When cats chew Catnip, it works as a Bug Spray Science

A cat chews and rounds the leaves of a silver vine.
Masao Miyazaki

Even for the most distant cats, a few cat leaves can cause chewing, kicking, and spinning.

Silver vine — or matatabi in Japanese — it evokes a similar euphoria caused by plants in our feline friends. The answer certainly sounds funny, but until recently, scientists weren’t sure if the cat’s behavior had any other benefit than pure pleasure.

New research, published this week Science, suggests that playing with (or harming) cats or silver grapes causes plant leaves to release higher levels of chemical compounds that actually have a benefit: repelling mosquitoes. Both plants can have a kind of natural beetle spray, and when cats chew the leaves, this bug spray is even more effective. Researchers at Iwate University in Japan have been investigating the interactions of cats with cats and silver grapes for several years..

But walking on leaves is just one component of the response that cats give to these plants. Masao Miyazaki, an animal behavior and research author at Iwate University, explains that cats have four main behaviors with cats or silver grapes: licking, chewing, rubbing and throwing. In a previous study, Miyazaki said that rubbing and rolling are very important in transferring iridoids — chemicals that cause cat endorphin speed — to transfer the cat’s tail and scare away mosquitoes. If rubbing the leaves of silver vines and rubbing them is a cat’s way to apply the beetle spray, this still doesn’t explain why, in addition to rising, cats also lick and chew the leaves.

In the new study, the researchers looked more closely at what happens at the chemical level when the leaves are damaged by cats. First, they collected the silver vine leaves, as well as the leaves that the cats chewed and the leaves that they wrinkled by hand. A chemical study showed that damage caused by both cats and humans caused the leaves to increase their emissions of various iridoids. The chemical cocktail of the affected leaves was also less prevalent in a single chemical, and had a more equal balance of five different chemicals.

The researchers then tested these different chemical cocktails to see how the cats and mosquitoes each responded. When they were given whole silver grape leaves and broken trays, the cats spent more time licking and turning on the damaged leaves. And when the researchers synthesized the chemical cocktails found in these leaves, the cats again spent more time with the leafy cocktail.

Cats preferred a more balanced mixture of iridoids compared to a simpler mixture, although the levels of nepetalactol, the main iridoid in the silver vine, were the same. Previously, nepetalactol was thought to attract cats, but this new finding revealed that the extraordinarily attractive chemical mixture had something special about it. “I was really surprised that the combination of iridoid compounds improved the feline response,” says Reiko Uenoyama, a graduate student at Iwate University and lead author of two studies.

The complex chemical mixture that was most appealing to cats was also scared away by mosquitoes. To compare the insect-repellent properties of the mixture, the researchers filled a transparent box with mosquitoes and placed a low plate inside. When a complex chemical mixture of spoiled leaves was added to the dish, mosquitoes escaped faster than when a simpler mixture of whole leaves was added.

The silver grape reacted to the damage caused by cats by diversifying its chemical profile, which catnip did not. The researchers repeated all the experiments with the cat and found very different results. The main chemical iridoid in Catnip is nepetalactone, not nepetalactol, which remains the same despite leaf damage. When cats chew catnip, the leaves only increase nepetalactone emissions.

Despite the different reactions to the damage, being wrinkled still made the cat’s leaves more attractive to cats and mosquitoes. But in this case, the answers were for higher levels of a single chemical. And when comparing plants to each other, a large dose of the cat’s cocktail was needed to trigger the same response as cats and mosquitoes, resulting in a very small dose of a silver grape cocktail. However, cat leaves were as attractive to cats as silver vines, because the amount of chemicals emitted by cat leaves is much higher overall.

Scientists are not clear why small amounts of complex chemical mixtures are so effective at triggering responses. “Unfortunately,” says Miyazaki, “we don’t know why the cocktail reacted more strongly to cats and mosquitoes.” But despite these persistent questions, Benjamin Lichman, a biochemical plant at York University who did not participate in the study, says the study “emphasizes the importance of chemical mixtures or cocktails in interacting with animals over single compounds.”

Scientists still do not know when this particular behavior of the cat evolved. In a previous study, researchers found that leopards and jaguars rub their heads on nepetalactol-soaked paper like domestic cats. This finding suggests that this behavior, which takes advantage of the insecticidal properties of certain plants, has evolved into a distant feline ancestor.

“I find it so interesting how cats have developed this inherent behavior to defend themselves in this way,” says Nadia Melo, a chemical ecologist at Lund University who did not participate in the study. He noted that other mammals are at risk for insect-like diseases, “but you don’t see that in dogs, because mosquitoes are obviously affected as well.”

Catnip and silver vines can also be useful in protecting humans from insects. The mosquito species used in this study transmit worms to cats and dogs and spread many human viruses, such as dengue and chikungunya. And Melo’s previous research suggests that other mosquitoes will have similar responses. “I think all mosquitoes would react the same way,” he says.

So catnip and silver vine chemicals may be useful in developing safer and more effective insect repellents for human use. They can also have a side effect on attracting cats. “If someone doesn’t like cats or is allergic to cats,” Miyazaki wrote in an email, “they shouldn’t use iridoids to scare them away!”

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