The fitness advantage of a high-performance weapon JERRY F. HUSAK 1 *, A. KRISTOPHER LAPPIN 2 and RONALD A. VAN DEN BUSSCHE 1 1 Department of Zoology, Oklahoma State University, Stillwater, OK 74078, USA 2 Biological Sciences Department, California State Polytechnic University, Pomona, CA 91768, USA Received 9 June 2008; accepted for publication 26 August 2008 Weapons used in combat between males are usually attributed to sexual selection, which operates via a fitness advantage for males with weapons of better ‘quality’. Because the performance capacity of morphological traits is typically considered the direct target of selection, Darwin’s intrasexual selection hypothesis can be modified to predict that variation in reproductive success should be explained by variation in performance traits relevant to combat. Despite such a straightforward prediction, tests of this hypothesis are conspicuously lacking. We show that territorial male collared lizards with greater bite-force capacity sire more offspring than weaker biting rivals but exhibit no survival advantage. We did not detect stabilizing or disruptive selection on bite-force capacity. Taken together, these results support the hypothesis that superior weapon performance provides a fitness advantage through increased success in male contests. Sexual selection on weapon performance therefore appears to be a force driving the evolution and maintenance of sexual dimorphism in head shape. © 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 96, 840–845. ADDITIONAL KEYWORDS: bite force – lizard – sexual dimorphism – sexual selection. INTRODUCTION Males of many animal species possess exaggerated structures that do not appear to serve a purpose in survival but that play a significant role during intrasexual conflicts over access to mates by inflict- ing injuries on rivals (Andersson, 1994; Berglund, Bisazza & Pilastro, 1996). Such structures constitute weapons. Subsequent to Darwin, the evolution of weapons has been hypothesized to be the result of sexual selection; a fitness advantage is enjoyed by males with higher ‘quality’ weapons due to an increase in the likelihood of success during conflicts (Darwin, 1871; Andersson, 1994). Classic studies of presumed sexually-selected traits have typically focused on the intensity of sexual selection on mea- sures of weapon anatomy (Andersson, 1994), leaving the term ‘quality’ ambiguous in an evolutionary context (Lailvaux & Irschick, 2006; Irschick et al., 2007). Recent work, however, has revealed that the performance capacity of complex structures (i.e. ‘whole-animal performance’; Arnold, 1983) is a direct target of selection, and the evolution of morphology underlying performance traits occurs secondarily (Arnold, 1983; Le Galliard, Clobert & Ferrière, 2004; Irschick et al., 2008). Thus, sexual selection on per- formance traits, and, indirectly, their underlying mor- phology, may ultimately contribute to the evolution and maintenance of sexual dimorphism. We propose that appropriately chosen measures of performance provide a quantitative metric of weapon ‘quality’ in a biologically relevant context (Lailvaux & Irschick, 2006). We note that how weapon perfor- mance is quantified will depend on the specific weapon for a given species. For example, the perfor- mance of an antler may be measured either as stiff- ness (Blob & LaBarbera, 2001), which could reflect susceptibility to breaking during male combat, or as its ability to pierce tough skin. Key to an appropriate performance measure of quality is knowledge of how the weapon is used. Quantification of the ‘quality’ of a weapon should be based on a metric(s) of its capacity to function as a weapon, namely its potential to injure and incur costs on rivals. We suggest that measures of *Corresponding author. Current address: Department of Biological Sciences, Virginia Tech, Blacksburg, VA 24061, USA. E-mail: husak@vt.edu Biological Journal of the Linnean Society, 2009, 96, 840–845. With 2 figures © 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 96, 840–845 840