The risks and benefits of signalling aggressive motivation: a study of cave-dwelling little blue penguins

Summary Cave-dwelling little blue penguins, Eudyptula minor, use a large repertoire of agonistic displays. Both qualitative and quantitative measures suggested that some displays were more dangerous to perform than others (estimated by the risk of being injured by one's opponent). A lag sequential analysis of interactions indicated a positive relationship between the risks of performing a display and the display's effectiveness (i.e., in deterring opponents). The cost or risk asymmetries that exist between displays may allow opponents to assess how motivated the actor is to secure or defend resources. Thus, high cost displays are more effective because they reveal a willingness to take risks and also place the opponent in a higher risk situation. Communicating in this way can be an evolutionarily stable strategy (ESS) because signalling a strong motivation entails using potentially costly behavior (i.e., there are costs associated with bluffing). Also, displays that were accompanied by calls were more effective in deterring opponents than the same displays performed silently. Calls may highlight the risks being taken by actors; this may be particularly important in nocturnal species like little blue penguins. Aggressive displays could also be used to predict what the actor was and was not likely to do following the performance of the display. Generally, aggressive displays predicted that the actor would escalate to a higher risk behavior if the opponent did not retreat, but that it would use a lower risk behavior if the opponent backed down. However, risk-based displays should be seen as revealing a certain level of motivation at the time of the display and not as a way of describing how great a risk the actor is willing to take following the display. The predictive value of displays appears to be the result of an escalation process which ensures that opponents are deterred by the lowest risk behavior possible.