Dogs are nature’s garbage disposals. They eat anything from table food to foreign objects to poop. Cats are more discriminating, though both of these domesticated animals seem to enjoy munching on grass. We have a pretty good idea why dogs do this—it’s often to relieve stomach discomfort and induce vomiting—but why cats like to nibble on lawns has largely remained a mystery. Now, scientists believe they have an answer.

A presentation last week at the International Society for Applied Ethology annual meeting in Bergen, Norway offered evidence on this peculiar behavior, which many cat owners attribute to the animals' urge to fix an upset stomach. Researchers at the University of California, Davis conducted a survey of 1021 cat owners who spent at least three hours a day observing their pet’s activities and found that of the 71 percent of cats caught chomping on grass, about a quarter wound up vomiting afterward—but roughly 91 percent of respondents reported that their cats did not appear to be ill before dining out on roughage.

So if they weren’t self-medicating a sick stomach, what happened? The scientists argue it’s evolutionary behavior that is not intended to provoke vomiting. Instead, cats are motivated to eat grass because this is how their ancestors expelled intestinal parasites. Grass consumption increases muscle activity in the digestive tract, which could force out unwanted contents. Cats have traditionally had to deal with parasites like hookworms or roundworms as a byproduct of devouring rodents, though it’s likely that most cats who aren’t on a diet of rat meat don’t have any parasites to treat. Still, the instinct to chew grass remains.

The survey also indicated cats younger than 3 years old were more likely to eat grass than older cats, but tend to vomit less afterward. If you have an outdoor cat who likes to supplement its diet with backyard salads, it might be best to offer up some grass grown indoors that is free of pesticides and other contaminants.

[h/t Science]