Why do cats eat plants?

In this post I’m going to talk about a question that has perplexed people for hundreds of years – why do cats eat plants? Even the ancient Greek philosopher, Aristotle, had something to say on this topic. In 350 BC, he wrote “dogs and cats, when they are ill, eat some kind of grass and produce vomiting”.

This is certainly something that plenty of pet parents ask us – is eating grass a sign of nausea or illness in cats? There have been some recent studies on this, and the short answer is – grass-eating can be associated with vomiting, but only in about 30% of cases.

We all know that the cat is an obligate carnivore. These animals are not anatomically suited to plant-eating, yet plant material is routinely found in the stomach contents and stools of wild carnivores. What potential reasons could there by for this? Do cats eat plants more frequently if their diet is deficient in essential nutrients? Could this behaviour relate to the fibre content of the diet? Or is it related to the presence of undiagnosed disease in some cats?

In a study by Hart et al. (2021), the authors found that 27-37% of cats frequently vomited after eating plants - so 63-73% of cats don't vomiting after eating plants. They also found that younger cats consumed plants more frequently, but were less likely to vomit than older cats. Interestingly, short-haired cats ate plants as frequently as long-haired cats, so this implies that cats don’t eat plants to try and help with hairballs.

Another study by Yoshimura et al. (2021) found that smaller carnivore species ate plants more frequently than larger species. They hypothesised that this could relate to self-medication, because energy loss caused by parasites has more consequences for smaller carnivores, compared with larger ones. The authors also noted that plant consumption might promote digestion or excretion of indigestible food items, like fur and bone, which are frequently consumed by small carnivores.

Most wild carnivores have intestinal parasites, so regular plant eating could be an adaptive behaviour to maintain a lower intestinal parasite load. Researchers have observed blades of grass wrapped around intestinal worms in wolf stools, and hypothesised that grass might have a “scouring effect” that helps to remove worms from the gut.

The science of animal self-medication is called zoopharmacognosy, and birds, bees, and chimpanzees all do it. The most important thing to know though, is that plant-eating seems to be a trait that domestic cats have inherited from their wild ancestors. It’s a common behaviour in normal cats that is unrelated to illness, and it doesn’t cause nausea or vomiting in a majority of cases.

Clients often ask us if it’s important to provide cats with access to plants, for nutritional reasons. Most cats and dogs in Australia are regularly wormed, so they don’t really need to eat grass to try and rid themselves of parasites. Could there be other benefits to offering plants to cats then?

Offering plants can definitely provide a lot of behavioural enrichment. You can use barley grass, oat grass or rye grass, cat nip, cat mint or even cat thyme, which can be a bit harder to find. Researchers have also found that silver vine, Tatarian honeysuckle and valerian root are appealing to some cats. We all know that indoor cats can become bored, so creating an indoor garden for them is a great way to add a bit more interest to their lives.

Another potential benefit to offering fresh plants to cats is that they may be a source of beneficial bacteria. In humans, a recent study showed that we consume over 100 million bacteria when we eat an apple, and that organically grown apples were a rich source of bacteria like lactobacilli.

Finally, we all known that most cats can become a bit loopy when offered fresh catnip - they roll, they scratch, they rub, and they often attack and shred the plant. A 2022 study by Uenoyama et al. found that damaged catnip leaves emit more volatile compounds called iridoids, compared with intact leaves, and this can act as an insect repellant. It also seems to encourage cats to continue rolling around in the remains of the plant, which means that they are potentially covering themselves in a fairly effective insect repellant.

So, to summarise - it seems like there's a few good reasons to provide indoor cats with a range of plants. It can provide valuable olfactory enrichment, and might help keep mosquitoes away as well!

Read the study by Uenoyama et al. here: https://pubmed.ncbi.nlm.nih.gov/35880027/

Read the study by Yoshimura et al. here: https://onlinelibrary.wiley.com/doi/full/10.1002/ece3.7885

Read the study by Hart et al. here: https://pubmed.ncbi.nlm.nih.gov/34206345/