The war of the sexes occurs even among insects, and it can be quite violent and unequal.
During mating in bugs called seed beetles, the females violently kick the males because the males hurt them with their spiky penises. And although researchers used to think this kicking was in the interests of the females, a new study shows that the opposite seems to be the case.
Male beetles appear to be controlling the kicking for their own evolutionary benefit, the researchers said. [Animal Sex: 7 Tales of Naughty Acts in the Wild]
"Everyone's assumption (including our own) was that the females kicked males to end the mating sooner," study author Carly Wilson, a doctoral researcher at The Centre for Evolutionary Biology at the University of Western Australia, told Live Science in an email.
"But our research showed that an earlier onset of kicking and doubling in kicking duration does not change copulation duration," Wilson said. In other words, kicking the males doesn't help the females to get the mating — which can injure the females — over with faster.
In the study, the researchers anesthetized half of the female beetles and, to prevent them from kicking, removed part of their hind legs. Then they mated all of the females with the males.
But apparently all that kicking wears out the females. The researchers also found that among the females that could kick, those that kicked for longer periods of time died sooner after they laid their eggs than those that kicked for a shorter time. But the females that kicked for longer did not lay any more eggs than those that kicked for shorter periods, Wilson said.
"This is good for males, because it is in his best interest to be kicked longer if his partner dies sooner, and therefore reduces the likelihood that she will mate with another male," Wilson said. In turn, such behavior could lower the percentage of the brood that is his, she said.
Although the researchers said they are not exactly sure how the kicking behavior evolved, Wilson said it could be that females evolved the kicking behavior to protect themselves, but that "males have somehow managed to gain control of the behavior," to the "detriment of the females."
Such an evolutionary battle of the sexes is called "sexual conflict," and occurs when male and female members of a species have different evolutionary interests, Wilson said. As part of this conflict, one sex may evolve a trait or behavior that increases their fitness, but comes at a cost to the other sex, she said.
In response, the other sex develops its own trait, which may in turn be harmful to the opposite sex. In the case of the seed beetle, males developed the spiky penises to increase their own fitness, and the females responded with the kicking behavior.
"And so the cycle continues," Wilson said.