What is it about?

Ecologists often classify animal species by the foods they are thought to eat to make sense of their roles in ecosystems. For large mammalian herbivores like bison and deer, diet categorization is typically based on grass consumption, ranging from grass-eating grazers to non-grass-eating browsers. But comprehensively studying large herbivore diets has presented significant challenges: they are dangerous to approach and observe closely, many of their food plants are small and difficult to distinguish from a distance—accurately differentiating between a blade of grass and another small herb through binoculars is nearly impossible—and even expert botanists may struggle to identify some of these plant species in the field. These observational limitations meant we still lacked a detailed picture of what these species were eating, leaving us uncertain whether traditional grass-based categories reflect their diets. We used DNA metabarcoding to construct detailed diet profiles for five large herbivore species at Yellowstone National Park and applied a machine learning algorithm to the data to test how well these traditional labels explain real patterns of dietary variation. The answer: not as well as we thought. While diets among species differ on average, individuals within a species can have very different diets such that some diet profiles from different species were as similar as those from within one species. Combining DNA technology with artificial intelligence enabled us to break through long-standing stereotypes about what these species eat, and our findings challenge the appropriateness of placing them into broad diet groupings.

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Why is it important?

The problem that this study addresses is a classic ecological paradox: “How do Yellowstone’s abundant herds of large herbivore species divvy up the available plant resources without starving each other out?” Traditionally, scientists have explained this by saying species divide up the menu: “grazers” eat primarily on grass, while “browsers” eat little to no grass. Finding that herbivore species’ menus overlap substantially in Yellowstone challenges us to consider the roles of other factors—like spatial separation, seasonal migration, and eating different plant parts—that allow them to inhabit the same landscape. Differences in diet among individuals of the same species may be shaped by interacting factors such as the nutrition, timing of growth, and local availability of plants. Our findings suggest that maintaining plant diversity is critical for supporting the diversity of migratory wildlife.

Perspectives

Imagine a herd of bison, which we typically think of as grass-eaters. But now and then, one or two might nibble on wildflowers or shrubs. Traditionally, scientists might explain this as a mistake—maybe the bison grabbed the wrong plant while aiming for grass, or maybe grass just wasn’t available. But what if this isn’t a mistake at all? What if it’s normal for some bison to eat differently, at least in certain situations? Our findings suggest that dietary variety is the norm, not the exception. Instead of asking, “Do bison eat grass?”, a better question might be, “Are they eating grass right now?” This small shift in perspective opens opportunities to better understand how animals really live—adaptively, flexibly, and often beyond the narrow categories we’ve put them in.

Hannah Hoff
Brown University

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This page is a summary of: The apportionment of dietary diversity in wildlife, Proceedings of the National Academy of Sciences, July 2025, Proceedings of the National Academy of Sciences,
DOI: 10.1073/pnas.2502691122.
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