Researchers Think They Know Why Cats Eat Grass

Contrary to popular belief, grass only occasionally makes cats vomit

cat nomming
Nom, nom, nom. Getty Images

Most cat owners will tell you if Fluffy noms on grass and then throws up, it must mean that kitty is having a bit of tummy trouble. But that’s not necessarily true. Cats actually eat grass all the time. People only notice the practice when they make a foaming green mess on the rug.

As David Shultz at Science reports, researchers from the University of California, Davis, School of Veterinary Medicine ran a web-based survey asking pet parents to report how often their purr pals ate vegetation. Qualified participants had to have been able to observe their kitty’s behavior for three or more hours a day. Cats that were indoor-only without access to plants and outdoor cats whose owners could not observe their behavior were excluded from the study.

The results were recently presented at the annual meeting of the International Society for Applied Ethology in Bergen, Norway.

The survey, which counted 1,021 cat owners, revealed that cats nibble on greenery quite often: 71 percent of cats were witnessed snacking on plants at least six times in their lifetime, 61 percent at least 10 times and only 11 percent of cats were never observed eating vegetation.

The interesting part, however, is that 91 percent of the time before eating plants, the cats were reportedly acting normally, with no signs of illness. And just around 27 percent of the cats were reported to frequently vomit after eating grass or plants. Among young cats, those under 3 years of age, only 11 percent were regular hoarkers, despite 39 percent of them eating plants every day, in comparison to 27 percent of cats 4 years or older.

The results suggest that it’s highly unlikely that the cats are using the grass as some sort of primitive stomach medicine. The findings also don’t support another hypothesis, that young cats learn to eat grass by watching older cats.

Instead, the team believes that eating grass is still part of health regimen, just one that most companion animals don’t need to engage in today. It turns out that field studies of wild carnivores and primates have found that they regularly eat non-digestible grass and other vegetation to purge parasitic worms from their systems. Cats, the team believes, regularly eat grass to stimulate muscle activity in their digestive tracts and force the parasites from their guts. “Given that virtually all wild carnivores carry an intestinal parasite load, regular, instinctive plant eating would have an adaptive role in maintaining a tolerable intestinal parasite load, whether or not the animal senses the parasites,” the authors conclude in their summary.

That conclusion mirrors what lead author Benjamin L. Hart found in a similar 2008 survey study looking at the frequency of plant eating in dogs. In that research, the team also found that dogs rarely presented illness before eating grass and that vomiting was a relatively rare byproduct of eating vegetation. The survey also found that the frequency of grass eating was not related to the dog’s diet or amount of fiber the animal ate, suggesting they weren't trying to make up for some dietary deficiency.

Hart and his team hypothesize that younger animals eat more grass because their immune systems aren’t as good at keeping parasites at bay and because nutritional stress is more detrimental to growing animals than older dogs and cats.

They also note that cats appear to eat less grass than dogs. This may be because parasite infections were less prevalent among feline ancestral species or it may be that cats' habit of burying and avoiding one another’s feces slowed the spread of parasites compared with dogs, which are known to get into each other’s business.

The upshot is that eating grass isn’t a telltale sign that an animal is sick, and more importantly, it’s likely an instinctual behavior that even the most diligent animal parent won’t be able to stop. The solution, the research team suggests, is to make sure indoor cats have enough safe, non-poisonous vegetation to gnaw on, like cat grass. And if they yak it up on the kitchen floor, that’s just a small price to pay for a little furry love.

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