The eland is a species of antelope native to the southern tip of Africa, and male’s horns can grow to 1.2 metres (four feet) long. Males are completely mute, but they do make sounds to attract females and avoid aggression from neighbouring males.
Eland males can ‘crack’ their knees by slipping the tendon over their carpal bone in the front legs. The tendon acts like a string being plucked, and the pitch of the resulting sound changes as the eland grows larger. The sound communicates the size and quality of the male, preventing attacks from smaller males and letting the females around know who to mate with.
Male elands also soak the tuft of hair on their foreheads with urine. It doesn’t matter whose urine they use, the chemicals serve a communicative function that lets others around know that they should not attack.
Image from www.flickr.com/photos/dkeats