Confused Why Your Cat Or Dog Is Eating Grass? Here's What Scientists Make Of The Behavior

Pet nutrition has gotten complicated in recent years. There’s the ethics of where that meat comes from (David Attenborough's OCEAN revealed how Antarctic krill is supplementing pet food). Then there’s the matter of whether raw meat is safe, or if pets can be healthy on a vegan diet.

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Owners put a lot of time, effort, and money into deciding what their precious pooches and pussy cats eat. That’s why it can be a little perplexing, then, when you take them outside and find them ravenous for grass.

If you’ve ever looked after a cat or dog you’ve probably witnessed the way they gnaw on grass like blissed-out cows. So, why do they do it? And is it a crucial addition to their diets?

Why do cats eat grass?

A 2019 study surveyed over 1,000 cat owners to find out more about cats’ vegetarian side. Each owner spent at least 3 hours a day hanging out with their cat, and reported back what they observed.

The results revealed that eating plants is actually a very common and normal behavior among cats, even if your precious spider plant disagrees. Of the cats included in the study, 71 percent were seen eating plants at least six times in their lifetime, and only 11 percent were never seen trying to munch on plants.

It’s commonly thought that cats eat grass to help them vomit, but 91 percent of those cat owners said their pet’s plant eating had nothing to do with feeling sick. It seems that vomiting is more of a common by-product of grass eating rather than the end goal.

Instead, eating grass is thought to be an evolutionary hangover from their ancestors for whom intestinal parasites were a big problem. Eating plants like grass increases muscle activity in the digestive tract, which is an effective way to evict freeloaders like parasites. Nowadays most cats cared for as pets don’t tend to pick up these bugs, but they can’t resist that ancestral urge to gorge on the green stuff.

Why do dogs eat grass?

A similar study in 2008 asked dog owners about their pet’s feeding habits. Of the 47 included in the research, 79 percent reported that yes, their dog liked eating grass or other plants.

They then took to the World Wide Web in search of owners of specifically plant-eating dogs to dig deeper into the behavior. Again, it’s often said that dogs also eat grass to help them be sick, but did it turn out to be true?

They were able to boost their size to a total of 3,340 surveys, 1,571 of which met the enrolment criteria for the research. Of those, 68 percent of dogs were eating plants daily or weekly, with grass being the most common leaf of choice for 79 percent.

Only a small number of dogs seemed to eat plants when we they were feeling ill (9 percent), and vomiting only followed the plant eating in 22 percent of dogs. There wasn’t much of a trend for sex, “gonadal status” (very tactful, scientists), breed, or diet, but younger pups do seem to eat a greater diversity of plants more frequently than older dogs, without it having much to do with illness or vomiting. So, why do it?

“Although this study only investigated plant-eating behaviour in domestic dogs, plant eating has also been observed in wolves and other wild canids,” wrote the authors. “Since the behaviour appears to have been conserved through domestication and artificial selection, it seems likely that it serves some biological purpose. Further studies are needed to determine what, if any, selective advantage plant eating may offer, such as an increase in gastrointestinal motility and/or the expulsion of intestinal parasites.”

Another potential explanation is that, like a lot of strange things dogs do, they simply eat grass because they like it. Which raises the question…

Do cats and dogs need to eat grass?

The general consensus at time of writing is that cats and dogs don’t need to eat grass, but there’s no real need to stop them from doing it, and they may even enjoy it. The only caveat to that is keeping them away from lawns that have been chemically treated. Otherwise, bon appetit!