A skeleton of the extinct giant ground sloth Megatherium americanum, on display at the Museo Argentino de Ciencias Naturales ‘Bernardino Rivadavia’ in Buenos Aires. Photo: Alberto Boscaini
Sloths once inhabited the open landscapes of the Americas in a range of sizes, but the arrival of humans left only small, tree-dwelling species behind.
Long before humans arrived, sloths flourished across the Americas for tens of millions of years, reaching a peak in diversity and body size during the Pleistocene. Variation in body size was driven primarily by available habitat – until, from around fifteen thousand years ago, the large ground-dwelling sloths underwent a dramatic decline, most likely due to human hunting, leaving just two genera of small, tree-dwelling sloths behind.
This is the picture painted by Argentine ecologists and evolutionary biologists, who reconstructed the evolutionary history of 67 sloth genera (suborder Folivora) (Science, 23 May). Using evolutionary models, they tested whether changes in sloth body size were linked to habitat, diet, climate or predation.
Habitat preference – whether sloths lived on the ground or in trees – turns out to be the dominant evolutionary factor shaping their body size. Early sloths were predominantly large, ground-dwelling grazers, but transitions to smaller forest-dwelling or arboreal forms occurred multiple times. Gigantism, as seen in well-known prehistoric giant sloths such as Megatherium and Mylodon, evolved independently in multiple lineages – most likely reflecting adaptations to cooling climates and ecological pressures.
The sudden and dramatic decline of megafauna that began fifteen thousand years ago cannot be adequately explained by climate change alone. All the evidence points to the rise of humans in the Americas as the cause of the large sloths’ extinction.
An artistic reconstruction showing three types of sloth: the arboreal Bradypus tridactylus (below), the forest-dwelling Hapalops elongatus (left), and the giant ground sloth Megatherium americanum. Illustration: Diego Barletta