Some cats go loopy for catnip. Not merely getting their face as close to it as possible, as they would treats, but biting and rolling all over it. Why? Researchers in Japan have found the way cats damage the plants causes the release of many more compounds, increasing a plant’s potency at repelling insects.
If you have a cat that enjoys rolling around on catnip or silver vine then, despite appearances, it’s being very sensible. Catnip and silvervine contain chemicals known as iridoids that repel mosquitoes. Rolling in the plants transfers some of these compounds into the cat’s fur. But why would a cat also lick or bite the plant?
Reiko Uenoyama and colleagues noticed that silver vine leaves crumpled and torn by feline licking and chewing appeared to have a much stronger aromatic odour than intact leaves. They set out to measure how much more chemicals the leaves released after a cat damaged them. A chemical analysis revealed two key changes.
First, a damaged leaf produces a lot more volatile compounds than an intact leaf. Damaging a silver vine leaf releases iridoids straight away and increases the emissions by ten times. Additionally, it changes the kind of iridoids that the leaves produce. “Nepetalactol accounts for over 90% of total iridoids in intact leaves, but this drops to about 45% in damaged leaves as other iridoids greatly increase,” says lead author Masao Miyazaki in a press release. “The altered iridoid mixture corresponding to damaged leaves promoted a much more prolonged response in cats.”
“We also examined whether feline licking and chewing of leaves has similar effects on the amount and composition of iridoids in the well-known cat attractant plant, catnip, as well as silver vine”, says Reiko Uenoyama, the paper’s first author in another press release. “Damaged catnip emitted 20-fold more total iridoids compared to intact leaves. However, in contrast to silver vine, leaf damage did not alter the composition of catnip iridoids. Both intact and damaged extracts of catnip consisted almost solely of nepetalactone, which is present at only very low levels in silver vine.”
The scientists set out to test if the felines were reacting to these compounds specifically. The researchers gave the cats dishes with pure nepetalactone and nepetalactol. “Cats show the same response to iridoid cocktails and natural plants except for chewing,” says 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.”
Finally, the research team showed that mosquitoes are more sensitive to the complex cocktail of iridoids induced by damage to silver vine leaves compared to the nepetalactol-dominated iridoids in the intact silver vine. The diversification of iridoids in damaged silver vine leaves provides a stimulus that is more repellent to mosquitoes at low concentration, inducing a faster aversive response than nepetalactol- or nepetalactone-dominated iridoids in plants.
The paper published in iScience suggests that botanists should try to identify how the leaves create the iridoids. Uenoyama and colleagues write: “Our findings may lead to new clues to identify key plant enzymes for the biosynthesis of plant iridoids that may be usefully utilized as repellents against a broad range of pests including mosquitoes. For example, it may be helpful to explore the enzymes with upregulated expression and/or activity that occur within 10 min of leaf damage (the typical duration of a feline response).”
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Uenoyama R, Miyazaki T, Adachi M, Nishikawa T, Hurst JL, and Miyazaki M. 2022. Domestic cat damage to plant leaves containing iridoids enhances chemical repellency to pests. iScience. https://doi.org/10.1016/j.isci.2022.104455