Why do cats eat grass if they only vomit it? Scientists may just have solved the mystery

Of all the strange quirks cats display, one of the most puzzling has been their habit of munching on grass. For years, pet owners and scientists alike have watched felines nibble at blades of green only to vomit them up soon after. Unlike dogs, who eat grass indiscriminately and often use it to purge something disagreeable, cats are typically far more selective about what they put in their mouths. This has raised an enduring question: why would such discerning creatures deliberately consume something that seems to make them ill?

Scientists have floated many theories. Some believed cats ate grass to induce vomiting when they felt unwell, while others suggested it was a way to rid themselves of intestinal parasites. However, both explanations have remained speculative, with little evidence to back them up. Now, recent research may offer the most convincing explanation yet—and it does not hinge on cats wanting to be sick.

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The clue lies in the grass

A team of researchers led by plant biologist Nicole Hughes at High Point University decided to look not at the cats, but at the plants themselves. As Hughes explained to Science, grass has a rough texture and under a microscope it is covered in jagged edges and tiny spikelike projections called trichomes. These features originally evolved as defences against herbivores, discouraging animals from eating the plants.

Yet cats may have found a different use for them. By examining hairballs coughed up by domestic cats, Hughes and her colleagues discovered that fragments of grass embedded in the fur were lined with these microscopic spikes. Under electron microscopes, the resemblance was striking: the jagged edges looked like miniature versions of “drain snakes,” the plastic tools humans use to extract hair from clogged sinks. This led the team to suggest that grasses may help snag and entangle fur clumps inside a cat’s digestive tract, making them easier to dislodge and cough up.

Grass as a hairball remedy

Cats naturally ingest large amounts of fur through grooming. The problem is magnified in long-haired breeds and in cats that hunt prey with fur of their own. The result is often the dreaded hairball, a mass of fur that can accumulate in the stomach and intestines. Left unchecked, these masses can obstruct the digestive system and cause serious health issues.

The new study, published in the Journal of Veterinary Behaviour, offers evidence that grass may serve as a mechanical aid for dealing with these blockages. Rather than passively waiting for hairballs to grow large enough to force expulsion, cats may be using the structural properties of certain grasses to hasten the process. In this sense, grass-eating may not be about nutrition or even self-induced illness, but about taking advantage of plant defences in a way that benefits the eater.

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An evolutionary twist

What makes this discovery particularly intriguing is that cats appear to be turning a plant’s defence mechanism into an advantage. The jagged structures of grasses are meant to deter herbivores from grazing, but cats may exploit those same features for their own digestive benefit. As T Michael Anderson, an ecologist at Wake Forest University, pointed out, this could represent another fascinating case of animals using plants in ways that have little to do with extracting calories or nutrients.

In evolutionary terms, it may show how flexible and opportunistic animal behaviour can be. While most associations between plants and animals are framed around diet, this suggests that plants can play a mechanical role in the biology of carnivores as well.

Grass and parasites: An older theory

Not all scientists are convinced hairballs are the entire story. A widely held alternative theory is that grass-eating is an ancient behaviour inherited from feline ancestors who used it to rid themselves of intestinal parasites. Supporting this view, a survey of more than 1,000 cat owners conducted by researchers at the University of California found that most cats—around 71 per cent—regularly ate grass, but only a minority vomited afterward, Jolly Pets Life said citing the study. The survey also noted that most cats did not appear sick before eating grass suggesting it is not always a deliberate purge.

Benjamin Hart, a veterinary behaviourist at the University of California, Davis, has argued that grass consumption could be a vestigial behaviour passed down through generations. While modern cats rarely face the same parasite loads as their wild ancestors, the instinct to consume rough greenery may persist as a kind of evolutionary “hard-wiring”.

A blend of explanations

The truth may lie somewhere in between. Cats could indeed be using grass both as a way to aid hairball expulsion and as a residual instinct connected to parasite control. Hughes’s work points strongly toward a mechanical role in helping with fur accumulation, especially given the evidence of plant microstructures binding to hair in actual hairballs. However, the fact that not all cats vomit after eating grass indicates the behaviour may serve multiple purposes—or none at all, beyond instinct.

What is clear is that cats are not eating grass for nutrition. Unlike herbivores, they gain little caloric or nutrient value from vegetation. Instead, the act seems to fall into a broader category of non-nutritional plant use. Just as some primates chew bitter leaves to combat parasites or use plants as tools, cats may be harnessing grass in a way that benefits their health indirectly.

Hughes and her team continue to explore this question, with further research planned to see whether grass might also help cats pass fur clumps through the digestive tract rather than simply coughing them up. She has even collected cat faeces from her own pets to test this possibility in future projects.

For now, the latest findings help explain a behaviour that has baffled cat owners for centuries. When your feline companion nibbles on the lawn and later hacks up a hairball, it may not be a sign of illness or randomness. Instead, it could be evidence of a clever evolutionary strategy—one that turns the defences of plants into a tool for survival.