Bats with delayed fertilization could be exposed by strong sperm competition during wintering, due to time lag between insemination and fertilization, and limitation of the number of fertilizable male. And this selection pressure would make hibernating bats active frequently to increase mating chance although arousing from hibernation is energetically too expensive. Rhinolophus cornutus has delayed fertilization. In the most heavy snowfall region of Japan, flight activity in this species during wintering has often been observed. We hypothesized that flight activity after arousal plays an important role in increasing mating chances because mating period in active season is limited to a few months in heavy snowfall region and bats may be exposed by strong sperm competition. We investigated bats’ behaviour during periodic arousals throughout hibernation period to reveal significance of flight activity in heavy snowfall region. 

     Field research was conducted from June 2011 to June 2015 in Osawa cave, Niigata. During wintering, the number of bats emerging from the cave was counted. Subsequently, both emerging bats and returned bats were captured by mist net. On the average, 30-120 bats emerged outside the roost despite of snowfall. DNA barcoding analysis using the feces collected from the returned bats showed that three genera of Diptera were used as diet during wintering. 89% of the emerging males showed distension of cauda epididymis. Interestingly, males kept spermatozoa in the cauda epididymis throughout hibernation period. Behavior tracking of bats after arousal revealed that males with distended cauda epididymis and females with opened vagina swarmed near the cave entrance. Furthermore, a microscopic examination on vaginal smear of females from this swarming cluster showed that spermatozoa existed in vagina. These results suggest that winter copulation may provide chances for insemination in addition to autumn copulation, and winter foraging may play an important role in retaining spermatozoa in male.