An artist's reconstruction of Mylodon darwinii. (Jorge Blanco)

This Ancient And Ginormous Sloth Had an Unexpected Supplement in Its Diet

CLARE WATSON
8 OCTOBER 2021

A giant ground sloth that lived during the last ice age was not largely vegetarian like its modern-day tree-dwelling relatives, but enjoyed munching on meat, according to a new study that has found telltale signs of its diet in fossilized hair samples.

 

Mylodon darwinii went extinct some 10,000 to 12,000 years ago along with most other megafauna, and scientists had presumed it ate only plants. But a comparison of chemical signatures in M. darwinii's hair to the diets of other extinct and living species of sloths and anteaters now suggests otherwise.

These results are the "first direct evidence of omnivory in an ancient sloth species," says paleontologist Julia Tejada of the University of Montpellier in France. Along with other xenarthrans, such as anteaters and armadillos, these sloths were a major part of South American ecosystems over the past 34 million years.

Given that all six species of living sloths are plant-eaters, it was long thought that M. darwinii – which was named after Charles Darwin who discovered its remains in Argentina in 1832 – was also a plant-loving herbivore. Its teeth, jaw, large foregut and dung all suggest M. darwinii was no active predator.

But this new research upends that thinking and suggests M. darwinii could have been a meat-curious scavenger picking up scraps, or even an opportunistic omnivore, chowing down meat or other animal protein if it was available.

 

"Whether they were sporadic scavengers or opportunistic consumers of animal protein can't be determined from our research," says Tejada. "But we now have strong evidence contradicting the long-standing presumption that all sloths were obligate herbivores."

In the past, some researchers have speculated that the ancient ecosystems of South America had more herbivores than could be supported by the available plants. Although that idea remains untested, this new study provides some clues about what else hefty animals like Mylodon were eating to supplement their diets.

The findings also have scientists rethinking where M. darwinii sits in the food chain, and reevaluating the ecological structure of ancient mammalian communities that lived in South America millions of years ago, before most megafauna became extinct.

In the study, Tejada and colleagues analyzed hair strands plucked from two sloth fossils, five modern zoo-fed xenarthrans, and eight wild omnivore species, including the screaming hairy armadillo and the black-capped squirrel monkey.

A three-toed sloth (Bradypus variegatus)
Paleontologist Julia Tejada with a three-toed sloth (Bradypus variegatus) in Peru. (Carmen Capuñay)

Like other megafauna, Darwin's ground sloths were truly huge creatures. Among hundreds of other fossil sloths that once roamed the ice-capped Americas, M. darwinii measured nearly 3 meters (10 ft) head to toe, and weighed in at an estimated 1,000 to 2,000 kilograms (2,200 to 4,400 pounds).

But these gentle giants, which lived near coastal areas, also had blond fur and skin riddled with bony deposits called osteoderms, and it's these types of tissues that preserve chemical markers available for analysis today.

 

Stable nitrogen isotopes in the sloths' hair were the target for Tejada and colleagues, as these chemical variants are found at different levels in foods such as plant matter and protein. As animals eat these foods, nitrogen isotopes are slowly incorporated into the building blocks of proteins (aka amino acids) and preserved in an animal's body tissues, including hairs.

Tejada and colleagues first analyzed amino-acid nitrogen levels in samples from modern herbivores and omnivores to find a clear signal of eating a mix of plants and animal protein versus plants alone, then analyzed the two fossils.

M. darwinii fossils on displayM. darwinii skin and dung on display at the American Museum of Natural History. (AMNH/D. Finnin)

While the other extinct ground sloth in the study, Nothrotheriops shastensis, was likely an obligate herbivore, the data suggest M. darwinii was not and probably consumed a diet similar to the modern-day American pine marten – a type of weasel found in the northern parts of North America.

"[Mylodon's] feeding behavior better fits that of an omnivore, consuming plant material but sometimes also incorporating items of animal origin in its diet," the researchers write in their paper.

Based on these results, and considering the icy conditions of the Americas when M. darwinii and other megafauna lived, the research team suspects the giant sloth supplemented its diet with energy-rich meat to meet its high energy demands, as a way to increase its metabolism to maintain a constant body temperature in cooler conditions.

Knowing how large plant-munching herbivores greatly impact the vegetation structure, soil moisture, and the carbon cycle of an ecosystem, finding out that at least one extinct sloth species ate more than just plants could change our understanding of the types of vegetation that dominated ancient landscapes at the time.

"This would be the case in particular if, in addition to Mylodon, other fossil sloth species also had more versatile feeding behaviors than traditionally thought," the research team concludes.

The study was published in Scientific Reports. You can also view a 3D model of the very first specimen of M. darwinii found by Charles Darwin here, courtesy of the UK's Natural History Museum.