Why Sloths Once Came in All Shapes and Sizes.

Sloths, once among the most diverse mammals in terms of body size and lifestyle, are now represented by only two small, tree-dwelling species. But the sloth family tree once included over a hundred different genera, ranging from lightweight arboreal climbers to hefty, bear-sized land walkers. A recent study now offers insight into why ancient sloths varied so dramatically in size — and why modern sloths do not.

Researchers have long speculated about the factors influencing sloth body size. Possibilities have included environmental changes, dietary shifts, predation, and human influence. But according to a new comprehensive study published May 22 in Science, the primary determinant appears to be habitat — specifically, whether the sloths lived in trees or on the ground.

Led by paleontologist Alberto Boscaini of the University of Buenos Aires, the research team analyzed fossils from 49 different sloth lineages, examining skeletal structures along with proteins and DNA preserved in bones. By reconstructing the evolutionary history of sloths, the scientists traced how different branches of the sloth family expanded or shrank over time and moved between forest canopies and open terrain. Their findings show a recurring evolutionary pattern: sloths that returned to treetop living shrank in size, while those that adapted to ground life grew larger.

Sloths are believed to have originated about 35 million years ago during the Late Eocene Epoch. They evolved in relative isolation on the island continent of South America, away from the larger predators of North America. This isolation allowed the group to flourish and diversify extensively, with some adapting to dense tropical forests and others to expansive grasslands.

Over the millennia, as global climates fluctuated and forests expanded and contracted, some sloth species transitioned between arboreal and terrestrial habitats. Each shift brought corresponding changes in body size, reinforcing the link between environment and morphology.

“Other animal groups face similar ecological pressures, but we don’t often see such dramatic variation in body size,” noted study coauthor Rachel Narducci of the Florida Museum of Natural History. “Sloths give us a rare perspective on how lifestyle and size are interconnected.”

The research also suggests that the most recent common ancestor of today’s sloths was likely a moderate to large terrestrial species — though direct fossil evidence for this ancestor has yet to be discovered. Juan Carrillo, a paleontologist at the National Museum of Natural History in Paris who was not involved in the study, supports this conclusion but urges caution, noting that early sloth fossils remain scarce and incomplete.

Greg McDonald, a former paleontologist with the U.S. Bureau of Land Management, praised the study for offering a broad evolutionary overview. “This kind of big-picture approach is refreshing,” he said. “We often get caught up in describing new discoveries one at a time. This study helps us see the long-term patterns that shaped these remarkable animals.”

By revealing the deep connection between habitat and size, the study brings new clarity to the evolutionary journey of sloths — a journey that once took them across a broad spectrum of forms, far beyond the slow-moving tree-dwellers we know today.

Source:https://www.sciencenews.org/article/sloths-array-of-sizes-heres-why

This is non-financial/medical advice and made using AI so could be wrong.