Why bats are nocturnal

Published: Monday 15 November 1999

Many of us believe that bats hunt at night to avoid predators. But, scientist John Speakman of the University of Aberdeen says his observation of bats in the 24-hour daylight of the Arctic summer casts doubts on that theory. Most probably, he says, bats fly at night to avoid competition with insectivorous birds. Flying consumes so much energy that each female bat is only able to produce a single off-spring a year. An insectivore, on the other hand, might produce five young ones every six weeks. If bats flew during the day, when most insects are active, they could eat enough to overcome this problem -- but they would then be vulnerable to raptors such as kestrels and sparrowhawks. In the Arctic summer, raptors are active around the clock. Speakman expected that Norway's northern bats, Eptesicus nilssonii , would respond when insects are most active. "But they did not shift their time of feeding at all," says Speakman. It is hard to say what causes this but Speakman suspects that competition is the main factor. Although at present sand martinis are too small in number to offer much competition, following their population crash in the 1980s, he suspects that the bats' continued nocturnal habit reflects "the ghost of competition past" ( New Scientist , Vol 163, No 2205).

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