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SEXUAL TENSION: DOES CONFLICT LEAD TO COSTLY MATE CHOICE?1

2006; Oxford University Press; Volume: 60; Issue: 2 Linguagem: Inglês

10.1554/br06-5.1

1558-5646

Michael D. Jennions,

Evolutionary Psychology and Human Behavior

Sexual Conflict is an enthusiastic introduction to sexually antagonistic coevolution. This is not surprising given that the authors are key promoters of its importance; however, their treatment is also remarkably well balanced. They begin by noting that although sexual conflict can be defined in many ways, it always boils down to the same fact. Evolutionary theory predicts that there are potential conflicts of interest whenever genetically distinct entities interact. This is demonstrably true even for such seemingly cooperative ventures as a mother feeding her child. Sexual reproduction offers no exception because it requires two participants, and potential mating partners are normally genetically unrelated. Consequently, genes that increase the reproductive success of males can be selected for even if they code for traits that are detrimental to genes expressed in females (and vice versa). The only exception to this interlocus sexual conflict is when there is strict monogamy and the genetic interests of parents exactly coincide. Even here, however, the potential for sexual conflict prior to pairing still exists if there is competition for mates. Love is truly a battlefield. Sexual Conflict begins by tracing the history of views on sex to identify when it went from being seen as a cooperative act to a conflict-ridden ordeal. The authors argue that sexual selection theory is still widely read as implying that mate choice reinforces natural selection and improves population fitness. This contrasts with their opposing view that sexual conflict can lead to mate choice for fecundity-reducing partners. Unfortunately, this distinction is somewhat forced. The first image that sexual selection invokes is usually of profligate excess-cue the peacock's train-and the evolution of beautiful but arbitrary traits. Well before sexual conflict became a buzzword, Helena Cronin (1992) documented that since the inception of sexual selection theory, researchers have been classified according to whether they think that mate choice opposes or reinforces natural selection. What is true, however-and the authors argue this extremely convincingly-is that the extent of direct selection on female mating preferences due to sexual conflict has been ignored. This creates a central motif, initially promoted by Holland and Rice (1998), that is replayed in numerous scenarios throughout the book: mate choice as a by-product of sexually antagonistic coevolution. When males impose costs on females that decrease their reproductive output (e.g., by inducing them to mate inopportunely), females that evolve resistance to males' advances do better. Ultimately, only males that evolve techniques to overcome female resistance will mate. Females will choose (fail to resist) persistent males and, because males need not share the same long-term interests as their mates, they may (or may not) have a more detrimental effect on female fecundity than the average male. Mate choice could be a by-product of a general cost-cutting strategy. A historic landmark that the authors identify is the chapter entitled Battle of the Sexes in Dawkin's The Selfish Gene. Thirty years have passed since then, so why has interest in sexual conflict suddenly surged now? One answer is provided by the authors' emphasis on the bizarre natural history of sexual encounters, from sperm digestion in flatworms to sexual asphyxiation in snakes and body-piercing genitalia in ricefish. They are too tactful to say so directly, but they make the case that sexual selection studies are stifled by an excessive bias toward searching for genetic benefits to mate choice. The associated models are intellectually satisfying, but their predominance may have caused us to neglect a more prosaic explanation for choosiness: males harass and harm females and continually cajole them to mate. Female resistance to reduce these costs may result in mate choice as a by-product. It is fair to say that, given the effort expended, the empirical evidence for genetic benefits of choosiness is rather paltry. Whether a focus on direct selection on mating preferences will be more fruitful is an open question, but the authors offer a well-reasoned argument that it is time to do so. This is an old plea packaged in the new framework of sexual conflict. To me, it offers a more convincing explanation for mating biases than male exploitation of female sensory systems constrained by natural selection for everyday tasks. Male coercion is an ever-present and unavoidable selective force. In contrast, if females prefer red fruit, it would seem to require only the slightest of neural adjustments to switch off this preference when assessing a male's plumage. Readers hoping for detailed mathematical models of sexual conflict will be disappointed. Given other papers by the authors (e.g., Gavrilets et al. 2001), they might expect a formal modeling framework, such as the recent one of Moore and Pizarri (2005), to match conventional Fisher-Zahavi models. In fact, not a single equation is presented. This is no loss, though, because the models are well described without the need for mathematics. However, the limitations of the model of Gavrilets et al. (2001), which is often cited in support of an endless arms race between the sexes, were understated. This model assumes that female resistance can only evolve through shifts in mating thresholds. When sensitivity (preference slopes) can also evolve, females may rapidly evolve insensitivity to, or even actively avoid, more persistent males. This makes arms races less likely than initially thought. This 1 Sexual Conflict. G. Arnqvist and L. Rowe. 2005. Princeton University Press, Princeton, New Jersey. xii + 330 pp. PB $39.50, ISBN 0-691-12218-0.