Sperm in Peromyscus mice have different shapes due to competition and swimming efficiency. The study looked at how the hook shape at the front of sperm affects competition and movement. They found that wider hooks are linked to less grouping of sperm and slower swimming, showing that big hooks might make it harder for sperm to move well. Some mice with abnormal hooks had trouble moving their sperm effectively. This suggests that the hook shape might play a role in how sperm interact with female cells during fertilization, rather than helping in competition between males.