Promiscuous mice make slender sperm

Sometimes, sperm competition gets very real. In mouse species that are highly promiscuous, a male mouse’s sperm cells may often find themselves sharing the same female reproductive tract with other males’ swimmers. In such mice, a new study in the Proceedings of the Royal Society B reports, it pays to have more hydrodynamic, and therefore faster-swimming, sperm. A Spanish-American team led by Lena Lüke found that the testicles of mouse species in which there is more competition among males over the available females, produce relatively less of a protein called Protamine 2. This causes the DNA in the sperm head to be packed more tightly, leading to a more sleek, faster-moving, spermatozoon. In the picture, the sperm head on the left comes from a less promiscuous, high-protamine-2 mouse (and is therefore more bulky, less streamlined), while the reverse is true for the sperm head on the right. (Mice have a characteristic “hook” on the sperm head, unlike humans.)

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