Cats go a bit crazy in the presence of catnip (Nepeta cataria) and its Asian counterpart, silver vine (Actinidia polygama). They roll on it, rub their bodies on it, chew it, lick it and play with it. It appears that cats are completely intoxicated by these plants, which induce a bout of this strange behavior that may last as long as 15 minutes. However, new research indicates that their behavior may not be as crazy as it looks.
Previous research has indicated that the leaves of catnip and silver vine plants contain chemicals known as iridoids that act as insect repellents. The unusual but characteristic response that cats have to these plants, including rubbing the head and face on the leaves and rolling on them, has the effect of coating the body in iridoids – which repel mosquitoes and other pests that can potentially transmit diseases. But the reason that cats lick, chew and otherwise damage the plant leaves as part of this characteristic behavioral response has remained a mystery.
Researchers in Japan have now conducted experiments to test whether damaging the leaves of catnip and silver vine by chewing, crumpling and tearing them has any effect on the chemicals that they release, and therefore on their efficacy in protecting cats from insect pests.
Cats have such an unusual reaction to catnip and silver vine, the world over, that lead author Masao Miyazaki, an animal behavior researcher at Iwate University, felt he had to investigate the phenomenon further. “Even in the famous musical ‘Cats’ there are scenes where you see a cat intoxicate another cat using catnip powder.”
Miyazaki began his career in veterinary medicine and developed an interest in how chemicals, such as pheromones, drive companion animals’ instinctual behaviors. Catnip and silver vine leaves contain the compounds nepetalactol and nepetalactone, amongst others, which are iridoids that have an insect repellent effect.
The research team started by measuring the concentration and composition of these iridoids in the space above intact leaves, and leaves that had been chewed or crumpled. They used gas chromatography/mass spectrometry for this and found that physical damage to silver vine and catnip leaves promotes an immediate emission of these plant compounds.
“We found that physical damage of silver vine by cats promoted the immediate emission of total iridoids, which was 10-fold higher than from intact leaves,” said Miyazaki.
Not only were more iridoids released by the leaves of both plants, but the chemical composition of the iridoid cocktail released from silver vine leaves also changed. “Nepetalactol accounts for over 90 percent of total iridoids in intact leaves, but this drops to about 45 percent in damaged leaves, as other iridoids greatly increase,” said Miyazaki. “The altered iridoid mixture corresponding to damaged leaves promoted a much more prolonged response in cats.”
According to the results, changes in both the amount of plant iridoid emission (both plant species) and composition (silver vine only) induced significantly longer response behavior by the cats, which included increased self-anointing (rubbing and rolling) that has the effect of transferring plant iridoids onto the cat’s fur.
To test if the felines were reacting to these compounds specifically, the cats were given dishes with pure nepetalactone and nepetalactol. “Cats show the same response to iridoid cocktails and natural plants except for chewing,” said Miyazaki. “They lick the chemicals on the plastic dish and rub against, and roll over on, the dish.”
“When iridoid cocktails were applied on the bottom of dishes that were then covered by a punctured plastic cover, cats still exhibited licking and chewing even though they couldn’t contact the chemicals directly. This means that licking and chewing is an instinctive behavior elicited by olfactory stimulation of iridoids.”
Lastly, the researchers tested the insect repellent properties of the iridoid cocktail from damaged catnip and silver vine leaves against Aedes albopictus, a common mosquito in Japan and China. This species of mosquito is a vector of Dirofilaria immitis, which infects the heart and pulmonary arteries of dogs and cats. Samples of iridoids representing extracts from intact and damaged leaves were introduced to groups of 18 – 26 mosquitoes in acrylic boxes, and the percentage of mosquitoes that moved away from the source and sought refuge in a small shelter about 15 cm away from the source was recorded after 10 and 20 minutes.
The results indicated that the iridoid cocktails from damaged leaves were more effective at repelling the insects than were the intact leaves. The effect of damaged silver vine iridoids was more rapid and occurred at lower concentrations of the cocktail, but the iridoids from damaged catnip leaves also had an effect, albeit at a stronger concentration and over a longer period of time.
The experts conclude that the physical damage of the plant leaves by the cats enhances the release of iridoids that are repellent to arthropods such as mosquitoes. This destructive behavior, in combination with enhanced rubbing and rolling activity, has the effect of enhancing the transfer of iridoids to the cats’ fur. In this manner, the cats obtain stronger chemical pest defense from the leaves of these plants. Their study also provides an insight into how humans could also use plants to obtain potent natural repellents against pest insects.
Next, Miyazaki and his team want to understand which gene is responsible for cats’ reaction to catnip and silver vine. “Our future studies promise to answer the key remaining questions of why this response is limited to Felidae species, and why some cats don’t respond to these plants,” said Miyazaki.
The results of the study are published today in the journal iScience.