Manipulating the appearance of a badge of status causes changes in true badge expression

Abstract

Signals of dominance and fighting ability (i.e. status signals) are found in a wide range of taxa and are used to settle disputes between competitive rivals. Most previous research has considered status-signal phenotype as an attribute of the individual; however, it is more likely that signal expression is an emergent property that also incorporates aspects of the social environment. Furthermore, because an individual's signal phenotype is likely to influence its social interactions, the relationships between status signals, social environment and individual quality are probably much more complex than previously appreciated. Here, we explore the dynamic relationship between social interactions and signal expression in a previously undescribed status signal, the frontal shield of the pukeko (Porphyrio porphyrio melanotus: Aves). We demonstrate that frontal shield size is a strong predictor of dominance status within social groups, even after controlling for potentially confounding variables. Then, we evaluate the relationship between social interactions and signal expression by testing whether manipulating apparent shield size influences (i) dominance interactions and (ii) future signal expression. By showing that decreasing apparent shield size causes both an increase in the amount of aggression received and a decrease in an individual's true shield size, we provide the first evidence of dynamic feedback between signal expression and social interactions. Our study provides important insight into the role of receiver-dependent (i.e. social) costs in maintaining signal honesty and demonstrates a unique approach to studying status signalling applicable to future studies on dynamic morphological signals.

Keywords: badges of status; dominance; handicap hypothesis; honest signalling; resource-holding potential.

Figure 1.

The relationship between shield width and David's score (an index of social dominance) is shown for 82 individuals in 11 pukeko social groups (one per panel). Higher David's scores indicate more dominant individuals. The bottom left panel shows all individuals across all social groups combined. See text for details of statistical analysis.

Figure 2.

Aggressive behaviours directed at focal individuals with natural (control) or decreased (reduction) apparent shield size. Data are the mean differences in the rate each type of behaviour was received (±s.e.) between the pretreatment and post-treatment observations. See text for details of statistical analysis. (Online version in colour.)

Figure 3.

The change in true shield width as a result of apparent shield manipulation. Data shown are the mean change (±s.e.) over a one-week period after manipulation for individuals that received the control (n = 9) and shield reduction treatments (n = 7). See text for details of statistical analysis. (Online version in colour.)