Scientists have discovered an extraordinary evolutionary battle of the sexes where the anatomy of the male diving beetle changes in response to changes in the anatomy of the female.
These anatomical changes are thought to be driven by sexual conflict and are in fact the female responding against having sex with the males.
Diving beetles are predatory aquatic insects. They don't have courtship before mating. The males simply attack passing females and the female resists by rapid and erratic swimming to try to dislodge the male from her back.
The male holds on using suction cups that have evolved on the bottom of their front feet. So mating does occur, but not always and not without aggression.
Johannes Bergsten, insect expert at the Natural History Museum, and his team wanted to find out how these modifications, such as the male suction cups and peculiar structures on the females' back, evolved. They studied the DNA of 13 different diving beetle species and were able to build an evolutionary tree.
The results showed that the male suction cups change in response to changes in the surface of the female's back.
'Male diving beetles evolved suction cups on their feet to better grip a slippery female in the water,' explains Johannes.
'As a response, females evolved a hairy or wrinkled back, difficult to grip with the suction cups, so she can more easily resist males.'
'As a response to the response, males evolve better suction cups to grip the new back surface of the female. In this way males and females can co-evolve in a never ending arms race.'
Charles Darwin also studied some of these beetles he interpreted the hairy furrows on the females' back as an aid for the male to grip her.
But the suction cups work best on smooth surfaces - think of how difficult it would be to get a sink plunger to work on a hairy bathroom mat!
The female furrows and hairs are a counter-weapon against the males' suction cups.
When sexual conflict in mating systems occurs, it is usually because the optimal time, length or the number of matings is not the same for males and females.
Matings can be very costly, even life threatening. When attached during mating the couple is more vulnerable, for example, from predation by fish.
For a male it can be worth the risk, if he then fathers all offspring of the female. If that female has already mated, it might not be worth the risk for her to mate again. Therefore, encounters of males and females can mean conflict, and the male tries to force the female to mate and the female tries to resist.
Even though the insects are aquatic, they depend on atmospheric oxygen to breathe and this is what Bergsten's team think may be the key to understanding these adaptations.
During mating, the females can't breathe because they are pushed under water by the male who is on top. Indeed, the males often shake the females before they mate, which increases the female's need for oxygen. The female, therefore, is exhausted and unable to resist.
After mating, the males guard the female for up to six hours, occasionally tilting the female upward to allow her to breathe.
'Since diving beetles normally go up to the surface to breathe every 5 to 15 minutes or so, this is a very long time,' says Johannes.
'It is possible female diving beetles can actually drown during matings. We have occasionally found dead newly mated females when they have been kept in the lab. The possibility of drowning could be the main reason for female resistance and therefore the fuel of the battle of the sexes in diving beetles.'
This research is published in the journal PLoS ONE