Among the many mysterious aspects of cat behavior, perhaps the biggest question is their persistent habit of eating grass, which often seems to only make them vomit. Scientists have proposed numerous hypotheses over the years, but none has been definitively confirmed—until now.
Hairball Removal: The Real Reason Cats Eat Grass
Authors of a new study published in Journal of Veterinary Behavior suggest that cats eat grass to remove hairballs from their digestive tract. Hairballs are an unavoidable part of a cat's life. Cats ingest large amounts of hair during grooming, and even more if they consume fluffy prey. This dense mass poses a threat to their health as it can block the digestive system, making regular expulsion essential.
Cats typically expel hairballs through vomiting, but consuming grass with a rough surface makes this process more effective.
Microscopic Evidence from Domestic Cats
Biologists from High Point University in the United States examined hairballs regurgitated by two black-and-white domestic cats, Mildred and Merle, living with one of the study’s authors, Nicole Hughes. Mildred and Merle regularly roam the yard around their home and have unrestricted access to the grass growing there.
Using electron microscopy, researchers obtained detailed images of plant fragments embedded in the hairballs. All of them had rough surfaces, serrated edges, and spike-like protrusions. The size of these plant microstructures was ideal for effectively catching and pulling hair.
Grass as a Natural Cleaning Tool
Researchers describe the grass strands that cats eat as functioning like sewer cables, clearing the digestive "pipes." This mechanism explains why cats favor grass with a specific texture and shape, optimized to extract hair efficiently.
Parasite Hypothesis Unlikely
Importantly, parasites such as roundworms and tapeworms are many times larger than the plant spikes and cannot be caught in the same way. This evidence makes the alternative hypothesis—that cats eat grass to rid themselves of worms—highly unlikely, according to the study.
