Mating and conflict

The notion that males and females of the same species may have different reproductive interests lies at the heart sexual selection theory (Bateman, 1948), and has recently been formalized under the heading of sexual conflict. Sexual conflict theory (Parker, 1979) was born out of Trivers' (1972) realization that the sexes invest differentially in parental effort and gametes. Today it is a flourishing field of enquiry providing insight into the nature of evolution and reproductive interactions (Arnvist and Rowe, 2005). The most obvious form of sexual conflict arises because males tend to have higher mating-frequency optima than females, sparking an evolutionary interaction between the sexes where males express traits that enable them to secure matings at rates that exceed the female optima. These, usually agonistic, traits are readily observable in a range of insect taxa (e.g. Thornhill and Alcock, 1983; Rowe et al., 1994). For example, the competitive struggles between male dungflies to mount and defend females often causes physical damage to the female (Hammer, 1941). Likewise several males of the digger-bee Centris pallida will converge on, and struggle to grasp newly emerged females (Thornhill and Alcock, 1983). In some species of insect the males are so tenacious they cut the female in two while exerting an ever-tighter grip (Holldobler, 1976), or bite through cuticular structures to maintain a better hold (Sivinski, 1981). Many male insects show specialized clasping organs which function to maintain a grip on either the female's external genitalia or delicate structures such as wings or eyes (e.g. MacKerras, 1970; Colless and McAlpine, 1970; Gerber et al, 1972; Thornhill, 1984). Some chironimid males have large, powerful, sclerotized grasping structures on their genitalia (Wirth and Sublette, 1970), and all odonate males grasp the female's head or prothorax prior to mating with special appendages on the end of the abdomen (Corbet, 1999). In two groups of odon-ates these male appendages tear the cuticle covering the compound eyes (Dunkle, 1984), resulting in reduced foraging efficiency and repair/immune costs (see Figure 15.2). As well as these manifestations of competition, males are also likely to damage females when selection drives males to mate with freshly eclosed females, that may be susceptible to physical damage. Male Drosophila

Figure 15.2 Damage to the vertex of the female's compound eyes caused by the male's abdominal claspers during pre-mating tandem formation in a dragonfly. The contact surface of the two disc-shaped projections can clearly be seen in the torn cuticle, as can the surface architecture of the ommatidia. The tear is approximately 1 mm across. The dotted rectangle in the inset photograph shows the region represented in the scanning electron micrograph.

melanogaster and Drosophila simulans mate with soft and vulnerable freshly eclosed adult females (Markow, 2000), and pupal mating is known in mosquitos (Slooten and Lambert, 1984) and helico-nids (Deinert et al., 1994). Each of these, and many other intersexual interactions during mating, are likely to cause external damage to females.

Sometimes mating is extremely prolonged in insects: up to 1 week in the Brimstone butterfly (Labitte, 1919). The male probably prolongs genital contact to prevent the take-over of mating by competitors (e.g. Parker, 1970). Although many insects maintain strong genital contact during copula, and often for prolonged periods, there are almost no studies that examine the effect these extended and mechanically powerful couplings have on the integrity of the cuticle in the female genital tract, especially in the propensity of males to wound the female.

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