It's not easy eating green, as most plants have chemical defenses to deter would-be grazers. Getting enough to eat, while minimizing exposure to toxins, is a persistent challenge that shapes an herbivore's foraging choices. Do they boost their survival by eating a bit of everything, bypass biological booby traps by specializing on one plant or adapt their strategy as environmental conditions change?
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. In a new study, researchers report findings from an eight-year, large-scale survey exploring the dietary choices of a model herbivore, the woodrat (genus: Neotoma). By analyzing plant DNA in the rodents' droppings, the scientists compared dietary breadth between individuals, within populations and across species of woodrats throughout North America.
Woodrats exhibited a wide spectrum of diet diversity that included both generalists and specialists. Species-level specialists stuck to narrow food niches, with little difference between individual diets. In contrast, generalist populations contained individuals with more varied diets. Even these individuals appeared to forage on a consistent subset of plants, which likely helps them to manage the risks of consuming potentially poisonous food.
"Most woodrat populations are generalists, but at the individual level, these generalists' diets may not be as broad as we previously assumed," said Sara Weinstein, biologist at Utah State University and lead author of the study. "We tend to think of generalists as being a jack-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 that 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 that the costs to introducing a new food source may be higher than maintaining a consistent, if more toxic, diet.
"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," said Denise Dearing, distinguished professor of biology at the University of Utah and co-author of the study. "This pattern reflects the constraints of the mammalian detoxification system-individuals forage for a diet that balances detoxification efficiency while limiting negative outcomes."
The findings may change how we think about resilience, food webs and invasive species, the authors assert. Scientists assume that generalist species are more tolerant of change or more effective invaders. The research reveals this may be untrue when generalists are comprised of locally adapted specialists.
The study published online on Sept. 15, 2025, in the journal Proceedings of the National Academy of Sciences of the United States of America. The work was supported by the National Science Foundation and the University of Utah.
Different woodrats exhibit both specialist and generalist diets
Woodrats (genus Neotoma) are a surprisingly perfect model for studying animal diets. 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. Earlier this year, two of the study's coauthors, Dearing and U postdoctoral researcher Dylan Klure, identified the specific genes and enzymes that allow woodrats to eat near-lethal food without harm.
"Woodrats are remarkable in their ability to eat truly awful, toxic plants," said Weinstein, "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. These droppings provide a treasure trove of information, with each sample representing a days-worth of food choices identified using DNA metabarcoding technology.
The scientists gathered data from 13 species, 57 populations and over 500 individuals to examine predictors of dietary breath and diet variation between individuals at a landscape scale. To test whether these patterns held across scales, the scientists also tracked the diets of a single woodrat population, analyzing their droppings over five years and mark-recapture data from individuals sampled at least three times.
The woodrats exhibited a continuum of dietary diversity, from the big-eared woodrat (N. macrotis) that prefer fibrous, bitter, tannin-rich leaves, to the desert woodrat (N. lepida) that consumes dozens of plant species across its range.
"Ultimately, 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 said.
