Sloth

The sloth has taken slowness further than any other mammal. It moves slowly, digests slowly, breathes slowly, and grows slowly — all because it lives on a food that barely pays for the effort of eating it.

Overview

There are six species of sloth in two groups: three-toed sloths (genus Bradypus) and the slightly larger, faster two-toed sloths (Choloepus). They live in the rainforest canopy from Central America to northern Argentina, hanging upside down from branches by long, hooked claws, and most of them spend almost their entire lives in the trees.

An extreme energy budget

Leaves are a poor food — low in energy, high in tough fibre, and often laced with toxins. A sloth’s response has been to slash its energy costs to the bone:

• It has roughly 30% less muscle mass than a mammal of its size.
• Its metabolic rate is among the lowest of any mammal, and its body temperature rises and falls with the surrounding air rather than being held constant.
• Its multi-chambered stomach ferments leaves so slowly that digesting one meal can take a month, and a full stomach can be a third of the animal’s weight.
• It sleeps 8–10 hours a day in the wild and moves only when it has to, at about four metres a minute.

Life upside down

Sloths are so well adapted to hanging that their internal organs are anchored to the ribcage to keep them from pressing on the lungs, and their fur grows the “wrong” way — parting along the belly and running toward the back — so rain sheds off while the animal is inverted.

That fur is a habitat in its own right. Grooved hairs trap moisture, and green algae grow in them, giving the sloth a greenish tint that helps it disappear into the canopy. Living in the fur are sloth moths and beetles found on no other animal, part of an ecosystem that travels with each sloth.

The one thing a sloth cannot do in the trees is defecate, so about once a week it makes the slow, risky climb to the forest floor, digs a small hole with its stubby tail, relieves itself, and climbs back up. Roughly half of all adult sloth deaths occur during this weekly trip, when the animal is exposed to jaguars, ocelots, and — increasingly — dogs and cars.

Reproduction and life cycle

Sloths are solitary. Females give birth to a single baby after a five- to six-month pregnancy (longer in two-toed sloths). The infant clings to the mother’s belly for several months, learning by licking her lips which of the canopy’s leaves are safe to eat — a knowledge that ties each sloth to a small number of specific trees for life. Young sloths become independent at around a year old.

Threats and conservation

Most sloth species are listed as Least Concern, but the tiny pygmy three-toed sloth, found only on one small island off Panama, is Critically Endangered with perhaps a few hundred individuals left.

The broader pressures on sloths are:

• Deforestation and forest fragmentation, which strand sloths in isolated trees and force the dangerous ground crossings.
• Roads and power lines — sloths climb power poles and are electrocuted, or are hit by vehicles.
• The illegal pet and tourism trade, which takes sloths from the wild for photo props.
• Dog attacks around the expanding edges of towns and farms.