BATS hunt flying insects by broadcasting sound waves, usually in the range 20 to 50 kilo-hertz, and listening for echoes. To combat this, some nocturnal moths, together with a few lacewings, have developed hearing organs that detect the frequencies used by most bats, to alert them to the need to take evasive action. However, the race between the hunters and their prey has not stopped there.
Jens Rydell of the University of Aberdeen and Raphael Arlettaz of the University of Lausanne in Switzerland found that the European free-tailed bat (Tadarida teniotis) searches for its prey using sound at a frequency of between 11 and 12 kHz, a frequency too low for most insects to hear. The disadvantage to the bats is that this frequency corresponds to a wavelength of about 3 centimetres, and since echolocation is insensitive to objects smaller than the wavelength of the sound being transmitted the bats are unlikely to detect small insects.
Rydell and Arlettaz decided to check the possibility that T. teniotis feeds mainly on relatively large insects that could not pick up its own lower-frequency pulses but could detect the echolocation pulses of most other bats (Proceedings of the Royal Society of London, Series B, vol 257, p 175). To do this, they examined bat droppings for insect remains. The researchers collected the droppings from under bat roosts in two locations: Sisteron in southeastern France and at an altitude of 850 metres in Kirghizia in central Asia.
In both sets of droppings Rydell and Arlettaz found that the majority of insect remains were of moths, or Lepidoptera – 68.3 per cent in the French collection and 86.8 per cent in the Kirghizian bats. Neuroptera such as lace-wings, which can also hear typical bats, accounted for 24 per cent of the European sample but were rare in the Asian sample, 12 per cent of which contained an unidentified insect. This is quite different from the diet of other bats, which feed mostly on flies. Moths make up less than 10 per cent of their diet.
These results clearly support the researchers’ idea that low-frequency echolocation allows T. teniotis to locate the relatively large insects, which are alerted to other bats by hearing their calls. It also explains why these bats are relatively rare. If they contributed too much to the selective pressure on the insects, they might trigger off the evolution of a wider hearing span in the moths.
T. teniotis also incurs an evolutionary cost. The low frequency of its echolocation call means that it cannot locate and eat small insects, such as flies, which are usually the more abundant than larger insects.