When cats chew catnip, it acts as an insect spray science

A cat chews and rolls in the leaves of the silver vine.
Masao Miyazaki

Even for the most remote cats, just a few leaves of catnip can cause agitated bouts of chewing, kicking and rolling.

Silver vine – or matatabi in Japanese – inspires a similar euphoria caused by plants in our feline friends. The answer certainly seems amusing, but until recently, scientists weren’t sure if cats’ behavior could actually have any benefits other than pure pleasure.

A new study published this week in iScience, suggests that when cats play (and harm) with a catnip or silver vine, the leaves of the plant actually emit higher levels of chemicals that are beneficial: repel mosquitoes. Both plants can act as something like a natural insect spray, and when cats chew the leaves, this insect spray becomes even more effective. Researchers at Iwate University in Japan, who have been studying the interactions of cats with catnip and silver vine for several years, are behind the study..

But rolling in the leaves is only one component of cats’ reactions to these plants. Masao Miyazaki, a behaviorist from Iwate University and author of the study, explained that cats are involved in four main behaviors with catnip or silver vine: licking, chewing, rubbing and rolling. In an earlier study, Miyazaki said they found that rubbing and rolling were very important in transporting iridoids – the chemicals that cause cat endorphins to rush – into cat hair and repel mosquitoes. If rubbing and rolling in silver vine leaves is the cat’s way of spraying insects, that still doesn’t explain why, in addition to inflating, cats lick and chew the leaves.

In the new study, researchers looked more closely at what happens at the chemical level when leaves are damaged by cats. They first collected intact silver vine leaves, as well as leaves chewed by cats and leaves that they crushed by hand. Chemical analysis has shown that damage to both cats and humans causes the leaves to increase their emissions of various iridoids. The chemical cocktail in the damaged leaves was also less dominated by one chemical and instead had a more even balance than five different chemicals.

The researchers then tested these different chemical cocktails to see how cats and mosquitoes reacted to them. When giving trays with intact and damaged silver vine leaves, cats spend more time licking and rolling over the damaged leaves. And when the researchers synthesized the chemical cocktails found in these leaves, the cats again spent more time with the cocktail with damaged leaves.

Cats prefer a better balanced mixture of iridoids than a simpler mixture, even when the levels of nepetalactol, the main iridoid in the silver vine, are the same. Nepetalactol was previously thought to attract cats to it, but this new discovery has revealed that there is something special about the mixture of chemicals that is extremely tempting. “I was really surprised that the combination of iridoid compounds increased the reaction of cats,” said Reiko Uenoyama, a graduate student at Iwate University and lead author of both studies.

The complex chemical mixture, which was most attractive to cats, was also the most repulsive to mosquitoes. To compare the insect-repellent properties of the mixtures, the researchers filled a transparent box with mosquitoes and placed a shallow plate inside. When the complex chemical mixture of damaged leaves was added to the container, the mosquitoes escaped faster than when the simpler mixture of intact leaves was added.

While the silver vine responds to damage caused by a cat by diversifying its chemical profile, catnip does not. The researchers repeated all their experiments with catnip and found many different results. The main iridoid chemical in catnip is nepetalactone – not nepetalactol – and it stays that way despite leaf damage. When cats chew catnip, the leaves significantly increase their nepetalactone emissions.

Despite this different reaction to damage, crushing still makes catnip leaves more attractive to cats and more repulsive to mosquitoes. But in this case, the answers are due to higher levels of a chemical. And when we compared the plants to each other, a large dose of catnip cocktail was needed to elicit the same response from cats and mosquitoes as a very small dose of silver vine cocktail. However, catnip leaves themselves are just as attractive to cats as silver vine leaves, as the amount of chemicals released from catnip leaves is much higher overall.

Why even small amounts of complex mixtures of chemicals are so effective in triggering reactions is not clear to scientists. “Unfortunately,” says Miyazaki, “we don’t know why the cocktail reacts more strongly to cats and mosquitoes.” But despite these long questions, Benjamin Leachman, a plant biochemist at York University who did not participate in the study, says research “emphasizes the importance of mixtures or cocktails of chemicals in the interaction with animals, as opposed to single compounds.”

Scientists are still unsure when this particular cat’s behavior first developed. In a previous study, researchers found that leopards and jaguars would rub their heads on paper impregnated with nepetalactol, just as domestic cats do. This finding suggests that this behavior, which takes advantage of the insect repellent characteristics of certain plants, may have evolved into a distant ancestor of cats.

“I’m just so interested in how cats have developed this innate behavior to defend themselves in this way,” said Nadia Melo, a chemical ecologist at Lund University who was not involved in the study. She said other mammals faced similar risks of insect disease, “but you don’t see that in dogs that are apparently also affected by mosquitoes.”

Catnip and silver vine can also be useful in protecting people from insects. The mosquito species used in this study transmit roundworms to cats and dogs, and also spread many human viruses, such as dengue and chikungunya. Melo’s previous research suggests that other mosquitoes are likely to have similar reactions. “I think all mosquitoes would react almost the same way,” she said.

Thus, catnip and silver vine chemicals may be useful in developing safer and more effective insect repellents for human consumption. They can simply have the side effect of attracting cats as well. “If someone doesn’t like cats or is allergic to cats,” Miyazaki wrote in an email, “I shouldn’t use iridoids as repellents!”

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