Photograph by Joel Sartore, Nat Geo Image Collection
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Built for the leisurely life: a three-toed sloth in an ambaibo tree.

Photograph by Joel Sartore, Nat Geo Image Collection

Why Do Some Animals Live Life in the Slow Lane?

When running away from danger isn’t an option, defense is crucial.

Not all animals feel the need for speed.

“Why are sloths so slow?” Vittorio Colonna recently asked us via Facebook. Sloths may be the slowpokes of Central and South American forests, but they are hardly the only animals that dawdle through life. So Saturday’s Weird Animal Question of the Week thought we’d look at some of the planet’s pokier animals and why, for them, slow and steady wins the evolutionary race.

Pacing themselves

The slowest mammals in the world, sloths, have a top speed of a mere mile (1.6 kilometers) per hour. Their biology makes them poke along at a rate that would get them honked at in traffic.

Sloths are “living on the edge of their energy budget,” Becky Cliffe, a zoologist at the Sloth Sanctuary of Costa Rica, says via email. They have a very slow metabolism and “every move has to be planned out.” The animals can’t even regulate their own body temperature, and that affects their digestion. Food takes an average of 16 days to pass through a sloth’s digestive system. But warmer weather helps that process go a little faster, Cliffe reported in a study published April 2 in Peerj. That lets sloths eat more when temperatures are higher.

Slowness helps makes sloths masters of disguise. They’re so slow algae grows on their fur, which may help them blend into the tree canopy, although, Cliffe explains, they face few predators in the wild since “big cats and harpy eagles are extremely rare now.” Still, “they move slowly to evade predators sight. Therefore camouflage is extremely important to them,” she says. (Read about the moths that live on sloths.)

Torpid turtles and sluggish slugs

The leisurely lifestyle can be dangerous. Without the ability to outrun predators, many slower animals have adaptations that make them difficult or unpleasant to pursue or eat.

There’s not much better defense than having armor fused to your body, like the iconically slow turtles and tortoises. ”Having that kind of architecture doesn’t make you speedy,” says Jeffrey Lovich, a reptile expert at the U.S. Geological Survey’s Southwest Biological Science Center. The desert tortoise, for example, has an average speed of 0.2 mile (0.3 kilometer) per hour. But the excellent protection on these herbivores has helped them survive for over 200 million years in the slow lane.

Mollusks, the group to which slugs and snails belong, have made it 550 million years without a speeding ticket. Traveling by muscular contractions called pedal waves makes slugs and snails pretty slow. Like turtles, snails rely on a defensive shell. Being nocturnal and having a mucus that smells and tastes nasty helps provide some additional safety.

“Generally animals get fast evolutionarily if they are pursued or if they pursue,” Chris Barnhart, a biologist at the Missouri State University, explains via email. “Fast” is a relative term, though. The Rosy Wolf snail follows the mucus trail of other snails and slugs and whizzes along at about 0.001 mile (0.0016 kilometer) per hour. The scent of these predators, Barnhart notes, will make other snails try to “run” and amp up their speed a bit (this poor fellow didn’t make it).

Shell-less slugs may compensate for their their lack of armor with an even “more copious and stickier defense mucus,” according to the Carnegie Museum of Natural History. The goo may work—domestic cats and dogs tend to avoid slugs.

Slow-moving sea life

Manatees, herbivorous marine mammals found in some shallow coastal areas and rivers, swim at about 5 miles (8 kilometers) per hour, notes George Burgess of the Florida Museum of Natural History. But sea cows, as they are also known, have a tough skin and are so big that even though their main predator, the bull shark, might take a bite out of a manatee tail, the entire animal usually proves too much of a meal.

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Slow-moving but thick-skinned, a Florida manatee in the Weeki Wachee River in Northwest Florida.

Brightly colored, soft-bodied marine mollusks called nudibranchs are “quite beautiful,” Burgess says, but slow moving, covering only about 30 feet (10 meters) a day. They use the toxins they ingest from their prey as a foul-tasting toxic defense against predators.

The slowest moving shark, the Greenland shark, swims as a leisurely pace of about 0.76 miles (1.22 kilometers) per hour, a 2012 study in the Journal of Experimental Marine Biology and Ecology found. This shark “gets up to 20 feet [6 meters] long and has the metabolism of a pine stump,” Burgess says. (Watch a NatGeo researcher realize he’s caught a Greenland shark on camera.)

The sharks’ biggest concern is not defense but offense. The sharks may snag seals while they sleep in the water, where the mammals can avoid predation by polar bears on land. To catch speedy seals, Burgess says, the Greenland sharks will wait at holes in the ice for animals returning to the water and “grab them as they go.”

Weird Animal Question of the Week answers your questions every Saturday. If you have a question about the weird and wild animal world, tweet me, leave me a note or photo in the comments below, or find me on Facebook