Scientists Reveal the Hairy Truth About Giant Ground Sloths

New York Times

03-04-2025

Ground sloths emerged in South America tens of million years ago, eventually ranging as far north as Canada. While their modern relatives dwell in trees and top out at the size of a dog, ground sloths also occupied land and seashores and, at their largest, rivaled today's elephants.
We know from preserved pelts that some ground sloths had lots of fur. But did other sloths evolve to be nearly hairless?
Research published earlier this year in The Journal of Mammalian Evolution offers new insights into what the extinct sloths may have looked like, and how furry they really were.
Developing a more accurate picture of ground sloths is not simple. Their habitats varied from tropical to cold climates across the Americas. And they came in a variety of sizes. The megatheres, known as Eremotherium and Megatherium, were among the biggest terrestrial mammals of their time.
The team started by determining sloth body temperature. They employed an analysis that two authors involved in the study, Robert Eagle and Aradhna Tripati of the University of California, Los Angeles, pioneered in earlier research that sampled fossilized teeth.
Using three megathere teeth and two teeth from relatively smaller ground sloths, they measured the abundance of certain isotopes within minerals inside the sloth teeth, Dr. Eagle explained.
'Isotopes are transformative to be able to illuminate the body temperature of extinct species,' Dr. Tripati said.
Remarkably, they found that, at 84 to 89 degrees Fahrenheit, these ground sloths had a lower core body temperature than most large land mammals today.
Randon Flores, another study co-author and a Ph.D. student at U.C.L.A., said that low body temperature 'allows us to make inferences about things like metabolic rate,' and from that, he added, 'we can draw comparisons' between the physiology of today's smaller sloths and their much larger extinct relatives.
Three-dimensional computer models created by paleoartists were used to estimate the surface area and body volume of the megatheres and the somewhat smaller species, Mylodon and Nothrotheriops.
Further analysis allowed the team to adjust for factors like wind speed, humidity and the amount of sunshine in various habitats. The goal, said Michael Deak, the paper's lead author, who is currently at Penn State Shenango, was to determine 'a thermal comfort zone' for each species, in which the animal is neither too hot nor too cold.
Megatherium produced the most surprising results. Regardless of the environment, each model indicated a preference for what Mr. Deak described as 'a fairly thick, dense, furry pelt.' Modeling it with sparse elephant-like hair, he continued, indicated the animal would 'be constantly cold-stressed.'
Eremotherium, which was slightly smaller than Megatherium, was the only sloth whose fur coverage might have changed depending on its habitat — without at least a centimeter of dense fur, it would have been too cold in the Andes Mountains and some northern habitats. But in tropical climates, sparse fur might have sufficed. The team suggests that Eremotherium may have had either varied fur coverage depending upon region or it may have shed its coat during warmer seasons.
Smaller Mylodon and Nothrotheriops would have needed one to five centimeters of denser fur year-round, an amount that matches mummified fur from those animals. But even with thick fur, the authors note, these species inhabited areas in South America where it wouldn't have been enough to combat the cold. That dovetails with fossil evidence suggesting the species sheltered in caves, or even hibernated.
The lifestyle of modern tree sloths, said Michael Butcher, a study author and a professor at Youngstown State University, 'is built around conservation of energy.' So he said it makes sense that extinct ground sloths were doing something similar, even 'at much larger body sizes.'
Mariana Di Giacomo, a natural history conservator at the Yale Peabody Museum who was not involved in the research, described its models as fun and necessary. But she wishes the team had sampled more than five teeth, and teeth from more varied regions.
'I think they could have been able to say these things with a lot more authority if there had been more data,' Dr. Di Giacomo explained, particularly if they had sampled more fossils, for example, from colder and more southern regions in Argentina or Chile.
Greg McDonald, a paleontologist formerly with the federal Bureau of Land Management who was also not involved in the research, said the results of this paper were achieved by synthesizing lots of information from various sources 'to see how well it fit together.'
'The isotope data is really a solid foundation,' he said. Obtaining the same results from disparate sources in the models indicates that this is 'probably a valid underlying explanation' about the amount of sloth fur.