Primates are mammals with menstruation cycle. The menstruation cycle is around 28-35 days in most primates. The cycle is synchronizing with moon period. Why does menstruation cycle synchronize moon cycle? Here, I propose that the phenomenon is because primates and bats need to increase the change of fertilization. Primates can stand with two legs. In majority of time, they maintain the straight position of their bodies. In this position, sperms will need swim against gravity toward Fallopian tube after intercourse. They will need to take 1-12 hours to meet the oocytes. Straight body position will increase the difficulty to let sperms to meet oocytes. Thus, gravity force can affect the chance of fertilization. Moon is the major source of tidal force. The moon cycle will affect the magnitude of gravity force. Thus, primates and bats evolve menstruation cycles to let the ovulation occur during the less-gravity period. So, menstruation cycle is synchronizing the moon cycle. That is the reason why primates and bats, not other mammals, have menstruation cycles.