Cats, so often, are a mystery, even to those that know them best. Why do they sleep so much? Why do they want your full attention one minute, none the next? How can they find their way back home after being stranded miles away for years? The writer Haruki Murakami, who is known for putting cats in his novels and essays, once confessed to not knowing why he does so; a cat “sort of naturally slips in,” he said.
Another mystery: Why do cats love catnip? When exposed to the plant, which is related to mint, the majority of domestic cats will lick it, rub against it, chew it and roll around in it. They brim with euphoria, getting high off the stuff. They also go wild for other plants, particularly silver vine, which is not closely related to catnip but elicits the same response from felines, including big cats like jaguars and tigers.
For years, this behavior was just another cat-related enigma. But a new study, published Tuesday in the journal iScience, suggests that the reaction to catnip and silver vine might be explained by the bug repellent effect of iridoids, the chemicals in the plants that induce the high.
Researchers, led by Masao Miyazaki, an animal behavior scientist at Iwate University in Japan, found that the amount of these iridoids released by the plant increased by more than 2,000 percent when the plant was damaged by cats. So perhaps kitty’s high confers an evolutionary advantage: keeping bloodsucking insects at bay.
Kristyn Vitale, a cat behavior expert at Unity College who was not associated with the research, noted that the study built on strong previous work. Last year, the same lab published a study that found that cats would try their best to coat themselves in DEET-like iridoids, whether by rolling on the chemicals or by rising up to nuzzle them with their cheeks. “This indicates there may be a benefit to the cat physically placing the compounds on their body,” Dr. Vitale said.
Carlo Siracusa, an animal behaviorist at the University of Pennsylvania who also was not involved in the research, concurred. “The evidence shows that they want to impregnate their body with the smell,” he said. But, he added, “keep in mind that a sizable chunk of cats don’t show this behavior. So why would they have been selected in this way?”
As an evolutionary adaptation, bug-repellent iridoids probably do more to protect plants from herbivorous insects than to help cats avoid bug bites. Plants often release irritants when damaged, which helps to ward off attackers, and they emit other chemicals that communicate danger to their neighbors. “Plants are masters of chemical warfare,” said Marco Gallio, a neurobiologist at Northwestern University who was not affiliated with the new study.
Last year, Dr. Gallio and his colleagues published a report that linked the primary bug repellent in catnip, nepetalactone, to a receptor protein that triggers irritation in mosquitoes and related insects. The receptor, which is also present in humans and cats, can be set off by tear gas. But Dr. Gallio found that although nepetalactone had no negative effect on humans and sent felines into spasms of ecstasy, it did activate this particular receptor (called TRPA1) in many insects — an added bonus for cats rolling around in their drug of choice.
In their most recent study, Dr. Miyazaki and his associates measured the chemical composition of the air immediately above leaves — both intact and damaged — of catnip and silver vine. Then they measured the iridoid levels in the leaves themselves. They found that catnip leaves mangled by cats released at least 20 times more nepetalactone than intact leaves did, while damaged silver vine leaves released at least eight times the amount of similar iridoids than did intact leaves. The cats’ interactions with silver vine also changed the composition of the plant’s bug-repelling cocktail, making it even more potent.
After rubbing their faces and bodies against the plants, cats are sure to be coated in a robust layer of Pest Begone.
This finding, paired with Dr. Miyazaki and his team’s previous research, supports nascent claims that at least part of the benefit of the kitty catnip craze is to stave off mosquitoes and flies. Such behavior, called “self-anointing,” would not be the first of its kind in the animal kingdom. Mexican spider monkeys have been known to smear themselves with different kinds of leaves, probably to serve a social or sexual purpose, and hedgehogs often rub toxins onto their spines.
Still, there are many questions left to be answered, including why seemingly only felines exhibit a euphoric response to catnip and silver vine, and why only some of these felines do so. Dr. Gallio, while enthusiastic about the new study, offered a cautious approach. “What do I know?” he said. “I wasn’t there to see evolution happen.”
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