Photograph by Joel Sartore

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In their native range in Central and South America, two sloth species are at risk: Brazil's maned three-toed sloth is vulnerable, and Panama's pygmy three-toed sloth is critically endangered. The four sloth species conservationists rank as being at the least risk include the Linné's two-toed sloth (above).

Photograph by Joel Sartore

You Can't Hurry Love: How Slow-Moving Sloths Mate (Video)

When one of Earth's slowest mammals mates, "apparently it's very quick."

How slow are sloths, generally considered Earth's slowest mammal? Distance moved in a day: often just a few yards. Time at rest: up to 20 hours out of each 24. Metabolism: so slow that the tree-dwelling herbivores climb down to defecate only about once a week. That's for the best, because their ungainliness on the ground makes them vulnerable to cars, humans, and other animals.

The sloth skeleton is suited for reclining or hanging upside down in trees. That's how sloths eat, sleep, give birth—and mate. Though the rain forest exhibit at Baltimore's National Aquarium has welcomed four sloth babies, the staff has never seen a sloth birth or copulation, says curator Ken Howell: "I think of them as having private lives." When seclusion does lead to sex, he says, "apparently it's very quick."

Well, yes and no, says Mark Rosenthal of Animal Magic, an exotic-animal rescue program in Michigan. With a smartphone and lucky timing, Rosenthal was able to capture "a very rare video of two of our sloths actually breeding" while hanging suspended from a branch in their habitat.

His halting narration describes the protracted scene: "The male keeps trying ... the girl ... is receptive ... He's going to try again ... Those of you watching, bear with me—they're sloths ..." (Read more about the three-toed sloth and Linné's two-toed sloth.)

Because his audience includes children, Rosenthal edited the video to finish before the sloths did. The eventual consummation, he says, "was upside down. And it didn't take very long."

The feature Basic Instincts: A genteel disquisition on love and lust in the animal kingdom appears every month in National Geographic magazine.