How threat displays work: species-specific fighting techniques, weaponry and proximity risk

Whether threat displays can reveal information about the strength or condition of the contestants is a long-debated issue. Enquist (1985, Animal Behaviour, 33, 1152–1161) showed that communication of such information is possible by means of choice of action in aggressive encounters. The key assumption of Enquist's model is that weak individuals signalling that they are strong (i.e. cheaters) cannot get away without fighting even if they want to, if they meet an honest strong individual. However, this assumption was not elaborated further and Enquist's model is often cited in support of Zahavi's handicap principle. Here I elaborate this assumption and show in terms of Enquist's model, by introducing spatial distance between opponents as a continuous variable, that it is the proximity of the opponent, what I call ‘proximity risk’, that maintains the honesty of threat displays. I show that the honest use of threat displays, sensu Enquist, is evolutionarily stable only within a certain distance threshold. Outside this threshold there may or may not be a zone where a mixture of honest and cheating displays can be evolutionarily stable. Outside this second zone threat displays are unreliable and thus expected not to be used and attended to. The model gives specific predictions about weaponry, species-specific fighting techniques and the value of these thresholds. Finally, I show that key predictions of the model, namely the relation between signal intensity, riskiness and proximity has strong support in the literature.