Trick or treat: the battle of the sexes

Abstract

Males and females differ in many ways due to theirdistinct roles in reproduction. A fundamental asym-metry, that in fact defines 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 conflict 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 conflictbetween the sexes, or sexually antagonistic selection,can affect either traits encoded by the same locus in bothsexes (intra-locus conflict), where different phenotypicvalues will be favoured, or male and female traitsgoverned by different loci (inter-locus conflict). Whileintralocus conflict may result in the evolution of sexlimitation and sexual dimorphism (de facto becoming aninterlocus effect), interlocus conflict 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 field wasparticularly stimulated by this insight that antagonisticselection on male and female traits can fuel chainreactions of reciprocal evolutionary change, i.e. lead toan intra-specific 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 conflict, 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-fifthof the book).As a young field of broad interest to biologists fromdifferent sub-disciplines, sexual conflict is not free ofconceptual heterogeneity. The possible meanings ofmetaphors such as conflict resolution or battle of sexesare fitted into the framework of intraspecific coevolution,where average male and female fitness cannot be inde-pendent of each other. The authors are honest about thefact that there are often identical expectations under asexual selectionandsexual conflict scenario;forexample,correlated male–female evolution. They note the difficul-ties this presents in determining the force at work in anygiven system. However, they also discuss the theoreticalcontributions that sexual conflict 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 conflict in any given system. For empiricists,the authors advocate a combined approach, encompas-sing economic studies to measure costs and benefits ofsexual interactions, phenotypic manipulation, experi-mental tests of optimality modeling, experimental evolu-tion, genetic experiments, and comparative studies. Theyalsodiscusshowtomeasurethestrengthofsexualconflictby the phenotypic selection gradient approach of Shuster& Wade (2003), and through inference of the cost ofsexually antagonistic adaptations to population fitness.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 conflict between the sexes can affect traitsexpressed both before and after mating. Pre-matingsexual conflict is illustrated by examples as diverse as