Sexual antagonistic co-evolution is the relationship between males and females where sexual morphology changes over time to counteract the opposite's sex traits to achieve the maximum reproductive success. This has been compared to an arms race between sexes. In many cases, male mating behavior is detrimental to the female's fitness. For example, when insects reproduce by means of traumatic insemination, it is very disadvantageous to the female's health. During mating, males will try to inseminate as many females as possible, however, the more times a female's abdomen is punctured, the less likely she is to survive. Females that possess traits to avoid multiple matings will be more likely to survive, resulting in a change in morphology. In males, genitalia is relatively simple and more likely to vary among generations compared to female genitalia. This results in a new trait that females have to avoid in order to survive.
Additionally, sexual antagonistic co-evolution can be the cause of rapid evolution, as is thought to be the case in seminal proteins known as Acps in species of Drosophila melanogaster. While Acps facilitate the mutually beneficial outcome of increased progeny production, several Acps have detrimental effects on female fitness as they are toxic and shorten her lifespan. This leads to antagonistic co-evolution, as the female must evolve in order to defend herself. When female Drosophila melanogaster are experimentally prevented from co-evolving with males, males rapidly adapt to the static female phenotype. This male adaptation leads to a reduction in female survivorship, which is mediated by an increased rate of remating and increased toxicity of Acps in seminal fluid. Since non-reproductive proteins do not feel the same evolutionary pressure as Acps, they are not evolving nearly as quickly. Consistent with the arms race theory, DNA analyses reveal a two-fold increase in Acp divergence relative to non-reproductive proteins.
Female co-evolution
For many females, reproduction can be very dangerous and disadvantageous as in the case of bed bugs mentioned previously. Therefore, females who possess traits where they can lessen the impacts of male behavior are the ones who will survive and go on to reproduce. There are many ways a female can "defend" herself to the onslaught of potential mates.
Spermatheca/pseudospermatheca
Females have a very complex and an extremely variable reproductive system, commonly known as a spermatheca. Some species do not have a spermatheca in the traditional sense, but do possess pseudospermatheca. Both forms play an essential role in sperm storage and fertilization. In the family Tingidae, pseudospermatheca are located at the base of the oviduct and are hypothesized to have functioned as spermatheca at one point in time. They now serve as storage units for sperm, where a female can introduce the stored sperm to her eggs when she finds it optimal. It is this factor that has put females in the driver seat of evolution. These organs give females the ability to pick and choose which sperm they will use to fertilize their eggs. Males now have another factor they need to overcome. In the case of D. melanogaster, females will mate multiple times and then expel the excess sperm that she does not need. However, neither the first or second mate know if it his sperm that was dispelled, because at any postcopulatory moment a female can store the sperm of more than one male.
Enzymes secreted by females
Enzymes secreted by female reproductive tracts may also play a role in sexual antagonistic coevolution with males. In Drosophila species, a large group of enzymes known as serine proteases have been associated with female sperm storage organs (most notably, the spermatheca) through genetic sequencing and analysis. It is hypothesized that these proteases break down various proteins in male seminal fluid. This would result in females choosing for males that can overcome these digestive enzymes, whether through genetic variation or physiological ability to produce greater quality or quantity of sperm.