Most people know sloths as slow-moving, bear-like creatures that dangle from trees, take nearly a month to digest a single meal, and defecate only once a week. Their closest relatives are anteaters and armadillos, which may sound like an unusual connection, but evolutionary history explains the link. Today only two sloth species exist, but in the past there were dozens, including one with a bottle-shaped snout specialized for eating ants and another that closely resembled early armadillos.

Many of these ancient sloths were far too large to live in trees. The giants of the group, belonging to the genus Megatherium, grew to the size of Asian bull elephants and weighed around 8,000 pounds [~3629kg].

“They looked like grizzly bears but five times larger,” said Rachel Narducci, collection manager of vertebrate paleontology at the Florida Museum of Natural History.

Narducci co-authored a study published in Science in which researchers examined ancient DNA and analyzed more than 400 fossils from 17 natural history museums to understand how and why some sloths reached such massive proportions.

Ground-dwelling sloths displayed an extraordinary range of body sizes. At one extreme was the enormous Megatherium, capable of stripping leaves from tall trees with its long, flexible tongue, serving as an ecological counterpart to giraffes. At the other was the comparatively smaller Shasta ground sloth, which thrived in the deserts of North America by feeding on cacti.

Tree-dwelling sloths, however, followed a different evolutionary path. Those that lived exclusively in the forest canopy have always been small, averaging about 14 pounds [~6.35kg]. Species that split their time between the ground and the trees were somewhat larger, weighing an average of 174 pounds [~79kg].

You don’t have to be a scientist to puzzle out why trees enforce a strict weight limit. It’s the same reason why modern tree sloths have a strange elastic quality to them: Branches break when put under too much strain, and sloths are not generally known for their ability to swiftly avert sudden disaster. Tree sloths have reportedly survived falls of up to 100 feet [~30m]. However, given that falls from even moderate heights can cause severe damage and some trees in the Amazon Rainforest top out at just under 300 feet [~91m], it makes evolutionary sense to be as small as possible when going out on a limb.

Why Did Ground Sloths Get So Big?
What’s less clear is why some ground sloths grew to such excessive sizes while others seemed content with being merely large. There may have been several reasons, which is why it’s been so hard for scientists to answer the question with confidence.

Larger sizes might have been advantageous for finding food or avoiding predators, for example. Ground sloths had a special fondness for caves, and their size undoubtedly played a role in their ability to find and make shelters. The moderately sized Shasta ground sloth favored small, natural caves bored by wind and water into the cliffsides of the Grand Canyon, like the alveoli of a gigantic, geologic lung. These also doubled as convenient latrines; in 1936, paleontologists discovered a mound of fossilized sloth poop, bat guano, and packrat middens more than 20 feet [~6m] thick in Rampart Cave, near Lake Mead.
Scientists analyzed ancient DNA and compared more than 400 fossils from 17 natural history museums to figure out how and why extinct sloths got so big.

Larger sloths weren’t restricted to pre-existing caves. Using claws that are among the largest of any known mammal, living or extinct, they could carve their own from bare earth and rock. Many of the caves they left behind are still around with claw-mark décor along the interior walls, evidence of their ancient nesting excavations.