The South American sloth, an evolutionary success story until humans arrived

Madrid, May 22 (EFE).- Sloths are small, slow-moving herbivores that live suspended from tree branches in the jungles of South America, but it wasn't always that way. About 15 million years ago, there were a wide variety of different species, some gigantic and others weighing only ten kilos.

A team of scientists led by the Universities of São Paulo (Brazil) and Buenos Aires (Argentina), with the collaboration of the National Museum of Natural Sciences (MNCN) in Madrid, has reconstructed the expansion and decline of these mammals, which had an incredible capacity for adaptation, at least until the arrival of Homo sapiens, which practically wiped them out.

By combining fossil research, genomic data, and advanced evolutionary models, the authors have reconstructed the evolutionary history of sloths and the size modifications they developed to adapt to climate change and survive both in grasslands (a terrestrial environment) and in trees.

Amazing transformations

The study, whose details were published this Thursday in the journal Science , recalls that sloths appeared 35 million years ago in South America, a region that evolved in isolation from the rest of the world for several million years, giving rise to numerous unique native mammals.

The first sloths were large, ground-dwelling animals that weighed between 80 and 350 kilos.

These animals adapted their size to different lifestyles, diets and habitats until around 15 million years ago there could have been up to a hundred genera distributed throughout the American continent, some weighing more than four tons (terrestrial ones) and others less than ten kilos (arboreal ones).

Extreme sizes and ecological adaptations

"Fifteen million years ago, there was a wide variety of sloth sizes. The first sloths were the size of an anteater, and then they evolved to much larger sizes, around a ton, and at the same time, to much smaller sizes of fifty kilos or less," Juan Cantalapiedra, a researcher at the MNCN and co-author of the study, told EFE.

Specifically, the study divides these mammals into "three major ecologies: 'terrestrial,' which were the largest; 'semi-arboreal,' with more intermediate sizes that did not allow for a fully arboreal life; and 'fully arboreal,' which needed to be very small, like today's mammals," notes the MNCN paleobiologist.

But around 14 or 15 million years ago, the climate began to change and the Earth began to cool. These environmental changes replaced forests with more open landscapes and wooded grasslands that favored the expansion of large mammals, especially large herbivores such as sloths, mastodons, and antelopes and buffalo.

As a result of this climate change, giant species, such as Megatherium americanum , reigned over the landscapes of the Pleistocene, between approximately 2.5 million years ago and 12,000 years ago.

Catastrophic for large sloths

However, the arrival of Homo sapiens to the American subcontinent some 15,000 years ago was catastrophic for the large sloths. The study details that during the transition between the Pleistocene and the Holocene, the giant sloths rapidly succumbed—first on the continent and then on the islands—unable to cope with human expansion. Only the smaller, tree-dwelling species remained.

Their extinction chronology goes hand in hand with human expansion. "No previous climate crisis affected them so radically, which points to anthropogenic pressure as the new variable and the final blow," explains Alberto Boscaini of the University of Buenos Aires (Argentina).

The little ones, the only survivors

Unlike the giants, the smaller lineages, which led a completely arboreal life—weighing between ten and twenty kilos, like today's—and were relatively recent (they emerged about three million years ago), managed to outlive humans and extend their lineage to the present day.

For the authors, the long evolutionary history of sloths demonstrates that they were able to adapt to geological, climatic, and ecological changes and reinvent themselves to survive in trees and on the ground. "This group turned versatility into opportunity," notes Daniel Casali of the University of São Paulo (Brazil) and co-author of the study.

However, its abrupt end highlights a harsh reality: "The arrival of something unexpected, in this case humans, is enough to put an end to a success story. Sloths are an example that in evolution, nothing is ever guaranteed or constant," Cantalapiedra concludes. EFE

ECG/ICN

A sloth bear in Costa Rica. EFE/Jeffrey Arguedas