Animal echolocation

Echolocation, also called Biosonar, is the biological sonar used by several mammals such as bats, 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.

Animals that use echolocation 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 somewhat like a sonar made by an animal. Ranging is most certainly done by measuring the time delay between the animal's own vocalization and any echoes that return from the environment. Unlike some sonar which relies on an extremely narrow beam to localize a target, animal echolocation relies on multiple receivers, the two ears, to perform localization. In sonar when a target is detected the narrow beam is oscillated around the target direction to maximize the echo return. The direction of maximum echo return is the target direction. 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/or loudness differences are used by the animals to deduce direction.

Echolocating Bats

Bats are the most famous examples for echolocation among animals. All microbats use echolocation. The only megabat which is known to echolocate is the genus Rousettus, which uses a different method of echolocation than that used by microbats. Lazzaro Spallanzani performed a series of experiments on bats in 1794 and concluded that they navigated by their sense of hearing, although the scientific community rejected these findings. The ultrasound echolocation used by bats was first described by zoologist Donald Griffin in 1938.

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 100,000 Hz, mostly beyond the range of the human ear (typical human hearing range is considered to be from 20Hz to 20,000 Hz). The emitted vocalizations form a relatively broad beam of sound that is used to probe the environment.

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

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.Cranford, T.W., (2000). In Search of Impulse Sound Sources in Odontocetes. In Hearing by Whales and Dolphins (Springer Handbook of Auditory Research series), W.W.L. Au, A.N. Popper and R.R. Fay, Eds. Springer-Verlag, New York. These sounds are reflected off of the dense concave bone of 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 recived 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 turning down the volume of the emitted sound. In bats this is known to happen, but here the hearing sensitivity is also reduced close to a target.

Oilbirds

Oilbirds 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

There is evidence that a number of shrew species use echolocation. These include the wandering shrew (Sorex vagrans), the common european shrew (Sorex araneus), and the short-tailed shrew (Blarina brevicauda). The shrews emit series of high-pitched squeaks. In contrast to bats, shrew probably use echolocation to investigate their habitat rather than search for food.

References

Reynolds JE III & Rommel SA (1999), Biology of Marine Mammals, Smithsonian Institution Press, ISBN 1-56-098375-2. 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".

Sonar