What Do Human Ears Miss?

Production team For many animals communication through a repertoire (range) of calls is an important part of their everyday lives and may be a factor in whether or not they pass on their genes. Some groups of animals, however, even have a mastery of sound utterly outside the usual human experience.

The pipistrelle bat is still one of the most widespread of bats in Britain, despite suffering major declines in the last 35 years (perhaps 70%). It is also one of the most often-encountered bats, despite its small size, as it inhabits suburban parks and gardens as well as woodland edges and watersides. Pipistrelles navigate their way through the night using echolocation, a series of short, high-frequency sounds (bats emit sounds between 9 and 200 kilohertz, most of which is well above the range of human hearing), often very loudly (about the volume of a smoke alarm, if we could hear them). These high-pitched (ultrasound) squeaks bounce off objects in the bats’ path and return, providing information an object’s size, shape, type, distance and direction, as well as whether or not it is moving. Tiny insects such as gnats can be detected, making nocturnal insectivorous bats effective aerial hunters! Such bats effectively “see” in sound, forming a picture of their dark world which enables them to avoid obstacles and capture their otherwise invisible prey.

In contrast to the “bouncy” ultrasound, low-frequency sound (infrasound – which is below the range of human hearing) is employed by animals precisely because it tends to roll around objects in its path. It is therefore a useful means of long-distance communication. Up until the mid-1980s, researchers marvelled at the ability of separated groups of African elephants to find one another on the savannah, as well as that of solitary males to locate groups of females for mating. It was then discovered that they stay in contact using “rumbles” largely in the infrasonic range (frequencies of between 1 and 20 hertz); although a component of elephant calls may be detectable to human ears. Low frequency sounds have a longer sound wave, and can therefore travel further without being absorbed or bounced back. Playback experiments using savannah and forest elephants suggest that they can stay in touch over distances of at least four kilometres, and that infrasound communication works just as well in dense forest.