As well as the range of scenarios in which the female's external body structures can become damaged, recent studies have revealed that males can, and do, damage female genitalia with specialized structures on the aedeagus. There has been an ongoing debate about whether such genital wounding is adaptive (i.e. males gain a direct benefit by wounding the female, e.g. delaying re-mating), or a by-product of competition between males (e.g. Merritt, 1989; Civetta and Clark, 2000; Morrow et al., 2003). Regardless of the ultimate reason(s) for these 'harmful' male traits, the proximate consequence is the wounding of the female's genital tract (e.g. Crudgington and Siva-Jothy, 2000; Blanckenhorn et al, 2002; Kamimura, 2007). That this has negative fitness consequences for the female has been demonstrated in the bean weevil Callosobruchus maculates (Crudgington and Siva-Jothy, 2000) and in the bedbug Cimex lectular-ius (Morrow and Arnqvist, 2003; Reinhardt et al., 2003). Genital wounding has been regarded as a restricted phenomenon, but it is likely to be more widespread than previously thought. Recent work by Kamimura (2007) on the Drosophila bipectinatta complex has revealed that the males of 10 out of the 13 studied species wounded the female genitalia during mating. His work showed that this phenomenon also occurred in male D. melanogaster (Figure 15.3) and had been overlooked despite this insect's central role in identifying mechanisms of sexual conflict (e.g. Chapman et al, 1995). Genital wounding has probably been overlooked in nature because of the lack of a conceptual framework: it is doubtless no coincidence that the recent explosion of interest in sexual conflict has coincided with the identification of this reproductive phenomenon.
Male insects have a wide range of melanized spines and bristles on their aedeagi that are interpreted as having a range of functions including the maintainance of genital contact. They may also generate copulatory wounding and/or traumatic insemination. Why would males 'want' to wound the female's genital tract? One proximate reason is that if they transfer physiologically active compounds that alter the female's receptivity or fecundity (e.g. Chapman et al., 1995) these have to diffuse across the female's genital-tract cuticle. The potency of these compound(s) could be enhanced by ensuring they get directly into the haemocoel by breaching the cuticle. This may well have been the first step in the path that led to full-blown traumatic insemination. However, such genital-tract damage leaves the door open to costs to the female via infection from opportunistic pathogens and requires the female to repair the wounds. Activation of the female's immune-effector systems and wound repair will have to be completed at the expense of investment in other traits and so are likely to reduce female fitness. In this scenario the harmful male trait evolves because of the advantage it confers on males. Ronn et al.'s (2007) comparison of bean weevil species shows that females may respond to this form of selection by evolving thicker genital-tract cuticle, thereby reducing the likelihood of damage and subsequent physiological immune activation.
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