Elands signal sexual prowess with clicky knees
BBC NEWS | Antelope's sex signal in the knee
It doesn't look like the original article is published yet, but this is actually quite interesting. The sound of these clicks goes very far, so they were well known. People just didn't know which part of the eland's body was doing the clicking (although since the clicks come in time with limb movements, parts of the limb were a reasonable guess), or why. The usual supposition was that the hooves click together as the animal walked. Turns out there was a more proximal answer.
And it's even a good pic of an eland.
Update 11/5: Ed Yong has a great post on this same article over at his blog, Not Exactly Rocket Science.
Bro-Jørgensen J, Dabelsteen T (2008) Knee-clicks and visual traits indicate fighting ability in eland antelopes: multiple messages and back-up signals. BMC Biology in press.
Abstract; provisional PDF of the original article (you may need a subscription).