Badges of status and the cost of aggression

Summary

The “badges of status” hypothesis suggests that frequency-dependent selection can maintain honest signalling of aggressiveness. Analysis of a simple ESS model reveals that this is only possible if aggressive individuals incur a cost independent of any particular fights they become involved in. Without such a cost, an honest population is vulnerable to invasion by a “modest” mutant bearing a dishonestly small badge. Analysis of a second model reveals that where individuals differ in their ability to bear the contest-independent cost of aggressiveness, badge size should reflect this ability. Under such circumstances, plumage badges which serve as honest status signals may have functional consequences beyond their role in settling contests. Thus females, when choosing a mate, could make good use of the information that plumage badges provide. Analysis of a third model shows that the existence of contest-independent benefits to badge size, such as increased mating success, is compatible with the maintenance of honest status signalling.