Why do pets do that? Eating grass, purring, yawning and catnip reactions (Proceedings)

Benjamin L. Hart, DVM, PhD, DACVB

Family veterinarians are expected to be all-around authorities on dogs and cats from the standpoint of physiology, pathophysiology, problem behaviors and even interesting tidbits of canine and feline behavior.

Family veterinarians are expected to be all-around authorities on dogs and cats from the standpoint of physiology, pathophysiology, problem behaviors and even interesting tidbits of canine and feline behavior. In the area of feline and canine behavioral tidbits, and of considerable interest to pet owners, are a number of topics. Just having something to say about these topics may contribute to your image as a "cool," or "with-it" veterinarian. I will share some of what has been scientifically learned about eating grass, purring, yawning and catnip reactions. This will give you a bit of information to pass on at a party, wedding reception, or in the exam room when you want to reveal your depth of knowledge of animal behavior.

Eating Grass

Both dogs and cats are frequently observed to eat grass and other plant items that presumably have no nutritional value. The explanations have been that plants provide fiber or that the animal is feeling ill and grass eating induces them to vomit. If you have seen dogs or cats eat grass yourself, it is likely that you could not detect any signs of illness and usually you have not seen them vomit afterwards. So what is the explanation for grass and plant eating? For one thing, it is known that wild felids and canids in nature are known to eat grass. For example, grass is seen in 5-10% of scats of wolves and cougars.

To seek answers to the grass-eating question, we first surveyed veterinary students about the frequency of grass eating in their own pet dogs and cats and whether they observed signs of sickness before eating grass and signs of vomiting afterwards. Next, we surveyed a group of outpatient clients, bringing their healthy pets to our teaching hospital, for their observations about their pets consuming plants and the behavior before and afterwards. Then we turned to a web-based survey of owners of plant-eating dogs and cats. The latter yielded about 3000 returns for dogs (Sueda, Hart and Cliff, 2008) and about the same number for cats (Torres de la Riva, Hart and Sueda, unpublished). Most dogs and cats eat grass as their preferred plant. After applying inclusion criteria to each type of survey, we found that, contrary to popular opinion, only about 10% of dog owners reported that their dog frequently showed signs of illness prior to plant eating and only 20% reported their dogs regularly vomited afterwards. The vomiting seemed to be correlated with, rather than caused by, the plant eating. There was no relationship between type of diet and plant eating, which does not support the dietary fiber idea. The data for cats are being analyzed; plant eating seems to be less common in cats than dogs. As most cat owners who provide an indoor grass source for their cats will testify, cats do not typically appear ill before eating plants nor do they regularly vomit afterwards.

Our hypothesis is that grass eating mostly occurs in normal dogs and cats, and is not associated with illness or a dietary need, but reflects an innate predisposition, inherited from wild ancestors, that grass eating has an intestinal parasite purging effect. Remember that animals in nature are always exposed to intestinal parasites. So, canids and felids living in nature have perhaps evolved their own anthelmintic; no patent, no drug resistance, no profits to be made. At least not yet, but check with our Office of Technology Transfer before trying to cash in on this.


Ask a client what he likes most about his cats and "purring" will be at the top of the list. So, why do cats purr? Do other felids such as lions and cheetah purr? How do cats purr? It turns out that the answers to these questions have been updated over the past few years. What you heard about purring as representing a feeling of contentment, or a mother and her kittens communicating, is not the complete picture. Nor is the theory that purring is produced by vibrations of the diaphragm or intercostal muscles or turbulence in the aorta.

First, the latest on the mechanism. Recordings of cats purring show that purring is produced by a neural oscillator signal originating in the brain, innervating the intrinsic laryngeal muscles, causing muscle twitches at the rate of 25 cycles per second. During both inspiration and expiration this produces a low volume sound at about 25 hertz (Hz). This fundamental frequency is great for producing vibrations but is barely audible. More audible harmonics are produced at frequencies up to 150 Hz. Cats can, and do, purr while also making vocalizations such as meow. Kittens purr at the same low frequency as adult cats; size makes no difference.

Ever since the writings of Darwin, it has been known that several wild felids, including pumas, cheetahs and ocelots purr. Darwin believed that lions, jaguars and leopards did not purr. Here again, Darwin was right. Research has added bobcats, lynx and the wild cat to the list of purrers and tigers, the snow leopard and the clouded leopard to the non-purrers. And, most of the large felids that purr do so at about the same frequency as the domestic cat. As to why cats and some other felids purr, we no longer have to "hand-wave" about some vague contentment thing. Every practitioner knows cats can, and do, purr when injured, sick, when giving birth, even when on their death bed; this is hardy contentment.

Findings from the mid-1990s showed that the application of low frequency sounds, at the 25 Hz frequency, or a harmonic of 25Hz, to the extremities of people and some animals increases bone density, promotes wound healing, relieves pain, increases muscle mass and helps in the repair of tendons and muscle. Elizabeth Muggenthaler, in a clever paper presented in 2001, argued that because the fundamental frequency of the purr of cats was in the 25 Hz, healing range, there was a health-related advantage for the evolution of purring. While felids are your prototypical couch potato, they do not lose muscle mass, or bone strength from doing nothing like humans. But they purr a lot and this seems to do the same as exercise in keeping up muscle and bone condition. Would that we humans could purr! When the felid goes for the all-out chase for a dinner, and comes back with sore muscles and overly-stretched tendons, purring is healing. On your exam table, the sick or injured kitty that purrs is not psychotic, but using the instinctual behavior from wild ancestor that promotes recovery. Just welcome the purring and tell your clients they have a smart cat. And yes, there is an advantage to mothers and kittens purring; it probably helps the mother recover from kitten-birth and the kittens grow stronger bones.

As to why the rest of the feline animal kingdom has not come up with purring, one can point to the interesting fact that felids that purr, such as the domestic cat and cheetah, cannot roar and felids that roar, namely lions, jaguars, leopards and tigers, roar but cannot purr. It boils down to the degree of ossification in the hyoid apparatus and the anterior-posterior positioning of the larynx. To make a long anatomy lesson short, the species with a fully ossified hyoid apparatus can, and do, purr; they thus enjoy the purring-related health benefits. The species with a non-ossified, elastic hyoid structure, can roar but can't enjoy the benefits of purring. Look at it this way, if roaring good and loud gets you your way more often with regard to resources in nature, you must have an elastic hyoid structure and you give up the benefits of purring. Like most things in life, you can't have it both ways.


Here is something that we share with our dogs and cats and lots of other animal species. So we can look at why we yawn, cats yawn and dogs yawn to come up with insights. Here is the leading old-school theory; we yawn because of a low oxygen level. The theory goes that when inactive, as in just before retiring or upon waking up, our lungs have not been fully used and the "involuntary" yawn fills unused alveoli, getting them active again. The problem is if you administer oxygen, we still yawn. Besides, unborn fetuses even yawn.

Recently, researchers, Gallup and Gallup put forth the theory that yawning cools the brain. When we, or animals, are inactive the blood flow to the brain slows down and the brain tends to heat up a bit. The brain, like any high-powered computer, works best when cooler. When we, or the pooch, are sleepy, a good yawn revs up the computer and we can put off sleeping awhile. Same thing upon awakening, the brain needs a boost.

A prominent aspect of yawning is that it is contagious; you yawn, I yawn. Psychologists take this as showing empathy. We used to think that only humans, chimpanzees and baboons could catch human yawns. Last year this myth was disproved. Psychologists Joly-Mascheroni, Senju and Shepherd showed, in controlled experiments, that dogs catch human yawns; in fact, of 29 pet dogs tested, 72% yawned when they observed an unfamiliar person yawning. That is better than the rate reported for humans (45-60%) and chimpanzees (33%) catching human yawns! So, are dogs showing empathy with people? This would fit with the recent finding that dogs can read our signals when we point to an object; they orient to where we point.

How do investigators explain the contagiousness of yawns? Well, the experts who proposed the cool-the-brain theory postulate that by arousing the brain we are alert to danger, and when others catch the yawn they also get ready for action. With a long history of co-evolution with humans, perhaps our dogs are getting ready to go into action with us.

Now if you want to see a dog catch your yawn, keep in mind that it takes several yawns in succession and it works in situations with somewhat heightened tension. I have yet to see it in my dog; she's probably too relaxed. You're likely to have better luck with your client's dog than your own.

Catnip Reactions

All cat owners know most cats love catnip or at least flip out over it. Why is this? Do all felids respond this way? Is there some adaptive function? Is the reaction from what they smell or consume? To fill you in with some useful insights, let's reflect on what has been learned. The catnip reaction involves sniffing and then chewing the catnip source. This may progress to rubbing their chins or cheeks on the source, and even rubbing their bodies on nearby ground very much like what a female does in courtship behavior. Pawing and digging at the catnip mouse is also common. Another reaction is biting into the mouse much as a cat bites into prey. Still another is batting the mouse about like a kitten playing with a leaf. Some cats will hold the mouse and kick it with the back legs. These reactions occur more-or-less at random. There is a satiation effect in that a reaction lasts 5-15 minutes and cannot be evoked again for an hour or so. Only about 50-70% of cats respond to catnip. We hypothesized in 1985 that catnip activates different brain areas controlling predatory, female-like courtship, and kitten-like play behavior at random. It has been shown that the catnip reaction is mediated by the sense of smell; catnip does not have to be ingested.

Felids other than cats known to respond to catnip are lions, leopards, jaguars, bobcats, pumas and ocelots. Tigers seem not to respond. A logical question is whether there is any possible adaptive value of a cat responding to catnip. The answer is probably not. In fact, it would seem to be a useless distraction to a felid living in nature. Although it has been transported around the world, the catnip plant (Nepeta cataria) is native to Eurasia. But most cats that trip out on the plant, or its active ingredient nepetalactone, did not evolve in Eurasia. Some are native to North, Central and/or South America and others native to sub-Saharan Africa. Thus, cats could not have evolved the reaction for some purpose. Interestingly, the tiger, which is native to Asia, seems not to respond to catnip. The catnip reaction appears to be an incidental, functionless accident of nature. Humans do make some functional use of catnip plant, a member of the mint family, by using the whole leaves to make a tasty, healthful tea.

Noting that a cat often bats around, and "plays" with, a real mouse it has just caught in a catnip-like fashion, I will offer an hypothesis. Mice, which are known to produce lactones in skin secretions when they are stressed, may have evolved a lactone secretion that is enough like nepetalactone, that when they are caught by a cat, and produce this secretion, the cat goes through a "catnip reaction." This gives the mouse a chance of escaping from the cat before the cat goes through the 5-15 minute catnip reaction and becomes refractory to the mouse's "magic potion."