There’s a new paper in Science Advances by a group of Japanese researchers who investigated the attraction of cats to catnip and silver vine. This involved isolating the compounds that attracts cats, showing that they activate the pleasure centers of cats (duh!), and, most important, proposing and testing an adaptive hypothesis for why cats rub all over catnip (Nepeta cataria, in the mint family) as well as on a related cat-drug plant, silver vine (Actinidia polygama, in a different family).
Their theory, which is theirs, is that the these compounds, which plants have evolved to repel insects (aphids), do a similar job for the cats, but act for them as a mosquito repellant. The paper is below, and free, but let me add that I don’t think their answer, while it might be correct, is strongly convincing—for reasons I’ll discuss.
Click on the screenshot to access the article. You can also get the pdf here, find the full reference is at the bottom, and see a News & Views about the paper in Science, “Why cats are crazy for catnip,” written by Sofial Moutinho, which doesn’t mention any problems with the paper and also omits a fascinating line of speculation.
First, some biogeography and history for cat owners. Catnip, while native to Europe, the Middle East, and Central Asia, has been planted widely throughout the world, and is the drug of choice for American and European cats. In contrast, silver vine is native to both Japan and China, and in Asia has supplanted catnip as the weed to give your moggy. Some cats who respond to one species won’t respond to the other, and for each species some cats do not show the typical euphoric rubbing and “getting stoned” reaction. In catnip, this is due to genetic variation among cats. More on that later.
With both plants the cat’s attraction is transitory. If you’ve given nip to your cats, you’ll know that they roll around and get stoned for a few minutes, but then recover and subsequently ignore the weed. This goes along with the authors’ hypothesis below (once you’ve put repellent on your fur, your job is done). Further, the “big cats” like lions, lynx, leopards, and bobcats also show a catnip response, and that has to be incorporated into any hypothesis about adaptation.
Here’s catnip (like mints, it has square stems):
There’s a bit of history in the paper that drove me to further investigation. The authors say this:
The first reports of the feline behavioral response to silver vine and catnip were described by a Japanese botanist in 1704 and by a British botanist in 1759, respectively. The behavioral response to silver vine has been captured in Japanese culture: An Ukiyo-e (a type of traditional painting) drawn in 1859 depicts a folk story concerning a battle between cats and mice, wherein mice use silver vine as a weapon to intoxicate cats
Well I simply had to find that 1859 painting, but it wasn’t easy. Finally I found it in a tweet by Tom Price. Behold: “Cats Tempted by a Delicious Smell” by Tsukioka Yoshitoshi. Look at those nefarious samurai mice!
Here’s a short Science video showing both housecats and big cats attracted to filter paper imbued with the isolated attractant, while d*gs ignore it (part of the experiment).
The experiment consisted of several distinct parts.
Isolation and testing of the attractant compounds. The authors chemically fractionated extracts of catnip and silver vine, isolating various compounds and testing them as cat attractants by putting the compounds on filter paper and comparing whether cats were drawn to the experimental papers versus control papers soaked in hexane. They found that the active ingredient in silver vine, which induced head and face rubbing in domestic and feral housecats, was nepetalactol. The attractant in catnip was a related compound, nepetalactone. These are part of the plants’ defensive systems against insects, especially aphids, so they evolved to protect plants from being chewed and sucked. Here are the two compounds so you can see their chemical similarity (they’re both part of a family called iridoids):
Here’s a sample photo and graph of the tests (the paper has lots of cool photos). Fraction 3 has the nepetalactol, of course:
The authors then synthesized these compounds so they’d have them in pure form for further tests. The tests:
Attraction of the plants to various cat species. As we know, house cats are polymorphic for the catnip and silver vine reactions, and that’s what the authors found with silver vine: only about two-thirds of both lab cats and feral cats were “postitive responders”. Because there was more nepetalactol than nepetalactone in plants, and because the silver vine compound was more potent than the catnip compound, most further tests used nepetalactol and its source plant, silver vine.
When filter paper soaked in this compound was given to captive leopards, jaguars, and lynx, all of them showed the face-rubbing and rolling seen in house cats. Dogs showed no reaction because they are no fun.
Activation of the “pleasure” system by nepetalactol. As the authors note, the μ-opioid system, which includes release of endorphins, “controls rewarding and euphoric effects in humans.” Sure enough, in house cats the beta-endorphins were significantly elevated in cats after sniffing nepetalactone—but not control papers. And a chemical that blocks the μ-opioid system, naloxone, significantly reduced the rubbing and rolling response. The authors conclude that the μ-opioid system is involved in the response to silver vines. In other words, the cats probably experience pleasure when they sniff the stuff. (Taste, by the way, doesn’t seem to play a role here; it’s all done through the nose.)
Mosquito repelling activity of nepetalactol. The compound was shown to be highly repellent to a local mosquito, Aedes albopictus, consistent with previous reports. The authors have in fact patented a mosquito repellent, something reported in Moutinho’s News & Views summary. It’s interesting to contemplate using a mosquito repellent that also attracts cats—a double benefit!
The authors then hypothesized that when cats rub silver vine on their heads and bodies, it acts to repel mosquitoes. They first tested whether cats actually got nepetalactol on their fur when rubbing impregnated filter papers. Unfortunately, they could not detect the compound on cats who had rubbed. So they did a bio-assay: they rubbed filter paper on the faces of cats who had rubbed against impregnated filter paper, and then tested the papers on other cats. Sure enough, the face-wiped papers showed a significantly higher attraction for the secondary cats than did the controls. The statistical significance was not high, though, with probabilities equal to 0.034 and 0.025—pretty close to the “standard cut off” level of significance, 0.05.
Finally, the crucial experiment: do cats who have had their heads treated with pure nepetalactol actually repel mosquitoes? The researchers rubbed the compound onto cat’s heads, and had a control where the heads were rubbed with solvents. They then anesthetized the cats and stuck their heads into cages containing A. albopictus mosquitoes, seeing how many skeeters landed on the cats’ heads. They also did the experiment with cats who had rubbed their heads on silver vine leaves.
In both cases the cats who had the compound on their heads showed significantly fewer mosquitoes landing on them than on the control cats (again, the results, while statistically significant, aren’t overwhelmingly so, with p values of 0.033 and 0.019 respectively). The authors conclude that “the characteristic rubbing and rolling response functions to transfer plant chemicals that provide mosquito repellency to cats.”
The upshot—and some issues:
The authors have a strong adaptationist bent in the paper, looking for the adaptive significance of the catnip reaction. And yes, they’ve shown a possible one, but there are lots of gaps in their story.
1.) Does resistance to this once geographically limited species of mosquito (now more widespread after human conveyance) confer higher fitness on the cats? There’s no evidence for this. The authors hypothesize that other mosquitoes that might be repelled carry diseases like yellow fever, dengue, and Zike viruses, but are these serious diseases of wild cats, including leopards and lynx? And are these other species of mosquitoes repelled by these iridoids? (Don’t forget, they didn’t test catnip, just silver vine, though I suspect they’d get similar results with nepetalactone from catnip.) Remember too that northern cats like lynx also show the response, but do not contract tropical diseases like dengue and yellow fever.
An alternative hypothesis floated by the authors is that cats, when stalking prey, have to remain motionless for long periods, and that might be hard if mosquitoes are biting you. If you’ve rolled on silver vine and catnip, you might be less plagued by mosquitoes, less likely to move, and thus less likely to be detected by prey. As the authors note, “Face rubbing against plant sources of the repellent will help to protect the face and head of the animal, as the mouth, eyelids, ears, and nose of felines have relatively little fur and are therefore easy targets for mosquitoes.” But they haven’t shown that the stalking behavior of treated and untreated cats differs. This would be fairly easy to do—or at least possible—with house cats and tethered rodents (you don’t want to kill the prey, of course).
So while the authors assert “we have uncovered an adaptive benefit of the behavioral response in cats”, they have uncovered a possible adaptive benefit, but haven’t shown any decisive reproductive advantage of cats who roll on silver vine or catnip.
2.) Do the wild cats who show catnip responses, like the ancestor of the housecat, and the jaguar, leopard, and lynx, coexist or coexisted in the past with silver vine or catnip? The fact that the big cats tested show rolling and rubbing implies that they either independently evolved that response or inherited it from a common ancestor. But for the response to be maintained over the millions of years since cat species diverged from that ancestor, a selective advantage should have been there pretty consistently. The authors don’t consider the problem of the geographic coexistence of these cat species, their ancestor, and of the two plants that evoke a response. That at least should have been mentioned.
3.) What about the polymorphism in house cats? Some house cats show the “nip response” to silver vine and catnip, while others don’t. This variation is known to be genetic, at least for catnip. How variable is it in other cats like lynx? And why have house cats lost a lot of their response?
One possible answer is that house cats no longer either coexist with wild catnip or are so domesticated that the proposed advantages of catnip no longer impose a selection pressure on cats. In other words, the variable response of house cats to catnip could be a “vestigial behavior.” We know that traits that were once useful but are no longer so tend to become more variable and even disappear. This variability is seen, for instance, in human wisdom teeth, considered a superfluous feature and also variable among people (some have them, others don’t, and their eruption is variable, which is why they are often pulled.)
And a fascinating topic the authors neglect: What about the adaptive significance of the pleasure response?
Both the authors and Moutinho in her N&V piece have a strange take on the fact that cats apparently get pleasure from catnip. The authors hypothesize this:
As many felids rely on stealth to stalk and ambush their prey, requiring them to remain cryptic and often unmoving, a repellent that reduces their susceptibility to both the irritation of biting mosquitoes and the diseases that these insect vectors carry is likely to provide a strong selective advantage. Stimulation of the μ-opioid system might further help by providing analgesia to reduce irritation where biting arthropods have not been repelled.
And Moutinho says this:
Most scientists and pet owners assumed the only reason that cats roll around in catnip was for the euphoric experience, Miyazaki says. “Our findings suggest instead that rolling is rather a functional behavior.”
But a “functional behavior” can also go hand in hand with the evolution of “a euphoric experience.” They are not alternative explanations, but complementary ones.
I wish the authors, and Moutinho, had gone down a fascinating byway here: the supposition that cats can evolve to feel pleasure from rubbing on catnip, for feeling pleasure constitutes a powerful impetus for them to rub.” That is, the writers don’t seem to have pondered that the pleasure need not be inherent in the behavior at the very beginning of its evolution, but could have itself evolved to facilitate the behavior.
This resembles the pleasure of the orgasm: it almost certainly evolved as a way to get us to want to copulate, so there’s powerful selective pressure on our pleasure systems to feel fantastic when we copulate. Anything that makes us want to pass on our genes will be selected for, including the great pleasure of orgasms.
Evolution can act not just on behaviors, but on the sensations attendant to them. We like sweets because sugar was good for us in our ancestral habitats, and so our taste system evolved to evoke pleasure when eating sugar. As I always says, “to a vulture, rotten meat probably tastes as good as ice cream sodas do to us.” It’s important to realize that sensations and feelings, good and bad, aren’t necessarily inherent in our physiology and neurology, but are themselves evolved. (Pain receptors, too, alert us to possible danger to our bodies.)
I should add that using plant compounds to repel insects and other arthropods is not a behavior unique to cats. Here are some examples cited by the authors (I love the cigarette-butt example and have written about it before):
There are other examples that nonhuman animals may exploit some chemicals emitted from other species for protection against insect pests: boat-tailed grackles (Quiscalus major) and white-nosed coatis (Nasua narica) rub fruits of Citrus spp. against themselves, chimpanzees (Pan troglodytes schweinfurthii) use sleeping platforms created from specific trees as a source of repellents , house sparrows (Passer domesticus) and house finches (Carpodacus mexicanus) living in urban habitats bring cigarette butts to the nest, and capuchin monkeys (Cebus olivaceus) anoint themselves with millipedes (Orthoporus dorsovittatus).
Finally here’s a photo of Jango, whose staff is reader Divy and her husband Ivan, trying to get to the jar of Cosmic Catnip put on the top shelf. Jango is a “positive reactor.”
h/t: Ginger K.
Uenoyama, R., T. Miyazaki, J. L. Hurst, R. J. Beynon, M. Adachi, T. Murooka, I. Onoda, Y. Miyazawa, R. Katayama, T. Yamashita, S. Kaneko, T. Nishikawa, and M. Miyazaki. 2021. The characteristic response of domestic cats to plant iridoids allows them to gain chemical defense against mosquitoes. Science Advances 7:eabd9135.