Ancient sloths ranged in size from tiny climbers to ground-dwelling giants. Now, researchers report this body size diversity was largely shaped by sloths’ habitats, and that these animals’ precipitous decline was likely a result of increasing human pressures, which also triggered the extinction of the large-bodied ground-dwelling animals. Today’s small arboreal sloths are the last remnants of a once-diverse group, surviving likely because they inhabited secluded forest canopies and avoided direct human pressures, say the authors. While only two small, tree-dwelling genera survive today – confined largely to the tropical rainforests of South and Central America – sloths (Folivora) represent a once-diverse, abundant, and widespread lineage of American mammals. During the late Cenozoic, more than 100 genera of sloths occupied a wide range of sizes and habitats, living across the Americas. Some terrestrial sloth species stood upwards of six meters tall and weighed several tons. However, by the end of the Pleistocene, the majority of these animals became extinct.

Alberto Boscaini and colleagues investigated the drivers behind the expansion and decline of body size variation in sloths over the past 35 million years, culminating in the eventual rapid collapse of the group. By combining fossil measurements, DNA and protein sequences, and advanced evolutionary modeling, Boscaini et al. reconstructed sloth evolutionary history across 67 genera and tested whether evolutionary changes in size were linked to habitat, diet, climate, predation, or other ecological pressures. The findings show that habitat preference – whether sloths lived in trees or on the ground – was the dominant factor shaping their body size evolution. Early sloths were large, ground-dwelling grazers. But transitions to tree-dwelling forms with smaller body sizes occurred multiple times, especially as open landscapes expanded. Gigantism evolved independently in several lineages, reflecting adaptive responses to cooling climates and ecological pressures. Yet, despite thriving for tens of millions of years, with body size diversity peaking in the Pleistocene, sloths experienced a sudden and dramatic decline beginning around 15,000 years ago. These declines do not align with the climatic changes of the time, but instead with the arrival of humans in the Americas. According to Boscaini et al., evidence suggests that human hunting drove the extinction of large-bodied terrestrial sloths.

For reporters interested in trends, a March 2019 Science Advances study by Gustavo Politis et al. highlights direct archaeological evidence of human giant sloth hunting and butchering in the Argentinian Pampas roughly 12,000 years ago. (https://www.science.org/doi/10.1126/sciadv.aau4546)