Sperm migration in complex environments

To fertilize the oocyte, sperm must navigate the complex and fluctuating environment of the female reproductive tract (FRT), surviving intensive selective pressures, undergoing physiological transitions, and searching for the oocyte. This inefficient long-distance migration, where only one sperm succeeds from billions, along with a poor statistical understanding of sperm motility, complicates the development of a comprehensive physical model. Here, we perform motility profiling for chemically stimulated bull sperm over extended periods and conduct stochastic simulations to propose a three-phase model for sperm migration inspired by a previous model for bird navigation. Our results reveal three phases of migration: 1) a long-distance phase with ballistic motion receiving directional cues, 2) an exploration phase with persistent random walks for navigating complex geometries, and 3) an exploitation phase with localized chiral swimming for target pinpointing. We also identify a mixed-phase indicating a smooth transition or trade-off between exploration and exploitation. This model offers a potential explanation for sperm migration in the FRT and could inform the next generation of assisted reproductive technologies.