Some people can't whistle at all, much less whistle without lips.

The walnut sphinx caterpillar has no such trouble. When disturbed, say by a predator, these North American insects compress their bodies and squeeze air out of holes in the sides of their bodies, called spiracles. The result is a shrill sound that can last for several seconds.

Now Jayne Yack, a neuroethologist at Carleton University who made the spiracle discovery, has found the high-pitched noises dissuade hungry birds. (Read: "Caterpillars Drum Their Anuses To Find New Friends.")

“There are different hypotheses to explain why a predator would be deterred, and we tested the startle hypothesis: Namely, that the whistle freaks out and frightens the bird so it doesn’t come back,”