Remember that '60s refrain, "Make love, not war"? For sperm, some scientists say, love and war are one and the same. According to the so-called kamikaze sperm hypothesis, sperm from different men hanging out in the same vagina may sabotage each other in the run for the egg. New findings, however, cast doubt on this idea.
In many species, from fruit flies to humans, females sometimes mate with multiple males in such quick succession that sperm of various appellations find themselves vying for the same prize. That phenomenon has prompted a sexual arms race: Mammals such as chimps have evolved larger testicles to churn out more sperm, for instance, while some insects use part of their reproductive apparatus to scrape out enemy sperm before depositing their own. Fruit flies poison the competition with a toxic protein in their seminal fluid.
Noticing that sperm in a mixed sample tends to clump together--making it less mobile--and to have a high mortality rate, reproductive biologist Robin Baker, formerly of the University of Manchester, proposed about a decade ago that some mammals, including humans, manufacture "killer" sperm whose only function is to attack foreign spermatozoa, destroying themselves in the process.
To test this idea, reproductive biologist Harry Moore and evolutionary ecologist Tim Birkhead of the University of Sheffield in the U.K. mixed sperm samples from 15 men in various combinations and checked for how the cells moved, clumped together, or developed abnormal shapes. "These are very simple experiments, but we tried to mimic what goes on in the reproductive tract," Moore says. The team found no excess casualties from any particular donor or other evidence of warring sperm, they report in the 7 December Proceedings of the Royal Society. "The kamikaze sperm hypothesis is probably not a mechanism in human sperm competition," says Birkhead.
The findings are "the nail in the coffin for the kamikaze hypothesis," says Michael Bedford, a reproductive biologist at Cornell University's Weill Medical Center in New York City. He says he had never given the idea much credence. But Birkhead maintains that kamikaze sperm might exist in species where promiscuity is the norm and competition is intense. The toxic sperm of fruit flies, he says, "tells us that almost anything is possible when it comes to reproduction."