Eyes and ears for bats

Returning from the office the other evening, I stopped on the Nun’s Bridges to scan the river. It was not quite dark and there was always the possibility that I might spot one of the local Otters. Needless to say, my luck was out and no dark shape was to be seen cutting it’s way across the surface of the slow-moving waterbody. However, other mammals were abroad and I was soon entranced, watching the dancing silhouettes of a dozen or so bats as they hawked for insects low above the water. This small cloud of tiny, delicate mammals, ‘chattered’ away as they circled too and fro to snatch midges and other small flies.

The chattering calls, whose intense high-pitched pulses I can still just about pick up with my hearing, are used to target prey and derive from a technique known as echolocation. This is really a highly developed form of sonar, the bat sending out short pulses of intense sound and then monitoring the returning echo with its sophisticated hearing to build up a ‘map’ of its surroundings. Although many people are bewildered by the apparent complexity of bat echolocation, to the bat it is just another component of its sensory armoury – much like how we might view our sense of sight or taste. Bats make and hear sounds in the same way as most mammals; the echolocation pulses are generated in the larynx and the resulting echoes are picked up by their ears. Admittedly, the larynx of a bat is proportionally bigger than our own, relative to body size, because the call has to carry a great deal of energy in order to produce a useful echo. Most bats emit the echolocation call through their mouth but there are species, like the horseshoe bats, where the sound is emitted through the nose.

Because the echolocation calls of bats often differ in their core frequency, it is possible to identify the different species by the pattern of the call and the frequencies over which it occurs. For example, the calls of Daubenton’s Bat start at about 85kHz and drop to about 32kHz, with a peak in intensity around 45kHz. Simple bat detectors, which convert the inaudible calls to a frequency we can hear, can help you split bats into rough groups but more complex detectors, coupled with computer software, are needed to separate the calls of species which are similar in their outputs.

The size and behaviour of the bats hawking over the river suggested that these were one of our pipistrelle species, although there may well have been a Daubenton’s Bat or two in with them. I will have to bring my bat detector down to the river in order to find out.