Recently we had renovations made to our cottage in Mfuwe. One of the changes involved raising the roof of our bedroom. This became a bit of an ordeal as we didn’t realise that there was a gap left in the wall which was very quickly inhabited by a colony of bats. Not only did the high pitched squeaking keep us awake at night, but bats were also were able to squeeze through the tiniest of gaps into the house and we would end up ducking and diving as these little creatures whizzed around our heads.
My absolute faith in bats’ ability to navigate without colliding with us was a little shaky after an incident a few years before at Lake Kariba. I was watching rugby match on television and whole heartedly reassuring the person next to me that the bat above would never crash into us. A few minutes later, the poor creature flew into the ceiling fan and splattered in a gory mess onto the seat between us!
This time we were able to resolve our bat problem without any unexpected messy moments. Once we were able to locate the “bat cave” in the wall, we blocked it up from the outside, opened it up from the inside and the bats were able to fly out of the cottage door with no ill effects.
The encounter prompt a little bat research on my part, and I thought it worth sharing some fascinating facts.
Bats belong to an order called Chiroptera, and in Southern Africa these are divided into two suborders, Megachiroptera (relatively large fruit eating bats) and Microchiroptera (smaller insectivores). Among the fruit bats, only one species – the Egyptian fruit bat – uses echo-location, but in cruder tongue-clicking form than the ultrasonic bursts of sound generated in the larynx of the insectivorous bats.
The insectivores of course use echo-location to hunt insects on the wing and can do so on the darkest of nights. Echo-location, very simply, is exactly what it says. It works very much like radar, but uses sound waves rather than radio waves. The bat emits these high pitched bursts of sound, and by processing the time taken to receive the echo, as well as the direction it comes from, the bat can locate the smallest of flying insects with astonishing accuracy.
The squeaking noises that interrupted our sleep were actually contact calls between the bats, not echo-location bursts. In fact, echo-location bursts are made at a frequency far too high for the human ear to detect. Frequency is measured in Hertz, and denotes the amount of vibrations or waves per second. Our upper limit is around 20,000hz, while bats employ frequencies of between 20,000 and 80,000hz, though frequencies as high as 210,000hz have been recorded; well beyond our range. And this is just as well, as the sounds are also extremely loud, around 150 decibels which are far louder than the 130 decibels produced by a gunshot or the 114 decibels of a lion’s roar. If we could hear the frequencies that bats produce, the sound would be absolutely deafening!
The high frequencies are used for a very good reason, not out of consideration to humans! It is all because of the size of bats’ prey. The prey is tiny, and so therefore the sound waves produced must also be tiny. Higher frequencies mean smaller wavelengths; a sound of 1hz must have a wavelength of 340 metres, while 2hz will be 170 metres. The sounds we can hear have wavelengths so long that they pass over small objects much like a wave on the beach flows over a pebble. We can only detect echos reflected by large objects – buildings, cliffs etc. Bats produce such high frequencies that the wavelengths are small enough to be reflected by their tiny insect prey.
All in all, a pretty incredible adaptation to exploit a feeding niche, and as species go, it has made bats very successful. As with everything in nature, though, it does not always go the bat’s way. A number of insects have very effective defence mechanisms. Some unpalatable insects warn bats off using ultrasonic clicks, much the same way a poisonous frog warns predators by being very brightly coloured.
Even more impressive is the ability of some moths to generate noises which mimic the effect of a bat’s echolocation sounds being reflected off a large object, fooling the attacking bat into breaking off at the last minute, thinking it is about to crash into a rock or a tree. Unfortunately this technology is not yet available for ceiling fans, so until it is, the bats must stay outside happily hunting insects where they belong.