Trick or treat: the battle of the sexes

Males and females differ in many ways due to theirdistinct roles in reproduction. A fundamental asym-metry, that in fact denes the genders, occurs over initialinvestment in gametes, with males producing many, tinyand females few, large gametes. Another very generalasymmetry concerns the intensity of selection for mul-tiple mating between the sexes: while males can max-imize their reproductive output by obtaining as manypartners as possible, females need fewer partners toachieve their maximum reproductive output (Bateman,1948). That these distinct roles can generate a battle ofthe sexes or sexual conict and can affect the evolu-tionary dynamics of reproductive traits and behaviourshas been recognized since the 1970s, and has nowdeveloped into an exciting and major area of investiga-tion in evolutionary biology. Evolutionary conictbetween the sexes, or sexually antagonistic selection,can affect either traits encoded by the same locus in bothsexes (intra-locus conict), where different phenotypicvalues will be favoured, or male and female traitsgoverned by different loci (inter-locus conict). Whileintralocus conict may result in the evolution of sexlimitation and sexual dimorphism (de facto becoming aninterlocus effect), interlocus conict gives wide scope forcontinual evolution of traits in males that manipulatefemale reproduction, and of female resistance to mani-pulation. The recent rise of interest for this eld wasparticularly stimulated by this insight that antagonisticselection on male and female traits can fuel chainreactions of reciprocal evolutionary change, i.e. lead toan intra-specic Red Queen process analogous to coev-olutionary arms races between hosts and parasites (Rice& Holland, 1997).The timely monograph by Arnqvist & Rowe (2005)synthesizes research on sexual conict, embeds it in thehistorical and conceptual context of sexual selectionresearch, explores many natural history examples acrosstaxa, and questions both the conceptual stringency andthe empirical testability of hypotheses emanating fromthe idea of diverging evolutionary optima in males andfemales, thus paving the way for future directions. Oneof the strengths of this book is the comprehensive lookat studies completed by a wide variety of scientists(on 75 pages, the reference list comprises over one-fthof the book).As a young eld of broad interest to biologists fromdifferent sub-disciplines, sexual conict is not free ofconceptual heterogeneity. The possible meanings ofmetaphors such as conict resolution or battle of sexesare tted into the framework of intraspecic coevolution,where average male and female tness cannot be inde-pendent of each other. The authors are honest about thefact that there are often identical expectations under asexual selectionandsexual conict scenario;forexample,correlated male–female evolution. They note the difcul-ties this presents in determining the force at work in anygiven system. However, they also discuss the theoreticalcontributions that sexual conict theory has made,including the distinction between a male trait experien-cing sexually antagonistic selection vs. sexual selection.Under sexually antagonistic selection, the male trait isadvantageous to the male but simultaneously disadvan-tageous to the female. In contrast, a male trait undertraditional sexual selection is advantageous to both themaleandthefemale.Thus,inthelattercasethemaletraitcan spread by female preference for the trait. Despite thistheoreticaldistinction,therecurrentlyisnogoldenruletodemonstrate conict in any given system. For empiricists,the authors advocate a combined approach, encompas-sing economic studies to measure costs and benets ofsexual interactions, phenotypic manipulation, experi-mental tests of optimality modeling, experimental evolu-tion, genetic experiments, and comparative studies. Theyalsodiscusshowtomeasurethestrengthofsexualconictby the phenotypic selection gradient approach of Shuster& Wade (2003), and through inference of the cost ofsexually antagonistic adaptations to population tness.The honesty in discussing how far empirical studies candiscriminate among hypotheses, and in identifying whereempirical evidence or theoretical developments arelacking, is one of the strengths of this book. Such honestyboth promotes further thought and discussion amongreaders and avoids the pitfalls of overinterpretation.The conict between the sexes can affect traitsexpressed both before and after mating. Pre-matingsexual conict is illustrated by examples as diverse as