For even the most aloof cats, just a few leaves of catnip can trigger excited fits of chewing, kicking and rolling around.
Silver vine—or matatabi in Japanese—inspires a similar plant-induced euphoria in our feline friends. The response certainly looks like fun, but until recently, scientists were unsure if the cats’ behavior might actually have other benefits than pure pleasure.
New research, published this week in iScience, suggests that when cats play with (and damage) either catnip or silver vine, the plants’ leaves actually emit higher levels of chemical compounds that do have a benefit: repelling mosquitoes. Both plants can act as a sort of natural bug spray, and when cats chew up the leaves, that bug spray becomes even more effective. Researchers at the University of Iwate in Japan, who have been investigating cats’ interactions with catnip and silver vine for several years, were behind the research.
But rolling around in the leaves is only one component of the cats’ response to these plants. Masao Miyazaki, an animal behaviorist at Iwate University and an author on the study, explained that cats engage in four main behaviors with either catnip or silver vine: licking, chewing, rubbing and rolling. In an earlier study, Miyazaki says they found that rubbing and rolling are very important to transfer iridoids—the chemicals that trigger the cat's endorphine rush—to cat fur and that repels mosquitoes. If rubbing and rolling in silver vine leaves is a cat’s way of applying bug spray, this still did not explain why, other than getting high, cats lick and chew the leaves as well.
In the new study, the researchers looked more closely at what happens on a chemical level when the leaves get damaged by cats. They first collected intact silver vine leaves as well as leaves that had been chewed on by cats and leaves that they crumpled up by hand. A chemical analysis showed that damage inflicted by both cats and humans caused the leaves to increase their emissions of various iridoids. The chemical cocktail in the damaged leaves was also less dominated by a single chemical, and instead had a more even balance of five different chemicals.
The researchers then tested out these different chemical cocktails to see how cats and mosquitoes each responded to them. When given trays with intact and damaged silver vine leaves, cats spent more time licking and rolling around on the damaged leaves. And when researchers synthesized the chemical cocktails found in these leaves, the cats again spent more time with the damaged-leaf cocktail.
The cats preferred the more well-balanced mixture of iridoids compared to the simpler mixture, even when the levels of nepetalactol, the main iridoid in silver vine, were the same. Previously, nepetalactol was thought to be what attracted cats to it, but this new finding revealed that there was something special about the mixture of chemicals that was extra enticing. “I was really surprised that the combination of iridoid compounds enhanced the feline response,” says Reiko Uenoyama, a graduate student at Iwate University and lead author on both studies.
The complex chemical mixture that was most attractive to cats was also most repellant to mosquitoes. To compare the insect-repellant properties of the mixtures, the researchers filled a transparent box with mosquitoes and placed a shallow dish inside. When the complex chemical mixture from damaged leaves was added to the dish, the mosquitoes fled more quickly than when the simpler mixture from intact leaves was added.
While silver vine reacted to cat-inflicted damage by diversifying its chemical profile, catnip did not. The researchers repeated all of their experiments with catnip and found very different results. The main iridoid chemical in catnip is nepetalactone—not nepetalactol—and this remains the case regardless of leaf damage. When cats chew on catnip, the leaves vastly increase their emissions of nepetalactone alone.
Despite this different reaction to damage, being crumpled-up still made catnip leaves more attractive to cats and more repellant to mosquitoes. But in this case, the responses were due to higher levels of a single chemical. And when comparing the plants to each other, a large dose of the catnip cocktail was needed to trigger the same response from cats and mosquitoes as a very small dose of the silver vine cocktail. Yet catnip leaves themselves were just as attractive to cats as silver vine leaves because the amount of chemicals catnip leaves emit are so much higher overall.
Why even small amounts of complex mixtures of chemicals are so effective at triggering responses is unclear to scientists. “Unfortunately,” Miyazaki states, “we don’t know why the cocktail reacted more strongly to cats and mosquitoes.” But despite these lingering questions, Benjamin Lichman, a plant biochemist at the University of York who was uninvolved in the study, says this research “highlights the importance of mixtures or cocktails of chemicals in interacting with animals as opposed to single compounds.”
Scientists are still unsure of when this particular cat behavior first evolved. In their previous study, the researchers found that leopards and jaguars will rub their heads on nepetalactol-soaked paper just like domestic cats do. This finding suggests that this behavior that takes advantage of the insect-repellant characteristics of certain plants might have evolved in a distant feline ancestor.
“I just find it so interesting how cats have developed this innate behavior of defending themselves this way,” says Nadia Melo, a chemical ecologist at Lund University who was not involved in the study. She points out that other mammals face similar disease risks from insects, “but you don’t see this in dogs, which are obviously also affected by mosquitoes.”
Catnip and silver vine could be useful for protecting humans from insects as well. The species of mosquito used in this study transmits roundworms to cats and dogs and also spreads many human viruses, like dengue and chikungunya. And Melo’s previous research suggests that other mosquitos are likely to have similar responses. “I think all mosquitoes would react pretty much the same way,” she says.
So the chemicals from catnip and silver vine could prove useful for developing safer and more effective insect repellants for human use. They just might have the side-effect of attracting cats as well. "If someone does not like cats or have allergy to cats,” writes Miyazaki in an email, “they should not use iridoids as repellents!"