Vulnerability and reliable signaling in conflicts between hermit crabs

When interacting organisms have opposing genetic interests, the integrity of communication systems may be undermined. For signaling in such conflict circumstances to remain evolutionarily stable, cheaters must be handicapped. Agonistic threat signals, however, are not always constrained or costly to produce, and yet these signals occur in the severest of conflicts where strong incentives exist for dishonesty. A leading hypothesis for how reliability is stabilized under these conditions is that signaling entails a risk, making signalers vulnerable to injury. Here I experimentally alter vulnerability to show how risk can modify organisms’ willingness to escalate disputes, affecting the use of threat signals. The vulnerability to injury of hermit crabs (Pagurus bernhardus) was manipulated by varying the exposure of their soft uncalcified abdomens. When faced with potentially damaging conspecific attacks, more vulnerable crabs were conflict averse, showing reluctance to claim ownership over contestable food, frequently retreating from threats, and refraining from threatening others. The risk an organism can bear in escalated conflict can thus mediate its agonistic behavior and usage of threats. Postural nuances can consequently provide reliable information about aggres-sive intentions despiteminimal production costs andopposing interests between communicationparties.Keywords: conflict, honest signaling, reliable communication, threat display, vulnerability handicap, evolutionary stability. [Behav Ecol 18:736–741 (2007)] Aside from special cases, as when organisms share genes orengage in reciprocal altruism, natural selection favors self-ish phenotypes that enable individuals to out compete others for fitness-enhancing resources (Dawkins 1976). Organismal