How male insects score

Abstract

Sperm Competition and its Evolutionary Consequences in the Insectsby Leigh Simmons. Princeton University Press, 2001. US$35.00/£24.95 pbk. (xiii + 434 pages) ISBN 0 691 05988 8A shot on goal does not ensure victory11Forgive the football metaphors but as I write The World Cup is still on.. Likewise, insemination does not guarantee paternity. When females store sperm from several males, each male's fitness depends on his ability to tilt the ensuing fertilization contest (sperm competition) in his favour. How males achieve this goal was the main focus of study for 30 years. Recently, however, there have been more questions about how females influence sperm competition (cryptic female choice). To answer them, we need to know why females mate multiply. What are the fitness benefits and consequent selection on female morphology, physiology and behaviour? In spite of the newfound popularity of cryptic female choice, driven largely by Bill Eberhard's book [1xEberhard, W.G. See all References][1] and more recent work on sexual conflict, the available data are still limited. It is testimony to Simmons’ hard-nosed empiricism that he pays more attention to questions from the traditional ‘male perspective’.The best chapter in the 1998 collection Sperm Competition and Sexual Selection [2xSperm competition in insects: mechanisms and the potential for selection. Simmons, L.W. and Siva-Jothy, M.T. : 341–434CrossrefSee all References][2] was one on insects by Simmons and Siva-Jothy. The current book does a fine job of elaborating on this earlier work and covers the burgeoning literature up to mid-2000. A table reporting P2 values (the second male's share of paternity from a doubly mated female) exemplifies the size of the undertaking, as the coverage has increased from 109 to 133 species. Sperm Competition and its Evolutionary Consequences in the Insects is remarkably comprehensive, sometimes overly so. Occasionally, a judicious focus on one or two key studies (with supplementary tables), would have better served the main point being made and allowed the writing to flow more smoothly. That said, 12 tables collate the available data on relationships such as correlates of ejaculate size, copulation duration and paternity levels. These are handy references and might reduce the likelihood of reading the overworked phrase ‘this is the first study’ in future papers.As any language-mangling sports commentator would observe, this is a book of two halves but one half is far bigger than the other22Forgive the football metaphors but as I write The World Cup is still on.. The first ‘half’ is six chapters devoted to males, covering the diverse ‘defensive’ tactics that they use to reduce the risk of sperm competition, such as removing or repositioning rivals’ sperm and post-copulatory mate-guarding. Describing these mechanisms leads naturally into a quantitative analysis of optimal male copulation duration. For P2, the fit between predicted and observed values from mechanistic models of time-dependent sperm displacement is impressive. By contrast, the wide scatter of data points around even the best-fitting models suggest that we have failed to identify major factors responsible for within-species variation in copulation duration. Of course, males often fail to prevent sperm competition. Two chapters present the current evidence that males ‘offensively’ adjust their ejaculates (number and/or type of sperm) in response to the intensity and risk of sperm competition. I found these chapters especially interesting. They refute the textbook wisdom that sperm is cheap. Prudent sperm allocation means that males are sperm limited and therefore choosy.Concerns about confounding effects of females on male ‘sperm games’ are sprinkled through these earlier chapters, but it is only in the final ‘half’ (actually a single chapter) that cryptic female choice is tackled directly. Treatment is generally balanced, although there is too much emphasis on sperm selection. Female biases in the uptake, transfer and storage of sperm related to male phenotype are more plausible, and certainly more detectable mechanisms of cryptic choice than is fine-scale discrimination between stored sperm with different genotypes. Whilst Simmons reviews evidence for the former mechanisms, the focus on sperm selection (half the chapter) inadvertently downplays the evolutionary importance of cryptic female choice. This is partly because most evidence for sperm selection involves selection for genetic compatibility. It therefore has no directional effect. Of course, this might simply reflect that showing sperm selection for genetic compatibility is the easiest way to demonstrate sperm selection.Simmons painstakingly chronicles the enormous success of models that optimize male fitness to predict paternity and copulation duration. Some might argue that incorporating cryptic female choice will vastly improve the predictive power of these model. Perhaps. I sense, however, that such research would be prompted by defiance of orthodoxy rather than a desire to maximize progress. The real trick is to ask fresh questions where selection on females cannot be ignored [3xAntagonistic coevolution between the sexes in a group of insects. Arnqvist, G. and Rowe, L. Nature. 2002; 415: 787–789Crossref | PubMedSee all References][3].