SLOTHS ARE MASTERS OF ILLUSION
I am frequently asked to explain how such an apparently flawed creature could have survived the rigors of natural selection, which ruthlessly weeds out the weak. This is when I gather my poise and explain that sloths are not a defective remnant. As a group, sloths have haunted the planet in one shape or another for around 64 million years, managing to outlive both the saber-toothed tiger and the woolly mammoth with its stratagem of stealthy living. Of the half-dozen species alive today, only two are considered endangered. That’s pretty good for a lazy loser. I’ve spent many hours watching sloths and can confirm they are mesmerizingly slow, moving as if they are in glue. Their average cruising speed is a leisurely 0.19 miles per hour, which is unlikely to challenge even a tortoise. They are physically incapable of going faster than their top speed of .93 miles per hour, since their muscles are engineered to be sluggish — up to 15 times slower than those of an equivalent-sized mammal. Sloths are the world’s only inverted quadrupeds. They have evolved to hook on and hang from trees as the animal incarnate of a hairy hammock. As a result, they have almost dispensed with the need for weight-bearing extensor muscles, like our triceps, which stiffen and protract the limbs. Instead they manage almost exclusively with retractor muscles, like our biceps, that pull them along branches. This unusual arrangement demands about half the muscle mass needed to prop up an upright existence and means sloths can hang out for long periods while expending hardly any energy. It also affords them surprising strength and agility. They can clasp a vertical trunk with only their hind limbs. They can also lean over backwards 90 degrees with freed forelimbs. Sloth bodies have an assortment of nifty modifications to enable their energy-saving, inverted existence. Their blood vessels and throat are uniquely adjusted to swallow food and circulate blood against the force of gravity. Their fur grows in the opposite direction to the norm, with a parting down the middle of their tummy so rain can run off easily. They even have sticky bits on their ribs to keep their stomach, which can hold up to a third of a sloth’s body weight in slowly digested leaves, from crushing their lungs. So how does a virtually immobile, somnolent bag of fermenting leaves avoid getting itself eaten? The sloth’s main predator is the harpy eagle, one of the world’s biggest and fastest raptors, with talons the size of grizzly bear claws. It can fly at speeds of up to 80 miles per hour. The sloth doesn’t seem like much of a match for this predator. Running from danger is clearly not an option. But the sloth, you see, is a master of illusion, capable of disappearing into the rain forest. Special grooves in the fur collect water and act as hydroponic gardens for 80 different species of algae and fungi, giving the sloth a greenish hue. It also supports a wealth of insects. Crawling with bugs and looking as if they have been dragged through a hedge backwards isn’t going to win the sloth any beauty contests, but it means they look and smell exactly like a tree. And most of the time they are as motionless as one too. Their arboreal ballet is silent and slow, [slipping] under the radar of the monstrous harpy as it swoops above the canopy, scanning for prey.