Infertile sperm die protecting their brothers

They are created in their thousands, for one purpose: to die for their brothers. They are the enigmatic "parasperm", infertile sperm that can dominate the ejaculate of some flies. 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. "They forgo reproduction to help their brothers."