Cats, so often, are a mystery, even to those who know them best. Why do they sleep so much? Why do they want your full attention in one minute, not the next? How will they find a way back home after being stuck miles away for years? Author Haruki Murakami, who is known for keeping cats in his novels and essays, once admitted to not knowing why he did so; A cat “slips naturally,” he said.
Another mystery: why do cats like catnip? Upon contact with mint-related plants, most domestic cats will lick it, rub against it, chew it, and move around. They slammed the edge, getting high off the stuff. These are also wild for other plants, especially the Silver creeper, which is not closely related to catnip but produces similar reactions from cats, including large cats such as jaguars and tigers.
Over the years, this behavior has only been a cat-related mystery. But a new study, published in the iScience Journal on Tuesday, suggests that the reaction of catnip and silver vine may be explained by the bug-resistant effects of irides, a plant chemical that induces altitude.
Researchers led by Masao Miyazaki, an animal behavioral scientist at the University of Iwate in Japan, found that the amount of this iridoid emitted by plants increased by more than 2,000 percent when plants were damaged by cats. So perhaps Kitty’s height offers an evolutionary advantage: keeping vampires away.
Christine Vitaly, a cat behavior specialist at Unity College who was not involved in the study, noted that the study is based on strong previous work. Last year, the same lab published a study that found that cats would try their best to wrap themselves in irides, such as DEET, rolling over a chemical or climbing up to rub their cheeks. “This suggests that cats may have a benefit for physically placing compounds in their bodies,” said Dr. Vitale.
Carlo Siracusa, an animal behaviorist at the University of Pennsylvania who was not involved in the research, agreed. “Evidence shows that they want to impregnate their bodies with odors,” he said. But, he added, “Remember that a large part of cats do not show this behavior. So why would they be selected that way?
As an evolutionary adaptation, bug-resistant iridesides probably do more to help cats avoid bug bites than to protect plants from herbivorous insects. Plants often release irritants when damaged, which helps repel attackers and they emit other chemicals that alert their neighbors to danger. “Plant is the master of chemical warfare,” said Marco Gallio, a neurobiologist at Northwestern University who was not involved in the new study.
Last year, Dr. Gallio and his colleagues published a report linking the primary bug resistance of catnip, neptalactone, to a receptor protein that causes irritation to mosquitoes and related insects. The receptor, which is also present in humans and cats, can be blocked by tear gas. But Dr. Galileo found that although neptalactone had no negative effects on humans and sent cats into ecstasy, it did activate this special receptor (called TRPA1) in many insects – an added bonus for cats to turn to their favorite medicine.
In their most recent study, Dr. Miyazaki and his colleagues measured the chemical composition of the air above the leaves – both intact and damaged – the catnip and the silver vine. They then measure the iridoid levels on the leaves themselves. They found that cat-driven catnip leaves emit at least 20 times more napetalactone than intact leaves, while damaged silver vine leaves emit at least eight times the amount of similar iridesides than intact leaves. The interaction of cats with the silver vine has also changed the texture of the plant’s bug-resistant cocktail, making it even stronger.
After rubbing their face and body against the plants, the cats must be covered with a hard layer of insects.
This discovery, linked to previous research by Dr. Miyazaki and his team, supports the new claim that at least part of the benefit of the Kitty Catnip craze is to repel mosquitoes and flies. This type of behavior, called “self-initiation”, is not the first in the animal kingdom. Mexican spider monkeys are known to stain themselves with a variety of leaves, perhaps for a social or sexual purpose, and hedgehogs often rub venom on their spines.
Still, many questions remain to be answered as to why ostensibly only cats react to catnip and silver vines, and why only some of these cats do so. Dr. Gallio, while enthusiastic about the new study, suggested a cautious approach. “Do I know?” He said. “I wasn’t there to see evolution happen.”