Most people are familiar with modern sloths — slow-moving, tree-dwelling mammals that digest food slowly and descend only occasionally. Their closest living relatives are anteaters and armadillos, a surprising family connection that hints at a complex evolutionary past.
Today, only two sloth species survive , but once there were dozens, ranging from small tree dwellers to enormous ground giants. Scientists have now uncovered why these extinct giant ground sloths grew to such immense sizes and what factors eventually led to their extinction.
How Sloth Sizes and Habits Were Twisted by Their Homes
Ancient sloths adapted to a wide range of environments, from dense forests to deserts and even mountainous regions. These habitat differences played a key role in the vast size variations among sloth species. The largest, such as those from the genus Megatherium , could weigh around 8,000 pounds — roughly the size of an Asian bull elephant. “They looked like grizzly bears but five times larger,” according to Rachel Narducci, collection manager of vertebrate paleontology at the Florida Museum of Natural History.
Unlike the tiny tree sloths we see today, many extinct sloths were too big to climb. Their sizes ranged dramatically, with some species like the Shasta ground sloth weighing significantly less but still robust enough to thrive in desert environments. Tree-dwelling sloths, on the other hand, remained small, averaging about 14 pounds , because branches impose a strict weight limit — they simply cannot support large animals without breaking.
Ancient DNA Reveals How Sloths Became Giants
The recent study published in combined ancient DNA with detailed measurements of over 400 fossils from 17 natural history museums to trace sloth evolution. Researchers created an extensive “ sloth tree of life ” spanning more than 35 million years, tracking shifts in size and habitat preference over time.
The research showed that the earliest sloths were small, ground-dwelling creatures about the size of a great Dane . Sloths' size changes strongly correlated with major climatic events. During the Mid-Miocene Climatic Optimum, around 15 million years ago, massive volcanic activity in the Pacific Northwest released greenhouse gases, causing global warming.
In response, sloths shrank in size, possibly because warmer temperatures expanded forest habitats that favored smaller, more agile sloths and because smaller bodies help manage heat stress. When the climate cooled again, sloths grew larger, reaching their peak size during the Pleistocene ice ages. This increase likely helped them conserve energy and water in challenging environments.
Did Humans Doom the Giant Ground Sloths?
Despite their size and adaptations, giant ground sloths vanished around 15,000 years ago , coinciding with the arrival of humans in North America. Their large bodies, which once protected them from most predators, made them vulnerable to early human hunters because they were neither fast nor well-defended.
Tree sloths avoided extinction longer, with some Caribbean species surviving until about 4,500 years ago — around the time humans first settled the islands . The disappearance of these last survivors roughly parallels the era when ancient Egyptians were building the pyramids