If you want to see some sex, violence and blackmail, don’t bother with soap operas – try looking at the surface of your local lake or stream. There, you’ll find small insects called water striders (or pond skaters), skimming across the water on outstretched legs. These legs can pick up the vibrations of prey, predators and mates, but they can also produce vibrations by tapping the water surface. And males use this ability to blackmail their way into sex. It’s a drama of sexual tension played out across the surface tension.
Water strider sex begins unceremoniously: the male mounts the female without any courtship rituals or foreplay. She may resist but if she does, he starts to actively strum the water surface with his legs. Each vibration risks attracting the attention of a hungry predator, like a fish or backswimmer (above). And because the female is underneath, she will bear the brunt of any assault. By creating dangerous vibes, the male intimidates the female into submitting to his advances. Faint heart, it is said, never did win fair lady.
One night of passion and you’re filled with a lifetime full of sperm with no need to ever mate again. As sex lives go, it doesn’t sound very appealing, but it’s what many ants, bees, wasps and termites experience. The queens of these social insects mate in a single “nuptial flight” that lasts for a few hours or days. They store the sperm from their suitors and use it to slowly fertilise their eggs over the rest of their lives. Males have this one and only shot at joining the Mile High Club and they compete fiercely for their chance to inseminate the queen. But even for the victors, the war isn’t over. Inside the queen’s body, their sperm continue the battle.
If the queen mates with several males during her maiden flight, the sperm of each individual find themselves swimming among competitors, and that can’t be tolerated. Susanne den Boer from the University of Copenhagen has found that these insects have evolved seminal fluids that can incapacitate the sperm of rivals while leaving their own guys unharmed. And in some species, like leafcutter ants, the queen steps into the fray herself, secreting chemicals that pacify the warring sperm and ease their competition.
The amazing thing about this chemical warfare is that it has evolved independently several times. Social insects evolved from ancestors that observed strictly monogamous relationships. Even now, the queens from many species mate with just one male during their entire lives. With just one set of sperm in their bodies, they have no problem with sperm conflict. The trouble starts when species start mating with several males during their nuptial flights, as honeybees, social wasps, leafcutter ants, army ants, and others do today.