water acts as a better heat conductor than air, so you lose heat more quickly in it.

Hence the yapok’s unique system. Other water-dwelling mammals face the same issue. But since they lack pouches, they’ve come up with other ways to hit that Goldilocks temperature.

In whales, dolphins, manatees, and seals, the testicles are internal, but they’re hooked up to a kind of cooling system that uses chilled blood from the extremities to prevent overheating. Sea lions do have scrotums, but they can be a bit flexible about the degree of dangling. They’re able to tuck their testicles up and in, and only poke them out into their scrotums as needed to cool off or during the breeding season.

And otters are considered seasonally scrotal—their testes are so small when they’re not breeding that they don’t really protrude from the body, so they don’t create much drag or catch on things. But since yapoks come from an evolutionary lineage where pouches are a thing, natural selection apparently co-opted a part already at hand to help males with their tender testes when they took to the water. Because if you’ve got a pocket right there, you might as well use it!

Thanks for watching this episode of SciShow, a Complexly production. If you really dig stories about amazing animals and their pouches, you might want to check out our sister channel, Animal Wonders, over at youtube.com/animalwondersmontana. On Animal Wonders, resident animal specialist Jessi Knudsen Castañeda talks about some of the most incredible creatures that share this planet with us and the challenges of caring for them as a wildlife rescuer. [♪OUTRO].