Sperm hyperactivation drives a circling-and-wandering migration strategy
During migration through the female reproductive tract, sperm undergo physiological changes known as capacitation, including a motility transition termed hyperactivation. Hyperactivation is essential for various aspects of fertilization, particularly effective migration within the tract. However, how hyperactivation facilitates this migration remains elusive. Here, we profiled bull sperm hyperactivation in Newtonian and complex fluids near microfluidic surfaces, mimicking generic swimming conditions in the tract. We identified three swim gaits: wandering (persistent random walks), circling, and an intriguing circling-and- wandering mode marked by stochastic transitions between the two. All gaits exhibit diffusive behavior over long time scales, with wandering showing a tenfold higher diffusivity than circling, and the diffusivity of circling-and-wandering falling in between. We found that while wandering sperm scatter from convex and concave surfaces, circling sperm become trapped around pillars, highlighting the distinctive nature of each phase. Additionally, stochastic simulations of swimming in porous media showed that as the geometrical complexity of the environment increases, circling-and-wandering outperforms either strategy alone in spreading through the media. Our findings suggest that while wandering promotes exploration and circling supports local exploitation, circling-and-wandering combines the strengths of both strategies by balancing exploration and exploitation to adapt motility, enhance migration, and potentially improve target search.