Knee-clicks and visual traits indicate fighting ability in eland antelopes: multiple messages and back-up signals - PubMed (original) (raw)

Knee-clicks and visual traits indicate fighting ability in eland antelopes: multiple messages and back-up signals

Jakob Bro-Jørgensen et al. BMC Biol. 2008.

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.

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Figures

Figure 1

Figure 1

A lateral photo of a mature eland bull indicating the measures taken: (1) body depth, (2) horn length, (3) dewlap droop, (4) frontal brush size, (5) darkness of the facemask, (6) greyness of the body, and (7) knee-click frequency.

Figure 2

Figure 2

The traits plotted against the three principal components using their numeric Pearson correlation coefficients.

Figure 3

Figure 3

The dominant frequency of knee-clicks in relation to body depth. Linear regression line shown (P < 0.001).

Figure 4

Figure 4

Narrow-band spectrograms (window length = 0.03 s, band width = 43 Hz, Gaussian window shape) of knee-clicks from the same bull recorded in two consecutive years. Note the drop in knee-click frequency between years compared with the constancy over the short term.

Figure 5

Figure 5

The shift after one year in the dominant frequency of the knee-click sound of individual bulls in relation to their change in body depth. Linear regression line shown (P < 0.001).

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