It’s not easy eating green. Most plants are heavily defended with chemicals to deter would-be grazers. Getting enough to eat while minimizing exposure to toxins is a persistent challenge that shapes herbivores’ foraging choices. Should they boost their survival by eating a little bit of everything, bypass biological booby traps by narrowing their choices to one plant, or adapt their strategy as environmental conditions change?
Understanding how these animals navigate these choices is fundamental to understanding what these creatures need to survive and how species respond to changing conditions, says Utah State University ecologist Sara Weinstein.
“The diversity of an animal’s diet — known as dietary niche breadth — is critical to a species’ resilience, yet it remains poorly understood in mammalian herbivores,” she says.
In a new study, Weinstein, along with University of Utah scientists Denise Dearing and Dylan Klure; USU adjunct faculty colleague Marc Mayes at Spatial Informatics Group-Natural Assets Laboratory in Pleasanton, California; and research partners at ARUP Laboratories in Salt Lake City, San Francisco State University and the North Carolina Museum of Natural Sciences, reports findings from a nearly eight-year, large-scale survey of woodrat (genus Neotoma) populations throughout North America in the Sept. 15 issue of the Proceedings of the American Academy of Sciences.
The research was supported by the National Science Foundation.
“Woodrats exhibited a wide spectrum of diet diversity that included both generalists and specialists,” says Weinstein, assistant professor in USU’s Department of Biology and Ecology Center, and lead author of the study.
Species-level specialists stuck to narrow food niches, with little difference between individual diets, she says. Even those individuals appeared to forage on a consistent subset of plants, which likely helps them to manage the risks of consuming potentially poisonous food.
“Most woodrats are generalists, but at the individual level, these generalists’ diets may not be as broad as we previously assumed,” Weinstein says. “We tend to think of generalists as being jacks-of-all-trades, master of none. However, it looks like most generalists are more aptly described as jacks-of-all-trades, master of some.”
The results show woodrats’ dietary breadth is driven by the costs of both specialization and generalization. For example, generalist woodrats continued to eat harmful creosote year-round, even when less toxic plants were available. This suggests the costs of introducing a new food may be higher than maintaining a consistent, if more toxic, diet.
“Even though we described the diets of hundreds of individuals across 13 species, we were surprised that we didn’t fully characterize woodrat diets — because there’s so much variety,” says co-author Dearing, distinguished professor of biology at the University of Utah. “Even the generalists are selective in what they choose to eat from the smorgasbord of available items. These results are consistent with there being costs associated with both narrow and diverse diets.”
Woodrats are a “surprisingly perfect” model for studying animal diets, Weinstein says. The herbivorous rodents occur in large numbers throughout the United States, with multiple species living together in the same area. Their habitats contain a diversity of food options, including many plants with extreme chemical defenses.
“Woodrats are remarkable in their ability to eat truly awful, toxic plants,” she says. “If there are no other options, woodrats can consume plants like creosote bush, mesquite and juniper, which are full of disagreeable compounds like alkaloids and terpenes.”
“Their small size makes them easy to capture and handle, and they have no qualms about defecating in traps,” Weinstein says. “These droppings provide a treasure trove of information, with each sample representing a day’s worth of food choices identified using DNA metabarcoding technology.”
Advancements with DNA metabarcoding, facilitated by next-generation sequencing over the past decade, have revolutionized the researchers’ ability to analyze animal diets, she says. “It’s a very powerful and accessible technique to characterize the unseen.”
The scientists gathered data from 13 species, 57 populations and more than 500 individuals to examine predictors of dietary breadth and diet variation between individuals at a landscape scale. To test whether or not these patterns held across scales, the researchers also tracked the diets of a single woodrat population, analyzing their droppings over five years and using mark-recapture data from individuals sampled multiple times.
“Ultimately, understanding these constraints on animal diets have important implications for our understanding of food webs, species interactions and which populations are more likely to persist in changing ecosystems,” Weinstein says.
Reference: Weinstein SB, et al. Large-scale surveys of woodrats (Neotoma spp.) reveal constraints on diet breadth in herbivorous mammals. Proc Natl Acad Sci. 2025. doi:10.1073/pnas.2413556122
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