Fossil teeth and other skeletal tissues hold many clues to an extinct animal’s diet. But a new study by Museum researchers challenges a long-held assumption that all plant-eating mammals, from the tiniest shrew to the largest sloth, can be approached the same way.
To glean details about the diets of long-extinct animals, paleontologists have historically focused on fossil teeth and jaws: tooth shape, tooth enamel wear-and-tear, and the shape of skull and jaws all offer clues.
One relatively recent method of analyzing teeth and other biological tissues hinges on a specific biomarker: stable carbon isotopes. Found in different proportions in various plants, stable carbon isotopes are also preserved in the tissues of the animals that consume vegetation, offering paleontologists a window into the diets and other features of extinct animals’ environment.
“Stable carbon isotope studies now are an essential part of our scientific toolkits, informing a wide array of paleontological and geological topics, from predator-prey interactions to ancient rainfall estimates,” says John Flynn, who is Frick Curator of Fossil Mammals in the Museum’s Division of Paleontology.
Until now, scientists had assumed that all plant-eating mammals absorbed carbon isotopes from plants similar to how cows and other large ungulates do today: at a constant proportion of 14 per mil (or 14 parts in a thousand.) That constant, called an enrichment value, is widely used in calculations for geochemical analyses of animal skeletal tissues. But a new study by Julia Tejada-Lara, a graduate student at Columbia University and the Museum, and Flynn challenges that assumption—and shows that enrichment value fluctuates based on body size.
“Using a single value for all mammal herbivores, from tiny monkeys to giant elephants, seemed like a large assumption to me,” says 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.”
Working with the Huachipa Zoo in Lima, Peru, Tejada-Lara tracked the diets of two modern sloth species, the three-toed sloth and the two-toed sloth, documenting the amount of stable carbon isotope they consumed. After sampling the sloths’ tooth dust, which was collected during routine veterinary care, the researchers found that the enrichment value for each species was different from the constant assumed for all herbivorous mammals—and from each other.
Additional measurements of the enrichment value of the extinct South American sloth species Mylodon darwinii turned up yet another variation—and pointed to significant differences among closely related species. Further research branched out beyond sloths to other mammals with known plant diets, revealing a pattern: enrichment value generally grows with body size and depends on the makeup of the digestive tract.
The researchers have proposed new models for how carbon is incorporated into mammalian herbivores’ teeth, detailed in a new study out this week from the Proceedings of the Royal Society B.