Wednesday, October 10, 2007

Tuning in to others

(discard from book ms)

In the autumn of 1985 I lay on my back staring at the sky near the entrance to a cave in Renfrew, Ontario. This spot is busy this time of year with bats preparing to enter the cave to hibernate for the long Canadian winter. It was late dusk, the perfect time to see bats emerging hungry for the evening’s first meal. Directly above me, silhouetted by the darkening sky, was a loudspeaker through which a tape recorder played a two minute playback of prerecorded bat calls, bracketed by a two minute control stimulus of tape recorded silence.

The bats’ response to the bat sounds was astonishing. I saw only a handful of random stragglers during the blank playback, but as soon as the echolocation calls kicked in, the speaker was swarmed. I counted hundreds during the two minute playback.

Why were they drawn to the speaker as if it were a magnet? I speculated that the responding bats were mainly youngsters listening in on other bats to find the best feeding spots. (Another theory is that they are just curious.) Several other playback studies by me and other biologists have documented that bats eavesdrop on the calls of other individuals to identify patches of insects. It’s a bit like choosing a good restaurant by looking to see how many people are eating there.

Fish, it appears, practice a similar art. Juvenile fish tune in to the crackling sounds that emanate from reefs. This cueing allows them to find reef habitats from the open ocean. The snapping of shrimp claws and other sounds distinctive to reefs can be heard up to 20 kilometers away. Scientists discovered this skill by setting up artificial “reefs,” broadcasting sounds recorded from reefs from underwater speakers (Stephen Simpson, Univ of Edinburgh, reported in NewSci, 16 April 2005, vol 186, p 19).

Some bats use the eavesdropping technique to muscle in on someone’s prospective meal. I’ve watched red bats dive-bombing towards a speaker playing the recorded feeding buzzes (rapid pulses made during an assault on a flying insect) of another red bat, just as they will dive after a juicy moth when another bat nearby is buzzing in for the kill (Balcombe & Fenton 1988; Griffin 1958). Such aerial piracy is well known in many bird species, whose daytime activities are more easily observed. It’s a hazardous business—I once watched as a dive bombing red bat miscued and collided with a patch of dirt. Fortunately, it was only a glancing blow and the little spitfire was soon airborne again.


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