They are created in their thousands, for one purpose: to die for their brothers. They are the enigmatic 鈥減arasperm鈥�, infertile sperm that can dominate the ejaculate of some flies.
In the fruit fly, Drosophila pseudoobscura, about half the sperm are infertile. Why waste energy producing dud sperm? and Rhonda Snook of the University of Sheffield, UK, may have the answer.
They measured how long sperm survived inside the reproductive tract of the female, which contains sperm-killing chemicals. They found that the more parasperm there was in the ejaculate, the longer the fertile sperm survived. 鈥淭hey forgo reproduction to help their brothers,鈥� says Holman.
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There is an infertile sperm 鈥渃aste鈥� in other fly species, and in butterflies and moths, and Holman suggests they may perform the same sacrificial function. Male humans seem to have evolved an alternative strategy, however. 鈥淭he most conspicuous adaptation is seminal fluid, which has pH buffers and suppresses female immune activity,鈥� says Holman.
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