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BiologyBats are the only true flying mammals. In Britain, they are insectivorous (eat insects), and contrary to popular misconception they are not blind; many can actually see very well (6). All British bats use echolocation to orient themselves at night; they emit bursts of sound that are of such high frequencies they are beyond the human range of hearing and are therefore called 'ultrasound' (7). They then listen to and interpret the echoes bounced back from objects, including prey, around them, allowing them to build up a 'sound-picture' of their surroundings (7). Noctule bats produce echolocation calls of frequencies between 25 and 45 kHz (5). They emerge relatively early, often when it is still light (2). They hunt for moths, beetles and other large flying insects with fast, high flight, making rapid turns and diving frequently (2). The noctule migrates to the south or southwest during autumn, in order to escape the worst of the harsh winter weather, although not all individuals migrate (6). Mating takes place between August and October (2); during this time a single male defends a mating roost of 4 to 5 females against other males (2). Fertilisation is delayed until the following spring (6), as females store sperm inside their uterus (womb) during the winter hibernation (8), which occurs between October and early April (2). During early summer males and females live together in summer roosts, but females begin to gather into maternity roosts of 20-50 (rarely up to a few hundred) females after late May (2). During this time males live in small groups (2). In England, a single young is usually produced in June or early July; the young can fly at 4 weeks of age and reaches independence at around 7 weeks (2). Noctule bats are known to live to a maximum of 12 years of age (2).