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  • In 1796, Thomas Jefferson received a box of bones he couldn't identify.

  • A long, sharp claw reminded him of a lion,

  • but the arm bones suggested a larger animal,

  • one about three meters long.

  • Thinking it might be huge unknown species of North American lion,

  • Jefferson warned explorers Lewis and Clark

  • to keep an eye out for this mysterious predator.

  • But Jefferson's box of bones didn't come from a lion.

  • They came from an extinct giant sloth.

  • Prehistoric ground sloths first appeared around 35 million years ago.

  • Dozens of species lived across North, Central and South America,

  • alongside other ancient creatures like mastodons

  • and giant armadillos.

  • Some ground sloths, like the megalonychidae, were cat-sized,

  • but many were massive.

  • Jefferson's sloth, Megalonyx, weighed about a ton,

  • and that was small compared to megatherium,

  • which could reach six metric tons, as much as an elephant.

  • They ambled through the forests and savannas using their strong arms

  • and sharp claws

  • to uproot plants and climb trees,

  • grazing on grasses, leaves, and prehistoric avocados.

  • In fact, we might not have avocados today if not for the giant sloths.

  • Smaller animals couldn't swallow the avocado's huge seed,

  • but the sloths could,

  • and they spread avocado trees far and wide.

  • Ground sloths flourished for millions of years,

  • but around 10,000 years ago, they started disappearing

  • along with the Western Hemisphere's other giant mammals.

  • Researchers think that ground sloths could have been pushed out

  • by an oncoming ice age,

  • or competition with other species, maybe humans,

  • who arrived in the region around the time most of the sloths went extinct.

  • Some of the smaller sloths did survive and migrated to the treetops.

  • Today, there are six species left living in the rainforest canopies of Central and South America.

  • Hanging out in the trees is a good way to avoid predators,

  • and there are plenty of leaves to eat.

  • But this diet has its drawbacks.

  • Animals extract energy from food and use that energy to move around,

  • maintain their body temperature,

  • keep their organs working,

  • and all the other activities necessary for survival.

  • But leaves don't contain much energy,

  • and that which they do have is tough to extract.

  • Most herbivores supplement a leafy diet with higher energy foods

  • like fruit and seeds.

  • But sloths, especially three-toed sloths, rely on leaves almost exclusively.

  • They've evolved finely tuned strategies for coping with this restricted diet.

  • First, they extract as much energy from their food as possible.

  • Sloths have a multi-chambered stomach that takes up a third of their body,

  • and depending on the species,

  • they can spend five to seven days, or even weeks, processing a meal.

  • The other piece of the puzzle is to use as little energy as possible.

  • One way sloths do this is, of course, by not moving very much.

  • They spend most of their time eating, resting, or sleeping.

  • They descend from the canopy just once a week for a bathroom break.

  • When sloths do move, it's not very fast.

  • It would take a sloth about five minutes to cross an average neighborhood street.

  • This unhurried approach to life means that sloths don't need very much muscle.

  • In fact, they have about 30% less muscle mass than other animals their size.

  • Sloths also use less energy to keep themselves warm

  • because their body temperature can fluctuate by about five degrees Celsius,

  • less than a cold-blooded reptile, but more than most mammals.

  • These physical and behavioral adaptations minimize the sloth's energy expenditure,

  • or metabolic rate.

  • Three-toed sloths have the slowest metabolism of any mammal.

  • The giant panda is second slowest,

  • and two-toed sloths come in third.

  • Moving slowly has allowed sloths to thrive in their treetop habitat.

  • But it's also made the sloths themselves a great habitat for other organisms,

  • including algae, which provides a little extra camouflage, and maybe even a snack.

  • Sloths may not be giant anymore,

  • but that doesn't make them any less remarkable.

In 1796, Thomas Jefferson received a box of bones he couldn't identify.

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B2 US TED-Ed sloth energy jefferson giant lion

【TED-Ed】Why are sloths so slow? - Kenny Coogan

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    Anita Lin posted on 2017/04/25
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