Even for the most distant cats, just a few sheets of catnip can trigger excited attacks of chewing, kicking, and rolling.
Silver Vine – or matatabi in Japanese – inspires a similar plant-induced euphoria in our feline friends. The answer certainly sounds like fun, but until recently, scientists weren’t sure whether cats’ behavior could actually bring benefits other than pure pleasure.
New research, published this week in iScience, suggests that when cats play with (and damage) catnip or vine, plant leaves actually emit higher levels of chemical compounds that have one benefit: repelling mosquitoes. Both plants can act as a kind of natural repellent, and when cats chew the leaves, this repellent 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 in leaves is just one component of cats’ response to these plants. Masao Miyazaki, an animal behaviorist at the University of Iwate and author of the study, explained that cats engage in four main behaviors with catnip or silver vine: licking, chewing, rubbing and rolling. In a previous study, Miyazaki says they found that rubbing and rolling are very important for transferring iridoids — the chemicals that trigger the cat’s endorphin rush — into the cat’s fur and repelling mosquitoes. If rubbing and rolling on silver vine leaves is a cat’s way of applying insect repellent, that still doesn’t explain why, in addition to getting high, cats lick and chew the leaves as well.
In the new study, the 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 leaves that had been chewed by cats and leaves that had been crushed by hand. A chemical analysis showed that the damage inflicted by 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 uniform 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 of intact and damaged silver vine leaves, the cats spent more time licking and rolling around on 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 damaged leaves.
The cats preferred the more balanced mixture of iridoids compared to the simpler mixture, even when the levels of nepetalactol, the main iridoid in the silver vine, were the same. Previously, nepetalactol was thought to be what attracted cats, but this new discovery has revealed that there was something special about the chemical mixture that was even more attractive. “I was really surprised that the combination of iridoid compounds increased the feline response,” says Reiko Uenoyama, a graduate student at the University of Iwate and lead author of both studies.
The complex chemical mixture that was most attractive to cats was also more repellent to mosquitoes. To compare the mixtures’ insect repellent properties, the researchers filled a clear 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 the silver vine reacted to the damage done by the cat by diversifying its chemical profile, the 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 catnip, the leaves greatly increase their nepetalactone emissions alone.
Despite this different reaction to damage, being crumpled still made the 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, it took a large dose of the catnip cocktail to trigger the same response from cats and mosquitoes as a very small dose of the silver vine cocktail. However, catnip leaves themselves were just as attractive to cats as silver vine leaves because the amount of chemicals that catnip leaves give off are much higher overall.
Why even small amounts of complex mixtures of chemicals are so effective at 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 “highlights the importance of chemical mixtures or cocktails in interacting with animals as opposed to single compounds.”
Scientists are still not sure when this particular cat behavior first evolved. In their previous study, the researchers found that leopards and jaguars rub their heads on paper soaked in nepetalactol, just like domestic cats. This finding suggests that this behavior that takes advantage of the insect repellent characteristics of certain plants may have evolved in a distant feline ancestor.
“I find it so interesting how cats developed this innate behavior of defending 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 risks of insect disease, “but you don’t see that in dogs, which are obviously also affected by mosquitoes.”
Catnip and silver vine can also be useful in protecting humans from insects. The mosquito species used in this study transmits roundworms to dogs and cats and also transmits 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 pretty much the same way,” she says.
Thus, the chemicals in catnip and silver vine could be useful in developing safer and more effective insect repellents for human use. They just might have the side effect of attracting cats too. “If someone doesn’t like cats or has an allergy to cats,” Miyazaki writes in an email, “they shouldn’t use iridoids as repellents!”