  We developed a mechanical model of spermatozoal swimming in bulk suspensions.
We traced the spatiotemporal elastohydrodynamic interactions and found that
spermatozoa engaged in self-organisation: flagellar undulatory motion generated
a stable polar order. Conventional microswimmer theories use time-averaged
stress fields to predict that the cellular structure is unstable. However, our
present results reveal a novel self-organisation mechanism attributable to
unsteady hydrodynamic interactions. We thus contribute to the theory of active
soft matter dynamics.
