Abstract The males of many butterfly species compete via pair-wise intrasexual contests, yet lack any obvious morphological traits conventionally associated with ani- mal aggression. The evolution of contest behavior in this group is therefore interesting because the means by which individuals are able to inflict costs upon each oth- er during combat are unclear. Here we review the rele- vant empirical information within a game-theoretical context in order to analyze contest-related costs and mechanisms of dispute settlement. Territorial butterfly contests carry obvious role asymmetries, and individuals in the ‘resident’ role are more likely to win across all studied species. We use this phenomenon as a framework for gaining further insight into the evolution of contest settlement in this group. Four principal hypotheses are evaluated: (1) that residency is used as an arbitrary con- vention, (2) that residents stand to gain a greater payoff if successful, (3) that residency bestows higher resource- holding potential (RHP) on males in that role, and (4) that residents are males of intrinsically higher RHP. We found most support for the latter explanation (the ‘supe- rior-competitor’ hypothesis), partly because the alterna- tives require a restrictive set of ecological conditions, but also due to positive support for the idea of intrinsic RHP. The strongest evidence is provided by species in which repeated interactions (between the same individuals) al- ways have the same outcome, regardless of changes in residency status. However, we also found a consistent ef- fect on contest duration due to apparent ‘confusion over residency,’ which suggests a conventional element to contest behavior. Although butterfly contests must be costly in some way, the literature provides limited in- sight into how costs accrue, and hence the determinants of contest RHP remain obscure. We suggest that careful experimentation, using ‘natural’ manipulations where possible, should prove most beneficial in identifying costs and exploring schedules of contest settlement in this group. Moreover, future investigations should be sensitive to the fact that butterfly territoriality and asso- ciated male-male interactions are seated within a broader ecological and life-historical context. Keywords Sexual selection · Intrasexual competition · Aggression · Evolutionary game theory · War of attrition, Lepidoptera Introduction Fighting between males over mating opportunities is a widespread and conspicuous component of animal mat- ing systems that has received much empirical attention (reviews in Huntingford and Turner 1987; Andersson 1994). Our understanding of the evolution of contest be- havior has now been considerably improved by the de- velopment and refinement of theoretical models based upon the principles of game theory (Maynard Smith and Price 1973; Maynard Smith 1982). This theory has al- lowed closer examination of how contestant fighting ability, resource value, uncorrelated ‘roles,’ and the pos- sibility for pair-wise assessment of these parameters should interact to direct the evolution of aggressive be- havior. Recent formulations of game theory models have become increasingly realistic (e.g., Enquist and Leimar 1987, 1990; Mesterton-Gibbons et al. 1996; Payne 1998) and, consequently, are able to provide more robust and testable predictions regarding the form, duration, and outcome of real animal fights. A fundamental principle of contest theory, and some- thing also suggested intuitively, is that asymmetries in fighting ability (termed resource-holding potential, RHP; Parker 1974) will greatly influence the form and out- Communicated by M. Elgar D. J. Kemp ( ✉ ) School of Tropical Biology, James Cook University, PO Box 6811, Cairns, Queensland 4870, Australia e-mail: darrell.kemp@jcu.edu.au Tel.: +61-7-40421386, Fax: +61-7-40421284 C. Wiklund Department of Zoology, University of Stockholm, 10691 Stockholm, Sweden Behav Ecol Sociobiol (2001) 49:429–442 DOI 10.1007/s002650100318 REVIEW Darrell J. Kemp · Christer Wiklund Fighting without weaponry: a review of male-male contest competition in butterflies Received: 22 May 2000 / Revised: 5 January 2001 / Accepted: 7 January 2001 / Published online: 16 March 2001 © Springer-Verlag 2001