When Cats Chew Catnip It Works As Insect Repellent | Science

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

Even for the most aloof cats, a few leaves of catnip can trigger tantrums of chewing, kicking, and rolling excitement.

Silver vine or matatabi in Japanese – inspires a similar plant-induced euphoria in our feline friends. The answer certainly sounds amusing, but until recently scientists didn’t know if cat behavior could actually have any benefits other than pure fun.

New research, published this week in iScience, suggests that when cats play with (and damage) catnip or silver vine, the leaves of the plants actually emit higher levels of chemical compounds that have one benefit: repelling mosquitoes. Both plants can act as a kind of natural bug spray, and when cats chew on the leaves, this bug spray becomes even more effective. Researchers from Iwate University in Japan, who have been studying cats’ interactions with catnip and silver vine for several years, are behind the research.

But rolling in leaves is only part of cats’ response to these plants. Masao Miyazaki, animal behaviorist at Iwate University and author of the study, explained that cats engage in four main behaviors with catnip or silver vine: licking, chewing, rubbing and rolling. In an earlier study, Miyazaki says he discovered that rubbing and rolling are very important in transferring iridoids — the chemicals that trigger the cat’s endorphin rush — to cat fur and repelling mosquitoes. While rubbing and rolling silver vine leaves is a cat’s way of applying insect repellent, that still doesn’t explain why, other than getting high, cats also lick and chew the leaves.

In the new study, researchers took a closer look at what happens on a chemical level when leaves are damaged by cats. They first collected intact silver vine leaves as well as cat-chewed leaves and hand-crumpled leaves. Chemical analysis showed that damage inflicted by cats and humans caused increased leaf emissions of various iridoids. The chemical cocktail in damaged leaves was also less dominated by a single chemical, and instead had a more balanced balance of five different chemicals.

The researchers then tested these different chemical cocktails to see how cats and mosquitoes responded to them. When given trays with intact and damaged silver vine leaves, the cats spent 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 from the damaged leaves.

Cats preferred the more balanced mixture of iridoids over the simpler mixture, even when levels of nepetalactol, the main iridoid in silver vine, were the same. Previously it was thought that nepetalactol was what attracted cats, but this new discovery revealed that there was something special about the mix of chemicals that was very appealing. “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 both studies.

The complex chemical mixture that most attracted cats was also the most repellent to mosquitoes. To compare the insect repellent properties of the mixtures, the researchers filled a transparent box with mosquitoes and placed a shallow dish inside. When the complex chemical mixture of damaged leaves was added to the dish, the mosquitoes fled faster than when the simpler mixture of intact leaves was added.

While silver vine responded to damage inflicted by cats by diversifying its chemical profile, catnip did not. The researchers repeated all their experiments with catnip and found very different results. The primary iridoid chemical in catnip is nepetalactone – not nepetalactol – and this remains the case regardless of leaf damage. When cats chew catnip, the leaves dramatically increase their emissions of nepetalactone alone.

Despite this different reaction to damage, being wrinkled made catnip leaves more attractive to cats and more repellent 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 high dose of catnip cocktail was needed to elicit the same response from cats and mosquitoes as a very small dose of silver vine cocktail. Still, catnip leaves themselves were just as appealing to cats as silver vine leaves, because the amount of chemicals emitted from catnip leaves is much higher overall.

Why even small amounts of complex mixtures of chemicals are so effective in triggering responses is unclear to scientists. “Unfortunately,” says Miyazaki, “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 not involved in the study, says this research “underscores the importance of mixtures or cocktails of chemicals in the interaction with animals as opposed to single compounds”.

Scientists still don’t know when this particular cat behavior first evolved. In their previous study, the researchers found that leopards and jaguars rub their heads on nepetalactol-soaked paper, just like house cats. This discovery suggests that this behavior, which takes advantage of the insect-repellent characteristics of certain plants, may have evolved from a distant feline ancestor.

“I find it so interesting how cats evolved this innate behavior to defend themselves in 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 that in dogs, which are obviously also affected by mosquitoes.”

Catnip and silver vine could also be useful in protecting humans from insects. The species of mosquito used in this study transmits roundworms to cats and dogs and also spreads many human viruses, such as dengue and chikungunya. And Melo’s previous research suggests that other mosquitoes are likely to have similar responses. “I think all mosquitoes would react in about the same way,” she says.

Thus, chemicals from catnip and silver vine could prove useful in developing safer and more effective insect repellents for human use. They could also have the side effect of attracting cats. “If someone doesn’t like cats or is allergic to cats,” Miyazaki wrote in an email, “they shouldn’t use iridoids as a repellent!”

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