Fighting without weaponry: a review of male-male contest competition in butterflies

The males of many butterfly species compete via pair-wise intrasexual conte sts, yet lack any obvious morphological traits conventionally associated wi th animal aggression. The evolution of contest behavior in this group is th erefore interesting because the means by which individuals are able to infl ict costs upon each other during combat are unclear. Here we review the rel evant empirical information within a game-theoretical context in order to a nalyze contest-related costs and mechanisms of dispute settlement. Territor ial butterfly contests carry obvious role asymmetries, and individuals in t he 'resident' role are more likely to win across all studied species. We us e this phenomenon as a framework for gaining further insight into the evolu tion of contest settlement in this group. Four principal hypotheses are eva luated: (1) that residency is used as an arbitrary convention, (2) that res idents stand to gain a greater payoff if successful, (3) that residency bes tows higher resource-holding potential (RHP) on males in that role, and (4) that residents are males of intrinsically higher RHP. We found most suppor t for the latter explanation (the 'superior-competitor' hypothesis), partly because the alternatives require a restrictive set of ecological condition s, but also due to positive support for the idea of intrinsic RHP. The stro ngest evidence is provided by species in which repeated interactions (betwe en the same individuals) always have the same outcome, regardless of change s in residency status. However, we also found a consistent effect on contes t duration due to apparent 'confusion over residency,' which suggests a con ventional element to contest behavior. Although butterfly contests must be costly in some way, the literature provides limited insight into how costs accrue, and hence the determinants of contest RHP remain obscure. We sugges t that careful experimentation, using 'natural' manipulations where possibl e, should prove most beneficial in identifying costs and exploring schedule s of contest settlement in this group. Moreover, future investigations shou ld be sensitive to the fact that butterfly territoriality and associated ma le-male interactions are seated within a broader ecological and life-histor ical context.