2002-07-13 16:34:21 --
After the first few minutes following ejaculation, the sperm began to hook up with each other, forming a pack of sorts, as they swam towards their goal: the elusive, alluring egg. Using a hook on their heads, they literally clung to each other: head to head, or head to tail. Looking at them, they were reminiscent of the carriages of a train, or a string of elephants in the circus.
Studying them, he found that sperm that traveled in groups were able to swim roughly 50 percent faster than those who went alone. In the viscous fluid that created the environment, that can mean the difference between getting to the egg first or allowing another man's sperm to get there before you do.
Wood mice are promiscuous creatures, and it's not unusual for more than one male's sperm to be making its way to the egg at the same time. Moore thinks that human sperm might behave in a very similar way, the front ones of the pack softening up the cervical mucous for those who follow after them. Sperm in the front of the pack have been observed to behave differently than their brothers behind them do.
Only individual sperm can enter the egg -- wood mouse trains start to decouple about 20-30 minutes after forming. Many sperm -- particularly those in the center of groups -- trigger a reaction in their head that is usually used to bore into the egg.
Sperm who blow their top before they get there become unable to fertilize the egg they came to find, sacrificing themselves on behalf of their brothers. The size of the trains can grow to thousands of sperm. At this point, scientists have several different theories about why the sperm acts the way it does.
Geoff Parker from the University of Liverpool explained them to Nature in an interview: "A male parent doesn't mind which of his sperm fertilizes the egg," he says. But every sperm wants to be the lucky one; although it would rather one of its brothers reached the egg than one from another male.
There could be conflict over which sperm sacrifice themselves, Parker speculates. This would depend on whether males mark sperm for doom before ejaculating, or whether the self-destruct sequence is initiated within the sperm itself. Research is ongoing to try and answer those questions.
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