MATT COLLINS

Life is going to present special challenges to any creature named for a deadly sin. Sloths really deserved better. They could have been called deliberates or contemplatives. How sloths conceive of themselves remains a secret—three-toed sloths (Bradypus variegatus) seem to smile a lot, but they’re not talking.

They didn’t say anything to me, anyway, at the Aviarios Sloth Rescue Center just north of Cahuita, along the Caribbean coast of Costa Rica, which I visited on March 6. How I got to Aviarios is a story of hardship and struggle. Not on my part, of course. I’m talking about the incredibly diligent and attentive staff on the luxury cruise ship Zuiderdam, which left Fort Lauderdale on February 27 for the Bahamas, Aruba, Curaçao, Panama and Costa Rica before returning to Florida. The biggest test of my mettle was seeing whether I could finish yet another extravagant dinner. If the creatures slowly digesting leaves in the trees of the Aviarios sanctuary are sloths, call me glutton. (Which, coincidentally, is the opening line of my planned novella Mopey Doc, about a melancholy physician with a body mass index of 37.)

The trip was the third in a continuing series of Scientific American cruises put together by InSight Cruises, in which a sterling faculty of expert scientists and the occasional freeloading journalist lecture to an enthusiastic and well-adapted audience. On this creative and intelligently designed voyage, the theme was evolution.

Along the way, the ship was lifted 85 feet in the legendary Gatun Locks, the Atlantic-side steps of the Panama Canal, into the vast Gatun Lake. Four cruise lecturers then took a small motorboat across the lake’s choppy surface to tour Barro Colorado Island, home to the Smithsonian Tropical Research Institute. A century ago, before the flooding of the island’s valley as part of the canal’s construction, Barro Colorado was a hilltop. Now it’s a giant island laboratory (that’s also home to sloths). A historic three-decades-old research effort includes tagging and periodically evaluating every single plant—a quarter of a million currently—on part of the island to try to get a picture of how tropical biodiversity is established and maintained. All that foliage provides ample cover for countless ticks, some of which temporarily enjoyed my patronage while masquerading as freckles.

The day after going through the Gatun Locks, I was face to incredibly cute face with Buttercup, the original rescued sloth at the Aviarios center. Husband-and-wife team Luis Arroyo and Judy Avey-Arroyo were bed and breakfast proprietors in 1992 when three neighborhood children brought them Buttercup, then an orphaned infant. The Arroyos nursed the three-fingered sloth to a healthy adulthood and became known as sloth savants. When local people found hurt or orphaned sloths, they brought them to Luis and Judy. By 1997 the Arroyos were in the sloth-saving business full-time.

The challenges are constant, as the scientific literature on sloths is sparse. The Arroyos are attempting basic studies, for example, on the makeup of sloth milk. Babies being fed by their mother “gain weight sometimes three or four times faster than the orphans we’re hand-raising,” Judy says. “So mother’s milk has something—maybe a high fat content. Our milk study has failed so far because we can’t take enough milk from the mother to analyze.” And that’s because mothers don’t bother to store milk, as the baby is always right there, hanging on. In business jargon, sloth milk production is a just-in-time inventory strategy—foxes may have the rep, but it’s sloths that are sly.

Despite their name, sloths are actually real go-getters, sleeping only about 10 hours a day as compared with the 24, minus meal breaks, of your average house cat. Their Muppety, monkeylike appearance belies their true taxonomical standing, more closely related to anteaters and even armadillos. And a sloth does indeed spend most of its time in the canopy, coming down to ground level about once a week to urinate and defecate. Its lifestyle is thus eerily similar to a rare variety of primate, the New York City penthouse-dwelling supermodel.