The dog days of summer are drawing to a close here in Seattle, and our dogs—and cats—are searching out the few remaining tufts of that moist grass they love to munch! At PugetPets, our dog walkers and pet sitters often get asked about this curious vegetarian habit of our favorite family carnivores. So why DO our pets eat grass?

The truth is, not everything is known about the dietary and nutritional requirements of our pets. In fact, some have put forth the idea that a dog’s desire to eat grass is a form of pica, a rare eating disorder characterized by the urge to consume non-edible items or substances. But you can relax. Your dog or cat probably is not suffering from an eating disorder. A study on dogs conducted in 2008* at the University of California, Davis (the only study of its kind) found that eating grass is a common behavior in normal dogs. Much simpler and more probable theories than pica disorder exist to explain our pets’ universal love of juicy green grass.

Cats, Dogs and Vegetables

Dogs and cats have different nutritional needs. Wild canines can derive some of their nutritional requirements from vegetation. According to wildlife photographer and researcher Wyman Meinzer, a coyote’s diet can consist of up to 90% juniper berry from October through January in areas where the berries are present. Wolves, the closest ancestors of our pet dogs, though not as given to a vegetarian diet as their coyote counterparts, are also opportunistic feeders. Dogs of all kinds can and will get their nutrients from any available source.

Cats, however, both wild and domestic, are a more specialized type of carnivore. These animals, referred to as obligate carnivores, must consume an all-meat diet. They necessarily derive all of their nutritional needs from their animal prey. Cats’ bodies, like those of raptors and many aquatic mammals, are not capable of making several key nutrients. These nutrients can be acquired only from the animals’ meat-only diet. Unlike dogs, cats simply cannot get their nutritional requirements from vegetable matter. Despite these differences, the reasons both dogs and cats eat grass, it turns out, may have surprising similarity.

Bellies and What’s in Them

Most theories about why dogs and cats eat grass have to do with digestive health. In the case of cats, this seems obvious. Cats ingest large amounts of hair when grooming and need to regurgitate it later, as it cannot be digested. When they eat grass, it helps form the hair balls and induce the vomiting necessary to expel the hair. Cats frequently do regurgitate the grass they eat and this is usually related to their need to eliminate those pesky hairballs, so it is not a cause for alarm. Dogs may also eat grass in an attempt to rid themselves of a grumpy belly, though evidence for this is anecdotal.

In both cats and dogs, the desire to eat grass may reflect an innate predisposition inherited from their wild ancestors. Research on wolf droppings supports this idea. These wild canines appear to eat grass in order to help purge intestinal parasites. As plant matter passes through their intestinal tracts, the fibrous material wraps around intestinal worms and increases intestinal contractions. In this way, eating grass helps to purge the system of harmful parasites. The same may also be true for wild cats. Our pets are typically free of these parasites, but they likely acquired from their ancestors a natural predisposition to eat grass.

Another digestion-related theory holds that because dogs and cats in the wild benefit from eating every part of their prey, including the stomach contents, they may have simply acquired a taste for grass in this form. Evolution resulted in their finding the stomach contents of grass-eating herbivores tasty. And we all know our dogs and cats find lots of things tasty that we don’t! Our pets may simply have retained this acquired taste.

Let Them Eat Grass!

Regardless of the underlying biological reasons our pets eat grass, they are probably doing so because they like the way it tastes. The wolf likely isn’t thinking, “Hmm…I think I should purge some nematodes.” But she probably is experiencing something like, “Boy, I could sure go for some grass right now.” Likewise, our dogs and cats probably aren’t analyzing the evolutionary biology behind their cravings any more than we do when we reach for one of our favorite snacks. They just know it tastes good. So unless your dog is consistently vomiting after eating grass (this could indicate a serious condition), or your pet shows signs of unusual behavior or illness, don’t worry when they eat grass. Avoid letting your pet eat grass that might be treated with pesticides or herbicides, which can be harmful to your pet’s health. Otherwise, you can safely allow your pet to indulge in this pleasurable, natural behavior.

*Sueda, K.L.C., Hart, B.L. & Cliff, K.D. (2008). Characterization of plant eating in dogs. Applied Animal Behavior Science, 111, 120-132.