Brown-throated three-toed sloth

High upon people’s lists of ‘must sees’ when visiting Costa Rica is the sloth. The sloth has an intrinsic charm, which people find endearing, rather like the koala bear of Australia. It is benign, odd looking, yet cute, and hangs upside down from branches. But let‘s face it; the sloth is not the most exciting animal in the Neotropics! Nonetheless, behind its rather boring façade lies a very interesting creature indeed.

If any animal were to symbolize the uniqueness of the Neotropical fauna it would be the sloth. It belongs to the oldest extant order of placental mammals to have originated from South America, the Xenarthra (formerly known as the Edentata) – an ancient group that, incidentally, also include the anteaters and armadillos. This once large and diverse order included giants like the huge armadillo-like glyptodont, and giant ground sloths, some as much as four tons in weight. Incidentally, these giants migrated north from South America once the Panamanian formed, 2.5 million years ago, and were a conspicuous component of the once impressive mega fauna that roamed the forests and plains of both continents. Their extinction, a mere few thousand years ago, coincided with the arrival people to the New World!

The Xenarthra evolved and radiated in isolation for over 50 million years on the South American continent into ten familial lineages. Today, alas, only four of those familial lineages remain. Curiously though, two of these families are represented by the sloths – the anteaters and the armadillos represent the remaining two families.

In the forests of Central America live two species of sloth: the Brown-throated Three-toed sloth (Bradypus variegatus) and Hoffmann’s Two-toed Sloth (Choloepus hoffmanni). Although both species look superficially similar, current taxonomic opinion considers them to be sufficiently different to warrant their inclusion in separate families: the former is placed in the family Bradypodidae, the latter in the Megalonychidae.

Hoffman's two-toed sloth

The smallest of the two species and the most commonly seen, is the Brown-throated Tree-toed Sloth. An adult may weigh up to 4.5 kgs (10 lbs) and has long, shaggy fur that is grizzled gray in color. The Brown-throated Three-toed Sloth has a cute, smiling face, with a small dark snout, a dark mask through the eyes and whitish fur on the forehead. As its name suggests, its throat is brown and it has three claws on its forelimbs. Hoffmann’s Two-toed Sloth is larger; adults can weigh up to 6 kgs (13 lbs). Its fur is dull brown in color and its snout is rather pig-like. But, the main distinguishing feature separating it from its smaller cousin is the presence of only two claws on its forelimbs. In both species their coloration can take on a greenish hue. This is due to algae coating their shaggy fur.

The word sloth is synonymous with laziness. Indeed, the local name for the sloth, ‘perezoso’, is used to describe a person of a rather lethargic disposition. However, there is a reason for the sloth’s apparent laziness: their diet. Both species dine exclusively on leaves. While the rainforest may appear to be one huge salad bowl for sloths, leaves are in fact hard to digest, low in nutrients and laced with toxins. To subsist on this low quality diet, sloths have undergone many physiological and behavioral adaptations: they are selective about the leaves they eat, often choosing younger leaves that are easier to digest; they digest leaves with the aid of bacteria in large, fermentation stomachs, similar to those of the ruminants; and sloths conserve their hard-won energy by being lethargic. Indeed, sloths maintain a low metabolic rate, almost akin to reptiles. Maintaining a high, constant body temperature, as do mammals and birds (endotherms), is extremely costly metabolically. Sloths are perhaps the most reptile-like mammals in their thermoregulatory behavior: they depend greatly (though not exclusively like reptiles) on basking in the sun to raise their body temperature. They are thus partial exotherms.

Little is known about the reproductive habits of Hoffmann’s two-toed Sloth. Because it is generally more common and more diurnally active than its larger cousin, the habits of the Brown-throated Three-toed Sloth are better known. This species reaches sexual maturity at about three years of age. Females produce one offspring per year after a gestation period of six months. The young remain with its mother for about one year before seeking its independence. Mortality rates can be high for sloths during their first of life, but once passed this vulnerable stage they can live to be 20 to 30 years in the wild. The main predators of sloths are harpy eagles and the larger cats.

Another behavioral curiosity of sloths is their habit of descending from the forest canopy once a week to defecate. No satisfactory explanation has been proposed for this odd behavior, but it does suit to veritable entourage of insect dependants that live in their fur. Several species of moths and beetles live exclusively in association with sloths. These insect scurry from the fur of the defecating sloth and lays their eggs on its dung. They then scurry back aboard the sloth before ascends again! The nature of the association between these insects and the sloth is not clear.

Despite the high population density of sloth in rainforest canopies, they can be difficult to see. Sloths will feed on many different species of trees, with individuals showing preferences for certain tree species. However, the Cecropia trees do seem to be a favorite among sloths, and it is in these rather open trees that sloths are most commonly seen.

One last curious fact about sloths is their dentition: their dental compliment does not include incisors and canines, and their remaining teeth, molars and premolars, lack enamel. Hoffmann’s Two-toed Sloth has what appear to be large canine teeth, but these are in fact enlarged premolars.

Sloths are tenacious survivors and are not in any immediate danger of extinction. So long as our Neotropical rainforests are conserved, even if only in National Parks and reserves, these curios of the natural, the sloths, have a secure future.

Mike Boston is a biologist, wilderness expedition guide, and the president of Osa Aventura. You can contact him at at info@osaaventura.com