Sperm competition influences how sperm is shaped in mammals. Scientists used a big dataset and looked at 226 mammal species to understand this connection between sperm competition and sperm design. They found that when there's more sperm competition, all parts of the sperm get bigger and sperm heads become longer. This makes the sperm faster when swimming, which helps them compete better. The findings suggest that sperm competition has driven changes in how mammal sperm is built, leading to longer and faster sperm to improve their chances in competitive mating situations.