For many members of the animal kingdom, sex isn’t quite as Hollywood movies would have you believe. Some males stab females with spikes or use “handcuff-like" devices to stop them escaping, others shoot females with a sharp “love dart” before mating to deliver a sperm-friendly substance. Some even engage in penis fencing, where hermaphrodites fight over who gets to stab the other first with their dagger-like genitals. These are all examples of sexual conflict, which arise from the evolutionary pressure to maximize reproduction.

It’s generally believed that defense mechanisms against such males are forms of resistance, which can actually lead to an exaggeration of these unpleasant traits. But interestingly, a new study has found that may not always be the case, at least for bed bugs. Male bed bugs fertilize the female by stabbing a hole in her abdomen, and scientists assumed that females would evolve self-defense strategies to prevent harm from this traumatic experience. However, according to new research, females actually opt for tolerance strategies, rather than resistance, which makes it easier for the male to pierce her. These intriguing new findings have been published in the Journal of the Royal Society Interface.

Sexual conflict is where the genetic and evolutionary interests of individuals of the two sexes diverge. This means that, under pressure to maximize fitness, some sexually reproducing organisms may evolve adaptations that function to enhance reproduction in one sex, but actually reduce fitness in the other. Traumatic insemination is a perfect example of this, where males use sharp needle-like genitalia to pierce and subsequently ejaculate inside females. This is costly to the females because it results in direct physical damage and increased risk of infection.

Models of sexual conflict predict that females would evolve tougher abdomens or other defense strategies as a form of resistance, which would in turn lead to the males evolving traits to counteract these tactics, such as a sharper penis. This results in a so-called “evolutionary arms race,” which leads to rapid exaggeration of traits. However, scientists wondered whether it might be beneficial for the female to tolerate, rather than resist, these traits, which wouldn’t require the male to evolve counter-adaptations. Tolerance, in this respect, is defined as the balance or repair of damage without harming the offender.

To find out if this is true, scientists examined traumatic insemination of female bed bugs by scanning them with high-powered laser microscopes. They found that the females possessed a rubber-like elastic protein called resilin in the region of the abdomen that is most likely to be punctured by males, but not anywhere else.

Interestingly, they found this wasn’t a resistance mechanism because less penetration force was required to breach this stretchy region compared to other areas. Furthermore, unlike some other known resistance mechanisms, the composition of this region didn’t seem to affect male survival or health, and thus doesn’t encourage the selection of males with counter-adaptations. Importantly, this means that tolerance, but not resistance, frees both sexes from the relentless evolutionary arms race that sexual conflict can drive.

[Via Royal Society Interface and Science]