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.
To understand the sperm wars, den Boer exposed sperm from different species to their own seminal fluids, those of brothers, or those of unrelated males. In two species of bees and three species of ants, she found that a male's seminal secretions are a boon to his own sperm. Even at small concentrations, they managed to boost the survival of sperm that had been stored in saline.
In species where queens mate with a single male, like bumblebees and Trachymyrmex zeteki ants, the seminal fluids had the same beneficial effect on the sperm of unrelated individuals. But these chemicals weren't so benign in species where queens store sperm from several males, like honeybees and the ants Atta coloimbica and Acromyrmex echinatior. There, they significantly reduced the survival rates of competitor sperm, slashing them from 6-18% after just 30 minutes.
How seminal fluids know to attack other sperm is a mystery. The fact that a brother's sperm also suffers, even though it shares much of the same DNA, suggests that the method involves a blanket attack on anything that isn't recognised as "self". And as with many wars, both sides suffer. It turns out that the protective chemicals from one set of seminal fluids can't counteract the destructive chemicals from another. If the two are mixed, no set of sperm survives very well.
From the queen's point of view, these battles are positively counter-productive. The more sperm she has, the more eggs she can fertilise and the more young she can raise. It's in her interest to stop the sperm from killing each other. Den Boer found that the queens of the leafcutter Atta colombica do just that. The fluids from a queen's spermathecae (the organ where she keeps her sperm supplies) can quell the destructive effect of rival seminal fluids. If they're added to the mix, survival rates for all the stored sperm shoot back up to normal levels. If the sperm wars get too heated, the queen evolves to restores peace for the sake of her future kingdom.
Reference: Science http://dx.doi.org/10.1126/science.1184709
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Images: all photos by Susanne den Boer
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