Attractive amplifiers in sexual selection: where efficacy meets honesty

How signals have evolved to convey reliable information is a major issue in the study of animal communication. The handicap principle states that recipients must impose differential costs on signallers. Here, we present an alternative hypothesis, the attractive amplifier hypothesis, according to which recipients acquire reliable information by imposing differential benefits on signallers. The attractive amplifier is a non-informative, low-cost phenotypic trait that allows recipients to increase the amount of reliable information acquired from other informative traits. We present a mating-decision model, in which mate attractiveness depends on the multiplicative-interaction between a trait that is positively correlated with mate quality and its attractive amplifier. The evaluation of both traits is assumed to be prone to error. The model shows that, to be an amplifier of cues, an attractive trait must show two well-known consequences of signal ritualization: it must be conspicuous and it must be perceived as a highly stereotyped trait, that is, it must show a low variation at both the within-individual and the within-population levels.