Knee-clicks and visual traits indicate fighting ability in eland antelopes: multiple messages and back-up signals
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BioMed Central
Open Access
Research article
Knee-clicks and visual traits indicate fighting ability in eland antelopes: multiple messages and back-up signals Jakob Bro-Jørgensen*1 and Torben Dabelsteen2 Address: 1Institute of Zoology, Zoological Society of London, London NW1 4RY, UK and Department of Biological and Environmental Science, University of Jyväskylä, Finland and 2Animal Behaviour Group, Department of Biology, University of Copenhagen, Denmark Email: Jakob Bro-Jørgensen* - [email protected]; Torben Dabelsteen - [email protected] * Corresponding author
Published: 5 November 2008 BMC Biology 2008, 6:47
doi:10.1186/1741-7007-6-47
Received: 27 June 2008 Accepted: 5 November 2008
This article is available from: http://www.biomedcentral.com/1741-7007/6/47 © 2008 Bro-Jørgensen and Dabelsteen; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Abstract Background: Given the costs of signalling, why do males often advertise their fighting ability to rivals using several signals rather than just one? Multiple signalling theories have developed largely in studies of sexual signals, and less is known about their applicability to intra-sexual communication. We here investigate the evolutionary basis for the intricate agonistic signalling system in eland antelopes, paying particular attention to the evolutionary phenomenon of loud knee-clicking. Results: A principal components analysis separated seven male traits into three groups. The dominant frequency of the knee-clicking sound honestly indicated body size, a main determinant of fighting ability. In contrast, the dewlap size increased with estimated age rather than body size, suggesting that, by magnifying the silhouette of older bulls disproportionately, the dewlap acts as an indicator of age-related traits such as fighting experience. Facemask darkness, frontal hairbrush size and body greyness aligned with a third underlying variable, presumed to be androgen-related aggression. A longitudinal study provided independent support of these findings. Conclusion: The results show that the multiple agonistic signals in eland reflect three separate components of fighting ability: (1) body size, (2) age and (3) presumably androgen-related aggression, which is reflected in three backup signals. The study highlights how complex agonistic signalling systems can evolve through the simultaneous action of several selective forces, each of which favours multiple signals. Specifically, loud knee-clicking is discovered to be an honest signal of body size, providing an exceptional example of the potential for non-vocal acoustic communication in mammals.
Background Rivals often use agonistic signals to broadcast their fighting ability and thereby settle conflicts without incurring the high costs associated with actual fighting
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