Mystery solved? Why cats eat grass

Cats do a lot of weird things. One of the biggies is eating grass, often to throw it up just a few minutes later. Now, after perhaps centuries of mystery, scientists think they know why.

Researchers surveyed more than 1000 cat owners on the internet who spent at least 3 hours a day watching and hanging out with their pet. As many cat parents suspected, eating plants is an extremely common behavior: Seventy-one percent of the animals were caught in the act at least six times in their lifetime, whereas only 11% were never observed gobbling greenery.

Many online explanations for grass eating posit that the behavior helps cats throw up when they’re feeling ill. But only about a quarter of grass eaters were observed vomiting afterward, and 91% of respondents said their cat did not appear sick before imbibing plant matter.

Instead, the vomiting is merely an occasional byproduct of eating grass—not the objective—the researchers say. Eating plants is instinctual and comes with an evolutionary benefit to felines—or at least it used to, they report this week at the annual convention of the International Society for Applied Ethology in Bergen, Norway. Their theory—based on research in chimps and other wild animals—is that grass munching helps animals expel intestinal parasites (p. 106) by increasing muscle activity in the digestive tract. Except, today’s cats likely don’t have these parasites anymore. The authors argue that the strategy probably first evolved in a distant ancestor. (The scientists did not test another common assumption: that eating grass helps cats throw up hairballs.)

The team’s advice to cat owners: Buy or cultivate some indoor grass for your pets to chew on. This will give them a chance to exercise this innate behavior with a safe source of nonpoisonous plant life. And if your cat throws up afterward, at least you can take solace in your knowledge that it wasn’t on purpose or out of spite … probably.