Plants and their predators have been evolving in step with each other since the earliest mosses and ferns first grew on land. Covering themselves in spines or producing harmful chemicals is a pretty good defense, but plants are a lot more active in their fight against herbivores than many tend to think. Now, a new study has found that young saplings are able to tell the difference between whether or not their buds have been damaged by the wind or have been nibbled by a deer.

As a young tree stuck to the spot growing on the forest floor, the ravaging appetite of a hungry deer could spell disaster, and ultimately the end. But the saplings don't go down without a fight, launching a chemical defense against the marauding herbivores by producing astringent tannins that taste bad and put the creatures off. But the plant needs to know whether or not damage to its buds is indeed caused by a munching deer, or more benignly caused by other things such as wind.

It turns out that when a bud is damaged, the trees can sense the animal's saliva in the wound. When it does, it triggers a response from the sapling, which produces a hormone known as salicylic acid, that in turn causes the plant to increase the concentration of tannins in that part of the plant. Not only that, but it also spurs the plant on to produce more growth hormones that cause the remaining buds to grow more vigorously, and make up for those that have been lost to the deer.

“On the other hand, if a leaf or a bud snaps off without a roe deer being involved, the tree stimulates neither its production of the salicylic acid signal hormone nor the tannic substances,” explains Bettina Ohse from the University of Leipzig, lead author of the study, published in Functional Ecology. “Instead, it predominantly produces wound hormones.”

The complexities of how plants deal with being eaten by animals may surprise some people. Rather than being passive organisms that are predated upon, plants are actively trying to fight off their predators. One study, for example, found that plants can in effect “hear” themselves being eaten, and mobilize a chemical defense in response. And not only can plants sense when they are being eaten by insects, they can even then warn others nearby as to the damage that is being done.

Initially dismissed as complete hokum, it now turns out that when an animal starts chewing on leaves, the plant produces what are known as volatile organic compounds into the air. These are then detected by other plants growing nearby, which respond appropriately to pre-empt the threat of being munched on themselves by increasing the concentration of unsavory chemicals, such as tannins, in their own leaves.

So next time you go for a wander in the great outdoors, bear in mind that while the trees and plants around you might look serene and passive, they are actually waging a bitter warfare with the animal kingdom.