A functional perspective on sexual selection: insights and future prospects SIMON P. LAILVAUX & DUNCAN J. IRSCHICK Department of Ecology and Evolutionary Biology, Tulane University (Received 3 January 2005; initial acceptance 18 March 2005; final acceptance 13 February 2006; published online 30 June 2006; MS. number: ARV-31R) A large number of sexual selection studies have focused on examining the morphological and behavioural factors involved in male combat and female choice, such as whether large males achieve higher reproduc- tive success compared with smaller males. However, until recently, the mechanistic reasons why such cues are linked to male dominance or female choice have been elusive. An emerging body of work shows that physiological and whole-organism performance capacities are important in individual reproductive suc- cess. Males with high performance or other physiological capacities (e.g. endurance, biting) often enjoy an advantage over males with poorer performance capacities during maleemale contests. In contrast, few studies have examined links between performance and female choice. Here, we highlight recent key literature integrating sexual selection, performance and physiology. We also point to areas where a more rigorous investigation of underlying physiological processes may yield insights into sexual selection. In particular, we note that current progress in several important areas may be hampered by an inadequate physiological understanding of condition. We suggest a conceptual approach that may shed light on the physiological factors underlying condition, and we point out several other potentially important avenues for future research. Ó 2006 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. Sexual selection theory lies at the heart of evolutionary biology and has been the subject of intense debates since Darwin (1871). Indeed, recent syntheses have stressed that sexual selection is not only important for understanding the often-striking differences between males and females, but may also be a major diversifying factor in adaptive ra- diations (Schluter 2000; Coyne & Orr 2004). The broad field of sexual selection encompasses several different con- cepts, but two processes are believed to be especially im- portant: maleemale competition and female choice. In turn, these processes are thought to be a driving force in the evolution of conspicuous sexual features (e.g. horns, bright colours) often considered to be hallmarks of sexual selection. Secondary sexual characters may act as arma- ments for resolving male conflicts, as ornaments in a female choice context, or both (Berglund et al. 1996). Although the relative importance of these two processes is not well understood, both probably play a significant role within many animal populations (Andersson 1994). To date, most studies of sexual selection have focused either on documenting the presence or intensity of maleemale competition and female choice within animal populations, or relating the evolution of reproductive strategies to these phenomena. In contrast, an emerging body of work has examined sexual selection from a func- tional perspective, largely because many aspects of sexual selection theory are rooted in functional concepts. For example, male fighting ability is likely to be affected by an individual’s physiological state and/or performance capac- ities. Furthermore, the ‘good genes’ model of sexual selection, or ‘viability indicator hypothesis’ (Andersson 1994), assumes a link between genetic quality and viabil- ity, frequently measured as survival (Møller & Alatalo 1999). Given the assumed general relationship between, for example, locomotor performance and survival (Jayne & Bennett 1990; Watkins 1996; O’Steen et al. 2002; Miles 2004), one might predict that males expressing good genes would also be good performers (Maynard Smith & Harper 2003). However, owing to life-history trade-offs, individuals who invest heavily in mate acquisition may Correspondence and present address: S. P. Lailvaux, Functional Mor- phology Laboratory, Department of Biology, University of Antwerp, Universiteitsplein 1, Wilrijk B-2610, Belgium (email: slailvaux@ gmail.com). D. J. Irschick is at the Department of Ecology and Evolu- tionary Biology, 310 Dinwiddie Hall, Tulane University, New Orleans, LA 70118, U.S.A. 263 0003e3472/06/$30.00/0 Ó 2006 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. ANIMAL BEHAVIOUR, 2006, 72, 263e273 doi:10.1016/j.anbehav.2006.02.003