The Flehman Response: Why Do Rutting Bucks Curl Their Lips?

If you've spent any time around deer in November, you've seen bucks exhibit a strange sneering expression. They'll strut up behind does, sniff their rumps or their urine, and then lift their noses in the air, curl their lips and...what?

What are they doing? You can see this behavior in the video. The behavior has a funny name: the flehmen response. For a long time I figured this was named after the biologist who first observed the phenomenon, some European naturalist named Victor Flehman or something.

I was wrong. The name is derived from the German verb "to curl." And the behavior is a way to get a deep whiff of something, exposing the vomeronasal organ to especially delicious smells. Horses do it. Cats do it. And every deer is doing it this month.

Now you know. Doesn't have anything to do with Victor Flehman, at all.