Insect semen kills rival sperm

If you've only got one shot, you better make it count. For some social insects, with only one chance to impregnate their queen, things can get nasty, but it's not the males that try to harm each other: it's their ejaculate.

Some female insects, such as honeybeesMovie Camera and leafcutter ants, have sex on only one day in their life. But what a day: they mate with multiple males and store enough sperm to fertilise eggs throughout their lives.

Now it seems that when honeybees and leafcutter ants inseminate the queen, their seminal fluid is harmful to rival sperm. Researchers looking at sexual selection often focus on the all-important sperm, says Boris Baer of the University of Western Australia. The seminal fluid tends to be discounted as merely a sugary liquid which provides energy, he says.

Baer and colleagues from the University of Copenhagen, Denmark, exposed the sperm of honeybees and leafcutter ants to their own seminal fluid, and the secretions of other males of the same species. The seminal fluid killed more than 50 per cent of the rival sperm within 15 minutes. "The males seemed to use the seminal fluid to harm the sperm," says Baer.