The Giant Ground Sloth Developed Three Distinct Rotations for a Common Purpose

The cool and arid climate has shaped sloths into giants—before humans potentially drove these large animals to extinction.

Today’s sloths are small, well-known herbivores that navigate through the lush canopy of tropical rainforests. However, for tens of millions of years, South America was home to an astonishing variety of sloths, many of which were massive ground dwellers, with some giants weighing close to five tonnes.

This remarkable range of sizes is of particular interest to Alberto Boscani from the University of Buenos Aires, Argentina, and his colleagues.

“Body size is correlated with all biological characteristics of an animal,” states Boscaini. “This provides a promising avenue for studying sloth evolution.”

Boscaini and his team have synthesized data on physical attributes, DNA, and proteins from 67 extinct and extant sloth genera (groups of closely related species) to construct a family tree that illustrates their evolutionary relationships.

They then analyzed this evolutionary timeline, spanning 35 million years, incorporating insights on habitat, diet, and lifestyle for each sloth. They also examined evolutionary patterns in body size and made weight estimates for 49 ancient and modern sloth groups.

The findings indicate that the evolution of sloth body sizes was significantly influenced by climate change and shifts in habitat. For instance, certain sloth genera began adapting to arboreal living, much like today’s sloths, resulting in a reduction in body size.

Simultaneously, three separate lineages of sloths evolved the proportion of elephant-like features independently. This adaptation appears to have occurred in the last few million years as global cooling and the uplift of the Andes transformed South America into a drier environment.

“The giants are more closely associated with colder and drier climates,” remarks team member Daniel Casari from the University of Sao Paulo, Brazil.

A significant number of these various sloths went extinct in two catastrophic phases: one around 12,000 years ago and another approximately 6,000 years ago, according to Boscaini.

“This aligns with the expansion of Homo sapiens across the American Supercontinent and subsequently into the Caribbean,” he explains, noting that many giant sloths lived in these regions. The surviving sloth species primarily inhabit trees, making them less accessible to humans compared to larger sloths.

The hypothesis that humans played a significant role in the extinction of ancient megafauna is strongly supported, states Thaís Rabito Pansani from the University of New Mexico, who was not part of the research.

“However, solid evidence is necessary to substantiate this theory, especially concerning unresolved and highly debatable issues such as megafauna extinction,” she emphasizes. Recent evidence adds context to this narrative.

“Sloths flourished for much of their history,” says Casari. “[The findings] indicate how a once-successful group can quickly become vulnerable.”