Not all sperm are created equal. Some swim like Michael Phelps and others swim like your 80-year-old grandmother. The difference, researchers have found, lies in their ability to compress their DNA. A sperm expressing less of a gene called protamine 2 relative to a gene called protamine 1 is able to more tightly wind up long strands of its genetic material in its nucleus, making its head sleeker—and more hydrodynamic. These differences have important consequences in the real world. Reporting in the current issue of the Proceedings of the Royal Society B, the team found that more promiscuous species of mice produce less protamine 2 relative to protamine 1 in their sperm, and thus more hydrodynamic sperm. The reason is that males of these species mate with females in quick succession, so males with the fastest sperm are more likely to sire pups. As a result, promiscuous species of mice are more likely to produce sperm that are champion swimmers.