Knee clicking – male antelope flexes pecs

© Jakob Bro-Jorgensen

This signal is used to establish mating rights amongst the males without the need to resort to physical fights.

The research, undertaken by the Zoological Society of London and University of Copenhagen and published in the open access journal BMC Biology, investigates the different signals that the male eland antelopes (Tragelaphus oryx) use to demonstrate their fighting ability and thus decide mating rights.

It found that the males (bulls) have a number of different signals, including knee-clicking to indicate body size, dewlap (skin fold under the throat) size to indicate age and hair darkness to indicate aggression, all of which provide an indication of fighting potential.

Dr Jakob Bro-Jorgensen, Research Fellow at ZSL and lead author of the paper, said, “We’re all familiar with macho posturing in humans – young men flexing their pecs – or with male lions demonstrating their fitness by roaring. The male eland antelope struts his stuff in a rather unusual way: by clicking his knees.”

Demonstrating fighting ability has obvious benefits to the males, as it allows bulls to assess their rivals and thus to avoid the high costs of actual fighting. The antelopes’ knee clicks, which can be heard several hundred metres away, are thought to be produced by a tendon slipping over one of the leg bones and, according to the authors, this can explain why they correlate with body size.

Dr Bro-Jorgensen explains: “The tendon in this case behaves like a string being plucked, and the frequency of the sound from a string correlates negatively with both its length and diameter. Thus, most importantly, depth of the sound is predicted to increase with skeletal measures and it can therefore be directly associated with body size.”

The research was undertaken in a 400km² area of Kenya, where the eland antelope is widespread.