"Perhaps there is something else that is influencing the evolution of the female reproductive tract," she added. "Perhaps it has to be big enough to handle an egg properly or something like that. The point is that the females are driving the sexual evolution. The sperm cells just have to keep up."
Pitnick added that given that no other cell type evolves as rapidly as sperm, variations in compatibility between sperm and the female reproductive system could be an important engine of speciation.
"If populations become separated and those traits are evolving rapidly in different directions, they will separate into two different species very quickly."
As in many other species, female diving beetles may not have a lot of control over with whom they mate, and that may be a driving factor in the evolution of intricate reproductive systems.
Being insects that spend most of their lives underwater, the beetles have to breathe air regularly.
"They come to the surface and take a little bubble with them when they swim back down," Higginson said. "Male diving beetles have suction cups on their feet that they use to grab on to the females. They can even prevent the females from breathing when they're holding on to their backs, which may be an attempt to pressure them into mating."
"One reason for this situation in which sperm from different males compete inside the female may be that females can mate with whomever they encounter and choose later," she said.
|Contact: Daniel Stolte|
University of Arizona