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
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!
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
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
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.
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.
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.
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
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.
a biologist, wilderness expedition guide, and the president of
Osa Aventura. You can contact him at at firstname.lastname@example.org