WHY do bats hunt at night? Most experts thought they were simply avoiding
predators, but John Speakman of the University of Aberdeen says his observations
of bats in the 24-hour daylight of the Arctic summer cast doubt on that theory.
Most probably, he says, bats fly at night to avoid competition with
insectivorous birds.
Insect-eating bats live on a knife edge. Flying consumes so much energy that
each female is only able to produce a single offspring a year. An insectivore on
the ground, such as a shrew, however, might produce five young every six weeks.
If bats flew during the day, when most insects are active, they could eat enough
to overcome this problem鈥攂ut they would then be vulnerable to raptors such
as kestrels and sparrowhawks.
In the Arctic summer, raptors are active round the clock. Speakman and his
colleagues expected that Norway鈥檚 northern bats, Eptesicus nilssonii,
would respond by switching their peak activity to the times at which insects
were most active. 鈥淏ut they didn鈥檛 shift their time of feeding at all,鈥� says
Speakman.
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It鈥檚 hard to say what causes this intransigence, but Speakman suspects that
competition is the main factor at work. Sand martins were least active between
11 pm and 3 am, during the bats鈥� period of peak activity. Although at present
there are too few sand martins to offer much competition, following a population
crash in the 1980s, he suspects that the bats鈥� continued nocturnal habit
reflects 鈥渢he ghost of competition past鈥�.