Female/male adaptation to traumatic insemination

"Sexual conflict coevolution" vs natural selection

Mesospermalege: the hemocyte-containing bed bug organ into which sperm are introduced (Siva-Jothy 2006).

The sexually antagonistic coevolution that probably occurs in these groups has generally not resulted in rapid, sustained evolutionary divergence in male and female external sexual morphology (Siva-Jothy 2001). Unlike many species, females have no control over mating frequencies; the act of mating is entirely dependent on male action. Taking this into consideration, it is possible that the evolution of a piercing intromittent organ which bypasses reproductive system may have provided males with the opportunity to avoid female adaptations that control copulation frequency. By passing along toxins in the seminal fluid, males may attempt to restrict female copulation in the future, shortening their life expectancy dramatically.

Multiple mating may provide fitness gains for females that offset or exceed the costs such that they get good genes or allelic diversity and their offspring may produce more viable offspring (Siva-Jothy 2001). Thus females have likely developed the mesospermalege to handle the costs of traumatic insemination (particularly STDs). However, there is growing support for the hypothesis that females gain proteins and other nutrients from insemination as well, such that it may not be so costly after all (Siva-Jothy 2006). Morrow & Arnqvist found that while mechanical copulatory damage reduced
female life span by 50%, the overall effect of mating was only a 30% life span reduction.
The recovery of 20% of the longevity costs in the treatment with ejaculates suggests some nutritive gain is shunted into female survival. This species is often cited as one of the best examples of sexual selection driving genital evolution, although little genital divergence can emerge from traumatic insemination.

Biologists hesitate to bring female mate choice theory into the arena of coercive reproduction because often the female costs and benefits are not obvious. One theory that might help explain the evolution of this traumatic insemination - or perhaps the lack of evolution of a particular counterattack on the part of the females - is the 'sexy son' theory. The idea then is that females do not mind being hurt by males because they view this as 'sexy' and thus will produce 'sexy sons' more likely to be able to reenact the sexy behavior and thus enjoy reproductive success. This, however, is a stretch, as we are hard-pressed to say that females "don't mind" traumatic insemination. The 'good gene' theory has also been used in conjunction with the idea of traumatic insemination, possibly giving rise to female choice, but the costs still seem too great (Siva-Jothy 2001).

In an attempt to tease out the problem of female choice in these species where trauma occurs, some researchers make assumptions about female choice to determine male gain. If harm were adaptive, female resistance to male harm would always compromise male interests. In contrast, female resistance to male adaptations that cause harm as a pleiotropic side effect may or may not compromise male interests. If female resistance to given male adaptation reduces its efficacy in terms of its selected primary function, this will compromise male interests. However, if resistance is instead targeted only at alleviating the costly side effects of a male adaptation, this will be in the interest of both sexes. We thus expect selection in both sexes to diminish negative pleiotropic side effects. For this reason, it is predicted that male traits that cause physical harm to females should only occur when harm represents a more or less unavoidable side effect of their primary function, and the magnitude of harm inflicted to females should be limited.
There is a possibility that sexual selection has nothing to do with the female evolution of mesospermalege; that in fact it is natural selection at work. However, this possibility remains to be tested.