Echolocation

In biology, echolocation, or biosonar, is the physiological process of emitting sound waves and interpreting the echoes reflected back to the emitter in order to identify objects and determine their direction and distance. Echolocation has been established in several groups of mammals, such as bats (not all of them), dolphins, and whales, as well as a few species of shrews and two kinds of birds that use it for navigating through caves (cave swiflets and oilbirds).

Echolocation adds to the amazing diversity in nature, with bats and birds being able to occupy unique niches, and dolphins being able to develop a remarkable acuity in distinguishing even small objects.

The term echolocation was coined in 1944 by Donald Griffin, who was the first to conclusively demonstrate its existence in bats. Echolocation is used for navigation and for foraging (or hunting) in various environments.

Contents

Basic principle

Echolocation works like active sonar, using sounds made by an animal. Sonar (sound, navigation, and ranging) is a technology that uses sound propagation for navigation, communication, and detection. In animal echolocation, 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 not only can see where it is going, but can also see how big another animal is, what kind of animal it is, and other features as well.

Bats

Microbats use echolocation to navigate and forage, often in total darkness.

The microbats constitute the suborder Microchiroptera within the order Chiroptera (bats). Other English names are "insectivorous bats," "echolocating bats," "small bats," or "true bats." These names are somewhat inaccurate, because not all microbats feed on insects, and some of them are larger than small megabats (fruit bats). The fact that microbats use echolocation, whereas megabats do not, are one of the distinctions between these groups. Most microbats feed on insects. Some of the larger species hunt birds, lizards, frogs, or even fish. Microbats that feed on the blood of large mammals (vampire bats) exist in South America.

Microbats 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 (which come out at night since there are less predators then), 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 calls 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, while cross-correlation 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, bats 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

Some cetaceans are capable of echolocation. The order Cetacea is divided into two suborders, Mysticeti (baleen whales) and Odontoceti (toothed whales). Mysticeti have little need of echolocation, as they filter plankton, which would be impractical to locate with echolocation. However, many toothed whales—a suborder that includes dolphins, porpoises, and whales with teeth and one blowhole—have been shown to use echolocation. They generally live in an underwater habitat that has favorable acoustic characteristics and where vision may be limited in range due to absorption or turbidity.

Many toothed whales emit clicks similar to those in echolocation, but it has not been demonstrated that they echolocate. Some members of Odontoceti, such as dolphins and porpoises, clearly do perform echolocation. These cetaceans use sound in the same way as bats: They emit a sound (called a click), which then bounces off an object and returns to them. From this, cetaceans can discern the size, shape, surface characteristics, and movement of the object, as well as how far away it is. With this ability, cetaceans can search for, chase, and catch fast-swimming prey in total darkness. Echolocation is so advanced in most Odontoceti that they can distinguish between prey and non-prey (such as humans or boats). Captive cetaceans can be trained to distinguish between, for example, balls of different sizes or shapes.

Echolocation seems to be an ability all dolphins have. Their teeth are arranged in a way that works as an array or antenna to receive the incoming sound and make it easier for them to pinpoint the exact location of an object (Goodson and Klinowska 1990).

In general, 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 (Cranford 2000). These sounds are reflected by the dense concave bone of the cranium and an air sac at its base. In some species, the focused beam is modulated by a large fatty organ known as the "melon," which acts like an acoustic lens because it is composed of lipids of differing densities. Delphinids (dolphins in the Odontoceti family Delphinidae) typically have a round, bulbous melon, but most porpoises lack a melon.

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.

Birds

Two bird groups employ echolocation for navigating through caves, the so called cave swiftlets in the genus Aerodramus (formerly Collocalia) and the unrelated oilbird Steatornis caripensis. This is 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 echolocate are two genera (Sorex and Blarina) of shrews and the tenrecs (Family Tenrecidae of Madagascar (Tomasi 1979). 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.

References

  • Au, W. W. L. 1993. The Sonar of Dolphins. New York: Springer-Verlag. ISBN 0387978356.
  • Cranford, T. W. 2000. "In search of impulse sound sources in odontocetes." In Hearing by Whales and Dolphins. edited by W. W. L. Au, A. N. Popper, and R. R. Fay. Spinger-Verlag, NY: Springer Handbook of Auditory Research series. ISBN 0387949062.
  • Goodson, A. D., and M. Klinowska. 1990. "A proposed echolocation receptor for the Bottlenose Dolphin (Tursiops truncatus): Modelling the receive directivity from tooth and lower jaw geometry" In Sensory Abilities of Cetaceans: Laboratory and Field Evidence. edited by J. A. Thomas, and R. A. Kastelein. New York: Plenum Press, vi.196:255-267. ISBN 0306436957.
  • Pack, A., and L. M. Herman. 1995. "Sensory integration in the bottlenosed dolphin: Immediate recognition of complex shapes across the senses of echolocation and vision" in J. Acoustical Society of America 98(2): 722-733.
  • Reynolds, J. E., and S. A. Rommel. 1999. Biology of Marine Mammals. Smithsonian Institution Press. ISBN 1560983752.
  • Tomasi, T. E. 1979. Echolocation by the short-tailed shrew "Blarina brevicauda". Journal of Mammalogy. 60(4): 751–759.

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