This ancient and gigantic sloth had an unexpected supplement in his diet

A giant land sloth who lived during the last ice age was not largely vegetarian like its modern arboreal parents, but enjoyed munching on meat, according to a new study that found telltale signs of its diet in samples of fossilized hair.

Mylodon darwinii had become extinct 10,000 to 12,000 years ago along with most other megafauna, and scientists assumed it only ate plants. But a comparison of the chemical signatures in Mr. 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 an omnivore in an ancient species of sloth”, explains paleontologist Julia Tejada from the University of Montpellier in France. Along with other xenarthrans, such as anteaters and armadillos, these sloths have been a significant part of South American ecosystems for the past 34 million years.

Since all six species of living sloths are herbivores, it has long been thought that Mr. darwinii – which was named after Charles Darwin who discovered his remains in Argentina in 1832 – was also a plant-loving herbivore. His teeth, jaw, large foregut, and feces all suggest Mr. darwinii was not an active predator.

But this new research upsets this thinking and suggests Mr. darwinii could have been a curious meat scavenger picking up scraps, or even an opportunistic omnivore, swallowing meat or other animal protein if available.

“Whether they are sporadic scavengers or opportunistic consumers of animal protein cannot be determined from our research,” explains Tejada. “But we now have strong evidence contradicting the long-held presumption that all sloths were obligate herbivores.”

In the past, some researchers have speculated that ancient South American ecosystems had more herbivores than available plants could support. While this idea has yet to be tested, this new study does provide some clues as to what other heavy animals like. Mylodon ate to supplement their diet.

The results also prompted scientists to rethink where Mr. darwinii is part of the food chain and reassesses the ecological structure of ancient mammal communities that lived in South America millions of years ago, before most of the megafauna became extinct.

In the study, Tejada and his colleagues analyzed strands of hair torn from two sloth fossils, five modern zoo-fed xenarthrans, and eight wild omnivorous species, including the squirrel armadillo and the black-capped squirrel monkey.

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

Like other megafauna, Darwin’s land sloths were truly huge creatures. Among the hundreds of other fossil sloths that once roamed the ice-covered Americas, Mr. darwinii measured almost 3 meters (10 feet) from head to toe and weighed approximately 1,000 to 2,000 kilograms (2,200 to 4,400 pounds).

But these gentle giants, who lived close to coastal areas, also had blond fur and skin riddled with bone deposits called osteoderms, and it is these types of tissue that preserve the chemical markers available for analysis today.

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

Tejada and his colleagues first analyzed amino acid nitrogen levels in samples from modern herbivores and omnivores to find a clear signal to eat a mixture of plants and animal proteins compared to plants alone, then analyzed the two fossils.

Mr. darwinii skin and feces on display at the American Museum of Natural History. (AMNH / D. Finnin)

While the other earthly sloth turned off from study, Nothrotheriops shastensis, was probably an obligate herbivore, data suggests Mr. darwinii was not and probably consumed a diet similar to that of the modern American marten – a type of weasel find in the northern parts of North America.

“[Mylodon’s] the feeding behavior corresponds better to that of an omnivore, consuming plant material but sometimes also incorporating elements of animal origin in its diet, ”the researchers write in their article.

Based on these results and taking into account the freezing conditions of the Americas when Mr. darwinii and other megafauna have lived, the research team suspects that the giant sloth supplemented its diet with high-energy meat to meet its high energy needs, in order to increase its metabolism to maintain a constant body temperature in cooler conditions.

Knowing how large plant-nibbling herbivores have a dramatic impact on vegetation structure, soil moisture and the carbon cycle of an ecosystem, discovering that at least one extinct sloth species ate more than 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 species of fossil sloths also had more versatile feeding behaviors than was 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 Mr. darwinii found by Charles Darwin here, courtesy of the UK Museum of Natural History.