Scientists have long determined what extinct animals ate by analyzing carbon isotopes locked inside their fossil teeth. And, for two decades, a key isotope value in these calculations has been assumed to be the same for all plant-eating mammals. But by comparing living captive South American sloths with long-gone cousins, researchers have upset this assumption; their study reveals that the value changes with body size. The finding, published this week in the journal Proceedings of the Royal Society B, might change some views of how mammals, including us, evolved.
For a long time, scientists inferred the diets of extinct vertebrates through anatomical data—for example, the shape of teeth, the height and amount of wear of tooth enamel, and the configuration of the skull and jaw. More recently, researchers started using stable carbon isotopes to directly determine diets. Stable carbon isotopes, which form in specific proportions inside plants, are preserved in the body tissues of the animals that consume them. By taking samples from bones, teeth, nails or other biological material, paleontologists can determine the kinds of plants that individual animals ate, how different species might have split up resources in the same habitat, and even overall rainfall or other environmental characteristics.
As an animal incorporates carbon from its diet into its tissues, an array of physiological processes affects how much of and in what proportions the carbon isotopes actually get absorbed. Based on studies of cows and other large ungulates, scientists have widely assumed that the skeletal tissues of all plant-eating mammals are “enriched” by about 14 parts per thousand in stable carbon isotopes relative to the food that they eat. That constant, called the enrichment value, is an essential part of the equation scientists use for analyses of animal diets. But lead author Julia Tejada-Lara, a graduate student at Columbia University’s Lamont-Doherty Earth Observatory and the American Museum of Natural History, was suspicious that something was overlooked with this one-size-fits-all equation.
“Using a single value for all mammal herbivores, from tiny monkeys to giant elephants, seemed like a large assumption to me,” said Tejada-Lara. “I decided to take a closer look at the enrichment value in living and fossil sloths, because they are outliers in a number of traits when compared to plant-eating mammals as a whole.”
In collaboration with the Huachipa Zoo in Lima, Peru, Tejada-Lara sampled the food and tooth dust—collected during routine veterinary dental care—of two species of modern sloths: the three-toed sloth (Bradypus variegatus) and the two-toed sloth (Choloepus hoffmanni). The sloths were kept under controlled and steady diets from a young age, giving researchers the ability to directly measure the amount of stable carbon isotope they ingested. When they calculated the enrichment value based on samples taken from the sloths’ teeth, they found it to be 10 parts per thousand for three-toed sloths, and 12 for two-toed sloths.
They then measured the enrichment value for the giant extinct ground sloth Mylodon darwinii, which lived in South America until about 10,000 years ago. By sampling the dung recovered with the fossil and the teeth of the specimen, the researchers determined that tooth tissue was enriched by about 15.5 parts per thousand relative to the proportions in the dietary carbon.
The researchers combined the sloth data with enrichment values calculated for a range of other mammalian herbivores with known diets, from prairie voles to pigs to giraffes, and found a clear pattern: the enrichment value generally increases along with body size. They found that at 9.1 parts per thousand, the house mouse had the lowest value of those measured, and the extinct Mylodon had the largest value. Based on this study, the research team has proposed new models of how carbon is incorporated into mammalian plant-eaters, from food to teeth.
The wide spread in values “can make a big difference in the interpretation of research results, including the potential of this discovery to provide more precision on how the evolution of the diet in our own lineage has evolved, because primates include small and large species,” Tejada-Lara said.
“Stable carbon isotope studies now are an essential part of our scientific toolkits, informing a wide array of paleontological and geological topics, [which] are essential for a more accurate understanding of the history of diverse species, ecosystems, and climate,” said John Flynn, a fossil curator at the American Museum and coauthor of the study.
Other authors of the study include Bruce MacFadden, from the University of Florida; Lizette Bermudez and Gianmarco Rojas from the Huachipa Zoo; and Rodolfo Salas-Gismondi from the Cayetano Heredia University and the San Marcos National University in Peru. Funding was provided by the Florida Museum of Natural History, U.S. National Science Foundation, Columbia University’s Graduate School of Arts and Sciences, and the American Museum of Natural History.
(This story adapted from a press release by the American Museum of Natural History.)