Some insects, such as ants, lead famously social lives, with massive colonies of individuals, cooperating for a common good. These insects also tend to have unusually large brains. For over 150 years, this link has been tacitly taken as support for the idea that social animals need extra smarts to keep track of all their many relationships. But Sarah Farris from West Virginia University and Susanne Schulmeister from the American Museum of Natural History aren’t convinced.
After comparing a wide range of species, they think that the large brains of these insect collectives have little to do with their cooperative societies. Instead, their enlarged brains may have been driven by a far grislier habit: body-snatching.
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.