D:        Yaël, I just discovered that trees know when deer have eaten their leaves.

Y:        How can they know that, Don? Trees don't have brains.

D:        It's a type of automatic chemical response. But it's still pretty impressive. Plants can't run to escape being eaten, so they need other ways to defend themselves from animals like deer that can eat enough leaves to kill young saplings.

Y:        You mean like having thorns on their stems.

D:        Yes. Thorns are just one type of defense. Plants can also produce different types of chemicals that make their leaves taste bad, and be difficult to digest. German scientists studying young beeches and maples knew plants responded to predation, but they wanted to know if plants could tell the difference between a deer chewing their leaves versus being damaged by other factors like storms, or being trampled upon.

Y:        Could they tell the difference?

D:        Yes. The scientists discovered that deer saliva was the key. When a deer feeds on leaves, saliva is left behind. The saliva sends a chemical signal to the trees' cells, and they increase their production of salicylic acid. The acid is a hormone that signals the tree to deposit more bad-tasting chemicals called tannins in its new leaves. It also increases the amount of growth hormone made to grow more leaves.

Y:        Are the scientists sure it was the saliva that caused the change?

D:        Yes. In their experiment, they broke leaves on the trees and either added saliva, or left them alone. The tannin and growth changes only occurred when the saliva was present. Only wound-healing hormones were produced when there was no saliva.