Male "N noctula" do not migrate, but scatter along the females' migration routes to have higher chances of attracting the first females migrating back to the wintering regions. The reproductive cycle of male bats seems to correspond directly to season and food availability and spermatogenesis can be affected by body condition. In early autumn, males develop large testes and aim to attract females with singing calls. While in the harem roosts, males often do not enter torpor, but stay awake and mobile to defend the females from other males trying to copulate. Another reason for the avoidance of torpor during late summer and early autumn can be that spermatogenesis in bats can be delayed when resting metabolic rate is depressed, i.e. during torpor and hibernation.