In the journey from the male to female reproductive tract, mammalian sperm experience a natural osmotic decrease (e.g., in mouse, from ~41 5 mOsm in the cauda epididymis to ~310 mOsm in the uterine cavity).At one hand, the hypotonic stress upon ejaculation is beneficial for mouse sperm motility "start-up" (evolutionary trait from fish sperm); However, the hypotonic stress could also cause potential harms to sperm function by inducing un-wanted cell swelling.