Three hypotheses on the relationship between personality traits (dashed boxes) and aggressive behaviours and signalling (solid boxes). The hypotheses are illustrated in the respective scatterplots of four hypothetical individuals marked by the different symbols. The diagonal line is x = y and is meant to illustrate a hypothetical relationship between signalling and aggression. (a) Represents the null hypothesis that there is no personality trait influencing aggressive behaviour or signalling behaviours, although these two are correlated with each other (i.e. signalling is honest). (b) Represents the first alternative hypothesis where aggressive and signalling behaviours are influenced by a single personality trait, aggressiveness. In this hypothesis, residual variation in signalling behaviours once aggressive behaviours are taken into account is not individually consistent. (c) Represents the second alternative hypothesis where signalling levels are influenced not only by aggressiveness but also by a second personality trait which we term communicativeness. The presence of the second personality factor is inferred from whether the residual variation in signalling levels—once aggressiveness is controlled for—is repeatable.
Average aggression and signalling scores of the 45 individuals with at least three trials. Each dot represents an individual and the error bars represent +1 s.e. Although the correlation between average signalling scores and average aggression scores is improved compared to when looking at single trials, there remains a significant portion of variance that is unexplained by this relationship. Individual differences in communicativeness seems to be responsible for at least part of this residual variation.