Friday, March 6, 2009

This Month’s Curious Critter: the Sloth

The remarkable adaptation of this mammal to its lofty environment is perhaps unfairly reflected in its name. Whereas in some circles, slothful behavior is regarded as a moral sin, for the sloth, it is a neatly refined way of conserving energy and avoiding the attention of potential predators.

Last year, a study came out in the Royal Society journal Biology Letters showing that sloths are not the lazy creatures they’d been held for. In the first study ever to investigate sleep in wild animals, Niels Rattenborg of the Max Planck Institute for Ornithology in Starnberg, Germany, discovered that sloths actually sleep less than 10 hours a day, instead of 16, as observed in captivity.

However, while sloths may not sleep any more than the average human teenager, they earn their name by holding the record as the slowest land mammals. The sloth can sprint short distances at 5 yards per minute, but its average ground speed is closer to 2-3 yards per minute.

Sloths are so slow and inconspicuous that a new species went undiscovered until less than a decade ago.

This remarkable lethargy is one of many tricks that help the sloth maximize the energy that it gets from the energy-poor leaves that make up its diet. Sloths also have large, specialized stomachs similar to those of cows, in which food is digested for up to a month. The contents of the sloth’s stomach contribute up to 30% of the animal’s body weight.

To further conserve energy, sloths have less than half the energy-burning muscle mass of mammals of comparable size. When it is cold, sloths don’t shiver; they roll up in a ball, and their specialized blood vessel systems keep the vital organs warm. Sloths have extremely low metabolic rates, and, although warm-blooded, their body temperature falls when they are inactive.

The sloth spends most of its life hanging upside-down. Eating and sleeping are done in this position. Females give birth in this position. The sloths’ long arms and claws enable them to hang effortlessly from tree branches—so effortlessly, in fact, that they often remain hanging after they die.

Sloths rarely venture from their home trees, except to urinate and defecate. It is during these weekly trips to the forest floor that the sloth’s slowness can prove dangerous.

Because sloths spend most of their time upside-down, their hair parts on their belly, allowing water to run off efficiently. Symbiotic algae growing in their hair causes it to appear greenish. In exchange for a warm place to grow, the algae supply the sloth with camouflage against predators.

This just shows that names can be deceiving. Sloths are not lazy; they are shrewdly tailored to life in the tropical treetops of South America’s rainforests.

Intrigued? Browse these links for more info:

A review of sloth biology
Rattenborg's article on sloth sleep
Speed of a sloth
Sloth fanpage
Video of the newly discovered pygmy sloth