Thursday, October 16, 2008

Extinct: Megatherium, Giant Ground Sloth

The world hides its secrets. Under rocks and water lie clues to a fantastic past. Our world today bears little resemblance to the world 10,000 years ago (a blink in the Earth's 4.6 billion year history). 10,000 years ago in the Americas, people lived along side a collection of incredible beasts - mammoths, saber-toothed cats, short-faced bears, giant beavers and condors, and the most recent installment in our Extinct series, Megatherium - the giant ground sloth.

Megatherium lived from around 2 million to around 10,000 years ago. It weighed about 4 tons and, when standing, measured nearly 20 feet tall. For comparison, modern tree sloths measure a puny 2-3 feet. Modern sloths eat mostly plants and sometimes insects. They occasionally scavenge. The giant sloth had similar habits, though on a larger scale. It is thought to have been primarily a plant eater and scavenger of kills but also possibly a hunter of the bizarre Glyptodon. While Megatherium may have moved faster than its modern sloth relatives, it was nevertheless lethargic. The giant sloth hunting the near comatose Glyptodon, a taxi-sized armadillo, must have been the most comically slow hunt in history (with the possible exception of every starfish hunt), a slow-motion battle of life and death.

If you'll allow a brief detour, it's worth considering for a moment the sloth's, well, sloth. The sloth represents a paradox of evolution and caution against anthropomorphizing evolution's methods and guessing its intentions (evolution doesn't have intentions, as the blind salamander can attest). At first glance, it's hard to imagine how moving absurdly slowly could be an advantage in the fight/flight world. The answer is that modern sloths enjoy a very low basal metabolic rate and body temperature that allows them to survive on the poor nutritional content of their leafy diet. Also related to the sloth's apathy is the flourishing of bacteria and algae in its fur. The algae renders the sloth green, offering camouflage in the trees. The algae/bacteria attract small insects that live on the sloth, including an abundance of beetles (beetles are themselves an amazing evolutionary success story). Here is a nice article for more on the sloth's truly bizarre existence.

Returning to the Megatherium, I find myself a little exhausted. Suffice to say it was an extraordinary animal and, like much of Pleistocene megafauna, was driven to extinction partly by man. The least we can do is remember it. Along with other huge animals, Megatherium makes a cameo appearance in the very enjoyable BBC series Walking with Beasts, narrated by Kenneth Branagh. The video clip below follows a Smilodon clan with the giant sloth appearing at the 8:20 mark.

(image credit: illustration of Megatherium, wikipedia.com)

Update: the prior video was removed. Here is a different Youtube clip of Walking with Beasts featuring Smilodon, Glyptodon and Megatherium.