Sexual conflict
Sexual conflict may lead to antagonistic co-evolution, in which one sex (usually males) evolves a favorable trait which is countered by a trait in the other sex. For example, male bean weevils (Callosobruchus maculatus) have spiny genitalia. These allow them to copulate for a longer time without getting dislodged, and so transfer more sperm. However, this damages the female and reduces her fitness. Females have evolved the counteradaptation of kicking at males during mating, which reduces the time spent in copulation.