Darwin’s Ground Sloth was Opportunistic Omnivore, Study Says

New research published in the journal Scientific Reports provides the first direct evidence of omnivory in an ancient sloth species.

Reconstruction of the Darwin’s ground sloth (Mylodon darwinii) feeding on the carcass of the hoofed native herbivore Macrauchenia. These extinct mammals roamed the Pleistocene landscape of Patagonia and other parts of high and mid-latitude South America, like this reconstructed scene from about 12,000 years ago in front of the famous Mylodon Cave (Cueva del Milodón) in southern Chile. Image credit: Jorge Blanco.

Reconstruction of the Darwin’s ground sloth (Mylodon darwinii) feeding on the carcass of the hoofed native herbivore Macrauchenia. These extinct mammals roamed the Pleistocene landscape of Patagonia and other parts of high and mid-latitude South America, like this reconstructed scene from about 12,000 years ago in front of the famous Mylodon Cave (Cueva del Milodón) in southern Chile. Image credit: Jorge Blanco.

Even though the six living sloth species all are relatively small herbivorous tree-dwellers restricted to tropical forests of Central and South America, hundreds of extinct sloth species — some as large as an elephant — roamed ancient landscapes from Alaska to the southern tip of South America.

One of these giants, the Darwin’s ground sloth (Mylodon darwinii), lived in South America between 1.8 million years and 12,000 years ago.

The extinct species was between 3 and 4 m (10-13 feet) long and weighed between 1 and 2 tons.

Based on dental characteristics, jaw biomechanics, preserved excrement from some very recent fossil species, and the fact that all living sloths are herbivores, the Darwin’s ground sloth and its extinct relatives have long been presumed to be herbivores as well.

“Whether they were sporadic scavengers or opportunistic consumers of animal protein can’t be determined from our research, but we now have strong evidence contradicting the long-standing presumption that all sloths were obligate herbivores,” said Dr. Julia Tejada, a researcher at the University of Montpellier, the American Museum of Natural History, Columbia University, the Museo de Historia Natural-UNMSM.

In the study, the researchers used an innovative approach called amino acid compound-specific isotope analysis.

Found in different proportions in the food consumed by an animal, stable nitrogen isotopes are also preserved in their body tissues, including hair and other keratinous tissues like fingernails, as well as in collagen like that found in teeth or bones.

By first analyzing the amino-acid nitrogen values in a wide range of modern herbivores and omnivores to determine a clear signal of eating a mix of plant and animal food, fossils can then be measured to determine the food they consumed.

This offers paleontologists a unique window directly into the diets of animals, enabling them to determine their trophic level — whether they were plant-eating herbivores, mixed-feeding omnivores, meat-eating carnivores, or specialized marine animal consumers.

“Prior methods relied solely on bulk analyses of nitrogen and complex formulas that have many untested or weakly supported assumptions,” said Dr. John Flynn, a researcher at the American Museum of Natural History and Columbia University.

“Our analytical approach and results show that many previous conclusions about tropic levels are poorly supported at best, or clearly wrong and misleading at worst.”

The scientists used samples from seven living and extinct species of sloths and anteaters as well as from a wide range of modern omnivores.

While the other extinct sloth in the study, the North American ground sloth (Nothrotheriops shastensis), was determined to be an exclusive herbivore, the data clearly flagged the Darwin’s ground sloth as an omnivore.

“These results, providing the first direct evidence of omnivory in an ancient sloth species, demands reevaluation of the entire ecological structure of ancient mammalian communities in South America, as sloths represented a major component of these ecosystems across the past 34 million years,” Dr. Tejada said.

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J.V. Tejada et al. 2021. Isotope data from amino acids indicate Darwin’s ground sloth was not an herbivore. Sci Rep 11, 18944; doi: 10.1038/s41598-021-97996-9

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