Echolocation

Echolocation, also called Biosonar, is the biological sonar used by several mammals such as bats (not all of them), dolphins and whales. The term was coined by Donald Griffin, who was the first to conclusively demonstrate its existence in bats. Two bird groups also employ this system for navigating through caves, the so called Cave Swiftlets in the genus Aerodramus (formerly Collocalia) and the unrelated Oilbird Steatornis caripensis.

They also emit calls out to the environment. They listen to the echoes that return from various objects in the environment. They use these echoes to locate, range and identify the objects. Echolocation is used for navigation and for foraging (or hunting) in various environments.

Basic Principle

Echolocation works like active sonar, using sounds made by an animal. Ranging is done by measuring the time delay between the animal's own sound emission and any echoes that return from the environment. Unlike some sonar that relies on an extremely narrow beam to localize a target, animal echolocation relies on multiple receivers. Echolocating animals have two ears positioned slightly apart. The echoes returning to the two ears arrive at different times and at different loudness levels, depending on the position of the object generating the echoes. The time and loudness differences are used by the animals to perceive direction. With echolocation the bat or other animal can see not only where it's going but can also see how big another animal is, what kind of animal it is, and other features as well.

Microbats use echolocation to navigate and forage, often in total darkness. They generally emerge from their roosts in caves or attics at dusk and forage for insects into the night. Their use of echolocation allows them to occupy a niche where there are often many insects (that come out at night since there are less predators then) and where there is less competition for food, and where there are fewer other species that may prey on the bats themselves.

Microbats generate ultrasound via the larynx and emit the sound through the nose or - much more commonly - the open mouth. Microbat range in frequency from 14,000 to well over 100,000 Hz, mostly beyond the range of the human ear (typical human hearing range is considered to be from 20 Hz to 20,000 Hz).

Individual bat species echolocate within specific frequency ranges that suit their environment and prey types. This has been sometimes been used by researchers to identify bats flying in an area simply by recording their calls with ultrasonic recorders known as 'bat detectors'. However echolocation calls are not species specific and some bats overlap in the type of calls they use so recordings of echolocation calls cannot be used to identify all bats. In recent years researchers in several countries have developed 'bat call libraries' that contain recordings of local bat species that have been identified known as 'reference calls' to assist with identification.

Since the 1970s there has been an ongoing controversy among researchers as to whether bats use a form of processing known from radar termed coherent cross-correlation. Coherence means that the phase of the echolocation signals is used by the bats, cross-correlation just implies that the outgoing signal is compared with the returning echoes in a running process. Today most - but not all - researchers believe that they use cross-correlation, but in an incoherent form, termed a filter bank receiver.

When searching for prey they produce sounds at a low rate (10-20/sec). During the search phase the sound emission is coupled to respiration, which is again coupled to the wingbeat. It is speculated that this coupling conserves energy. After detecting a potential prey item, microbats increase the rate of pulses, ending with the terminal buzz, at rates as high as 200/sec. During approach to a detected target, the duration of the sounds is gradually decreasing, as is the energy of the sound.

Toothed whales

Diagram illustrating sound generation, propagation and reception in a toothed whale. Outgoing sounds are red and incoming ones are green

Toothed whales (suborder odontoceti), including dolphins, porpoises, river dolphins, orcas and sperm whales, use biosonar because they live in an underwater habitat that has favourable acoustic characteristics and where vision is extremely limited in range due to absorption or turbidity.

Toothed whales emit a focused beam of high-frequency clicks in the direction that their head is pointing. Sounds are generated by passing air from the bony nares through the phonic lips. These sounds are reflected by the dense concave bone of the cranium and an air sac at its base. The focussed beam is modulated by a large fatty organ known as the 'melon'. This acts like an acoustic lens because it is composed of lipids of differing densities. Most toothed whales use clicks in a series, or click train, for echolocation, while the sperm whale may produce clicks individually. Toothed whale whistles do not appear to be used in echolocation. Different rates of click production in a click train give rise to the familiar barks, squeals and growls of the bottlenose dolphin. A click train with a repetition rate over 600 per second is called a burst pulse. In bottlenose dolphins, the auditory brain response resolves individual clicks up to 600 per second, but yields a graded response for higher repetition rates.

Echoes are received using the lower jaw as the primary reception path, from where they are transmitted to the inner ear via a continuous fat body. Lateral sound may be received though fatty lobes surrounding the ears with a similar acoustic density to bone. Some researchers believe that when they approach the object of interest, they protect themselves against the louder echo by quietening the emitted sound. In bats this is known to happen, but here the hearing sensitivity is also reduced close to a target.

Oilbirds

Oilbirds and some species of swiftlet are known to use a crude form of biosonar (compared to the capabilities of bats and dolphins). These nocturnal birds emit calls while flying and use the calls to navigate through trees and caves where they live.

Echolocating shrews

The only terrestrial mammals known to echoloate are two genera (Sorex and Blarina) of shrews and the tenrecs of Madagascar. These include the wandering shrew (Sorex vagrans), the common or Eurasian shrew (Sorex araneus), and the short-tailed shrew (Blarina brevicauda). The shrews emit series of ultrasonic squeaks. In contrast to bats, shrews probably use echolocation to investigate their habitat rather than to pinpoint food.

See also

• Human echolocation

References

ISBN links support NWE through referral fees

• Reynolds JE III & Rommel SA (1999), Biology of Marine Mammals, Smithsonian Institution Press, ISBN 1-56098-375-2. Authoritative work on marine mammals with in depth sections on marine mammal acoustics written by eminent experts in the field.
• Au, Whitlow W. L. (1993). The Sonar of Dolphins. New York: Springer-Verlag. Provides a variety of findings on signal strength, directionality, discrimination, biology and more.
• Pack, Adam A. & Herman, Louis M. (1995). "Sensory integration in the bottlenosed dolphin: Immediate recognition of complex shapes across the senses of echolocation and vision", J. Acoustical Society of America, 98(2), 722-733. Shows evidence for the sensory integration of shape information between echolocation and vision, and presents the hypothesis of the existence of the mental representation of an "echoic image".

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