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.
You’ve got to feel sorry for the female seed beetle. Whenever she mates with a male, she has to contend with his spiked, nightmarish penis (remember this picture?). And despite the damage that it inflicts, one liaison just isn’t enough; female seed beetles typically mate with many males before they lay their eggs. Surely, she must benefit in some way?
The most likely idea is that she somehow ensures that her eggs are fertilised by sperm from males with the “best” genes – those that either make for particularly fit and healthy young, or that are a compatible match for the female’s own genes. Perhaps these sperm outrace their weaker peers, or maybe the female has a way of selectively letting through the best quality sperm. It would be a reasonable explanation were it actually true. Sadly, reality isn’t that kind to the female seed beetle.
Trine Bilde from the University of Uppsala has found that after females mate with two different males, it’s actually the sperm from the lower-quality specimen that fertilises most of her eggs. Even though the paragon’s sperm would sire more successful offspring, it’s the loser who ends up fathering most of her progeny.
If you’ve ever complained about having bad sex, you really have no idea. Human women may have to complain about poor stamina or incompetent technique but the female seed beetle (or bean weevil; Callosobruchus maculatus) has to contend with her partner’s nightmarish penis – an organ covered in hard, sharp spikes. Just see if you can look at the picture on the right without wincing.
It’s no surprise then that females sustain heavy injuries during sex. But why have male beetles evolved such hellish genitals? What benefits do they gain by physically harming their partners?
It’s possible that the injuries directly benefit the males, either because they stop the females from mating again or spend more efforts in raising their fertilised eggs to avoid the strain of future liaisons.
The alternative is that the spikes could give the males an edge in “sperm competitions“, where they compete with rivals not through direct combat, but through fertilising as many eggs as possible. In this theory, the spines are important for winning these competitions, and the wounds they inflict are simply a nasty side-effect.
Cosima Hotzy and Goran Arnqvist from Uppsala University think that the latter theory is right. They have found that the penile spines are vital to a male’s success – those with the longest spikes fertilise the most eggs and father the most young. Size, it seems, really does matter.