This is a brief article designed to inform the reader about how and why bats ‘see’ in the dark, and how to ‘listen’ out for them. It specifically deals with echolocation and the mechanism Chiroptera (bats) have evolved for detecting prey. In monochrome rather than in full colour, bats can see as well as humans.
However, research shows that bats have evolved to make use of the cover of night in order to predate on flying insects—their main prey. This has the added value of not disclosing their whereabouts to both prey and predators, though, even for an animal with excellent colour vision, hunting a night is difficult.
For a bat, monochrome vision alone is not going to cut it. In consequence, many bats—particularly those in temperate climates and especially those in the UK—use echolocation, a kind of natural sonar, to locate prey and get a view of the world around them.
Echolocation of course, does not require daylight and so is used by bats to compensate for the lack of visibility at night when they emerge, rather than to compensate for having poor vision. Bats emit echolocation calls (produced either from their nose or mouth), and as with some modern technological devices such speed cameras, or radar, the time taken for the wave emitted to be reflected back allows bats’ brains the interpret the world around in three dimensions, even enabling them to accurately judge distances to such fine tolerances that they can catch insects on the wing—if you’ve ever tried to swat a fly in mid-air, you know this is no mean feat.
This ability provides them with much in the same advantages that forward facing eyes produce, in daylight of course, through a phenomenon known as stereoscopic vision in e.g. humans. As with all things in nature, predator and prey are locked in a constant battle to out gun one another.
All British bats have echolocation to ‘see’ in the dark, however, in response to this, many insects have evolved to ‘listen’ for this echolocation signal, and once they have detected it, can flee from danger. In response, both the brown long eared bat (Plecotus auritus) and the Barbastelle (Barbastella barbastellus) species have developed a softer, quieter echolocation call, which cannot be detected as well by the insects, so providing them with the advantage.
At least, until the next evolution of insect defences against predation by bats. Humans of a very young age can sometimes hear these calls, though by adulthood most have lost this ability. Normally, bat calls are identified by using a bat detector, which converts the echolocation call into a sound that we can hear ourselves, and so use such information for scientific study and conservation purposes.
Detectors such as heterodyne, frequency division and time expansion, all alter the sound wave in different ways, so are usually recorded for proper identification using computer software at a later date, unless your bat surveyors are particularly experienced and can identify them simply from the call alone.
Examples of this would be common pipistrelle (Pipistrellus pipistrellus) and soprano pipistrelle (Pipistrellus pygmaeus) species, which can be identified by listening for the peak frequency on the bat detector, where their frequencies are 45KHz and 55KHz (hence soprano!) respectively.