Echolocation, also called biosonar, is a biological sonar used by several animal species. Echolocating animals emit calls out to the environment and listen to the echoes of those calls that return from various objects near them. They use these echoes to locate and identify the objects. Echolocation is used for navigation, foraging, and hunting in multiple environments.
Echolocation is a technique used by many different animals to determine the position of objects using reflected sound. Echolocation allows the animals to move around in pitch darkness to navigate, hunt, recognize friends and enemies, and avoid difficulties. This technique enables them to see in muddy waters or dark ocean pits and may even have evolved for dolphins and whales so that they can chase squid and other deep-diving groups. Echolocation lets bats fly at night as well as in dark caves. Echolocation is a skill bats probably developed so they could find night-flying insects that birds can’t see. In the early 1900’s it was discovered that when bats fly at night, they rely on some sense besides vision, but it was not discovered that the other sense was hearing.
Echolocation is the same as active sonar, using sounds made by the animal itself. Ranging is done by measuring the time delay between the animal’s own sound emission and any echoes that return from the environment. The reasonable intensity of sound received between each ear and the time delay between arrival at the two ears provide information about the horizontal angle from which the reflected sound waves arrive.
Unlike some human-made sonars that depend on many extremely narrow beams and many receivers to identify a target, animal echolocation has only one transmitter, and two receivers positioned slightly apart. The echoes returning to the ears arrive at different times and different intensities, depending on the object’s position generating the echoes. The animals use the time and loudness differences to perceive distance and direction. With echolocation, the bat or other animal can see where it is going and how big another animal is, what kind of animal it is, and other features.
Echolocating bats fill a diverse set of ecological conditions. They can live in environments as different as Europe and Madagascar and hunt for food sources as various as insects, frogs, nectar, fruit, and blood. Also, the characteristics of an echolocation call are adapted to the specific environment, hunting behavior, and food source of the particular bat. However, this adaptation of echolocation calls to environmental factors is limited by the phylogenetic relationship of the bats, leading to a process known as descent with modification and subsequent in the diversity of the Chiroptera today.
Flying insects are a common food source for echolocating bats, and some insects, moths, in particular, can hear the calls of predatory bats. There is evidence that moth hearing has evolved in response to bat echolocation to avoid capture. Furthermore, these moth adaptations provide selective pressure for bats to improve their insect-hunting systems and this cycle culminates in a moth-bat “evolutionary arms race.”
Bats can identify an insect up to five meters away, work out its size and hardness, and avoid wires as fine as human hairs. As a bat swarms in for its prey, it turns up its calls to pinpoint the target.
A bat turns off its middle ear just before calling, fixing its hearing a split second later to listen for echoes to avoid being deafened by its own calls. Bats make echolocating sounds in their larynxes and radiate them through their mouths. Luckily, most are too high-pitched for humans to hear, although some bats can scream at up to one hundred and forty decibels, as loud as a jet engine thirty meters away.
A few species, though, click their tongues. These sounds are usually produced through their mouths, but horseshoe bats and old-world leaf-nosed bats release their echolocation calls through their nostrils. There they have basal fleshy horseshoe or leaf-like structures that are well-adapted to behave as megaphones. Echolocation calls are usually ultrasonic–ranging in frequency from 20 to 200 kilohertz, while human hearing usually tops out at around 20 kilohertz. Even so, we can hear echolocation clicks from some bats, such as the spotted bat.
These noises seem like the sounds made by hitting two round stones together. In terms of loudness, bats radiate calls as low as fifty decibels and as high as one hundred and twenty decibels, louder than a smoke detector 10 centimeters from your ear. That is not just loud but harmful to human hearing. The little brown bat can give off such an intense sound. The good news is that because this call has an ultrasonic frequency, we cannot hear it. The outer structure of bats’ ears also plays an important role in getting echoes. The large variation in sizes, shapes, folds, and wrinkles are considered to help in the receiving and directing of echoes and sounds radiated from prey.
Hunting bats face a variety of problems when flying to their hunting grounds and searching for prey. These problems vary depending on where the bats hunt, what they eat, and how they develop their food. For example, bats hunting for insects in the open meet conditions different from those that search for prey near the edges of vegetation, in vegetation gaps, in the forest, or near the ground. The problems also vary depending upon whether or not they catch moving prey in flight or mostly inactive prey from the ground such as leaves, dirt, or water.
Bat calls can usually be classed into two groups: narrowband and broadband. Narrowband calls are those of almost constant frequency, while broadband calls sweep a large range of frequencies in a very short time (from 100 kilohertz down to 20 kilohertz in a couple of seconds). Broadband calls are used to check the landscape, while narrowband calls are used to detect and provide information on possible prey objects.
The search calls tend to be intense, with some 10 to 15 calls—or pulses—every second in some species, getting faster and faster, up to 200 calls per second, to home in once an insect has been detected. Bats only spend a certain period of their time, about 20%, echolocating because they can’t listen for returning echoes while shouting new pulses. However, this is not true of all bats; horseshoe bats can call and listen simultaneously, letting them spend as much as half their time echolocating. The bat (voice box) is large and hardened with bone, allowing a high pressure on the vocal cords to be preserved. Many microbats have a ‘spike’ of cartilage sticking up from the base of their ear, which scientists believe helps give the bat better sound detection in a given plane.
The echolocation of bats is remarkably correct. It seems that the ability to echolocate is mostly a trait of microbats; megabats, such as fruit bats or flying foxes, with few exceptions, don’t echolocate because they have suitably good vision to find fruit by sight. Hence, their scent is probably also involved. One exception to this is the Egyptian fruit bat, which uses echolocation to find its way in caves. Newborn bats appear to pick up echolocation rapidly. Two-week-old mustache bats pups were capable of suddenly producing CF and FM signals.