Animal echolocation

Echolocation, also called bio sonar, is a biological active sonar used by several animal groups, both in the air and underwater. Echolocating animals emit calls 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 prey.
Echolocation calls can be frequency modulated or constant frequency. FM offers precise range discrimination to localize the prey, at the cost of reduced operational range. CF allows both the prey's velocity and its movements to be detected by means of the Doppler effect. FM may be best for close, cluttered environments, while CF may be better in open environments or for hunting while perched.
Echolocating animals include mammals, especially odontocetes and some bat species, and, using simpler forms, species in other groups such as shrews. A few bird species in two cave-dwelling bird groups echolocate, namely cave swiftlets and the oilbird.
Some prey animals that are hunted by echolocating bats take active countermeasures to avoid capture. These include predator avoidance, attack deflection, and the use of ultrasonic clicks, which have evolved multiple functions including aposematism, mimicry of chemically defended species, and echolocation jamming.

Early research

The term echolocation had been coined by 1944 by the American zoologist Donald Griffin, who, with Robert Galambos, first demonstrated the phenomenon in bats. As Griffin described in his book, the 18th century Italian scientist Lazzaro Spallanzani had, by means of a series of elaborate experiments, concluded that when bats fly at night, they rely on some sense besides vision, but he did not discover that the other sense was hearing. The Swiss physician and naturalist Louis Jurine repeated Spallanzani's experiments, and concluded that when bats hunt at night, they rely on hearing. In 1908, Walter Louis Hahn confirmed Spallanzani's and Jurine's findings.
In 1912, the inventor Hiram Maxim independently proposed that bats used sound below the human auditory range to avoid obstacles. In 1920, the English physiologist Hamilton Hartridge correctly proposed instead that bats used frequencies above the range of human hearing.
Echolocation in odontocetes was not properly described until two decades after Griffin and Galambos' work, by Schevill and McBride in 1956. However, in 1953, Jacques Yves Cousteau suggested in his first book, The Silent World, that porpoises had something like sonar, judging by their navigational abilities.

Principles

Echolocation is active sonar, using sounds made by the animal itself. Ranging is achieved by measuring the time delay between the animal's own sound emission and any echoes that return from the environment. The relative intensity of sound received at each ear, as well as 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 rely on many extremely narrow beams and many receivers to localize 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 at different intensities, depending on the position of the object generating the echoes. The time and loudness differences are used by the animals to perceive distance and direction. With echolocation, the bat or other animal can tell, not only where it is going, but also how big another animal is, what kind of animal it is, and other features.

Acoustic features

Describing the diversity of echolocation calls requires examination of the frequency and temporal features of the calls. It is the variations in these aspects that produce echolocation calls suited for different acoustic environments and hunting behaviors. The calls of bats have been most intensively researched, but the principles apply to all echolocation calls.
Bat call frequencies range from as low as 11 kHz to as high as 212 kHz. Insectivorous aerial-hawking bats, those that chase prey in the open air, have a call frequency between 20 kHz and 60 kHz, because it is the frequency that gives the best range and image acuity and makes them less conspicuous to insects. However, low frequencies are adaptive for some species with different prey and environments. Euderma maculatum, a bat species that feeds on moths, uses a particularly low frequency of 12.7 kHz that cannot be heard by moths.
Echolocation calls can be composed of two different types of frequency structure: frequency modulated sweeps, and constant frequency tones. A particular call can consist of one, the other, or both structures. An FM sweep is a broadband signal – that is, it contains a downward sweep through a range of frequencies. A CF tone is a narrowband signal: the sound stays constant at one frequency throughout its duration.
Echolocation calls in bats have been measured at intensities anywhere between 60 and 140 decibels. Certain bat species can modify their call intensity mid-call, lowering the intensity as they approach objects that reflect sound strongly. This prevents the returning echo from deafening the bat. High-intensity calls such as those from aerial-hawking bats are adaptive to hunting in open skies. Their high intensity calls are necessary to even have moderate detection of surroundings because air has a high absorption of ultrasound and because insects' size only provide a small target for sound reflection. Additionally, the so-called "whispering bats" have adapted low-amplitude echolocation so that their prey, moths, which are able to hear echolocation calls, are less able to detect and avoid an oncoming bat.
A single echolocation call can last anywhere from less than 3 to over 50 milliseconds in duration. Pulse duration is around 3 milliseconds in FM bats such as Phyllostomidae and some Vespertilionidae; between 7 and 16 milliseconds in Quasi-constant-frequency bats such as other Vespertilionidae, Emballonuridae, and Molossidae; and between 11 milliseconds and 52 milliseconds in CF bats.
Duration depends also on the stage of prey-catching behavior that the bat is engaged in, usually decreasing when the bat is in the final stages of prey capture – this enables the bat to call more rapidly without overlap of call and echo. Reducing duration comes at the cost of having less total sound available for reflecting off objects and being heard by the bat.
The time interval between subsequent echolocation calls determines two aspects of a bat's perception. First, it establishes how quickly the bat's auditory scene information is updated. For example, bats increase the repetition rate of their calls as they home in on a target. This allows the bat to get new information regarding the target's location at a faster rate when it needs it most. Secondly, the pulse interval determines the maximum range that bats can detect objects. This is because bats can only keep track of the echoes from one call at a time; as soon as they make another call they stop listening for echoes from the previously made call. For example, a pulse interval of 100 ms allows sound to travel in air roughly 34 meters so a bat can only detect objects as far away as 17 meters. With a pulse interval of 5 ms, the bat can only detect objects up to 85 cm away. Therefore, the bat constantly has to make a choice between getting new information updated quickly and detecting objects far away.

Tradeoff between FM and CF

FM signal advantages

The major advantage conferred by an FM signal is extremely precise range discrimination, or localization, of the target. J. A. Simmons demonstrated this effect with a series of experiments that showed how bats using FM signals could distinguish between two separate targets even when the targets were less than half a millimeter apart. This ability is due to the broadband sweep of the signal, which allows for better resolution of the time delay between the call and the returning echo, thereby improving the cross correlation of the two. If harmonic frequencies are added to the FM signal, then this localization becomes even more precise.
One possible disadvantage of the FM signal is a decreased operational range of the call. Because the energy of the call is spread out among many frequencies, the distance at which the FM-bat can detect targets is limited. This is in part because any echo returning at a particular frequency can only be evaluated for a brief fraction of a millisecond, as the fast downward sweep of the call does not remain at any one frequency for long.

CF signal advantages

The structure of a CF signal is adaptive in that it allows the CF-bat to detect both the velocity of a target, and the fluttering of a target's wings as Doppler shifted frequencies. A Doppler shift is an alteration in sound wave frequency, and is produced in two relevant situations: when the bat and its target are moving relative to each other, and when the target's wings are oscillating back and forth. CF-bats must compensate for Doppler shifts, lowering the frequency of their call in response to echoes of elevated frequency – this ensures that the returning echo remains at the frequency to which the ears of the bat are most finely tuned. The oscillation of a target's wings also produces amplitude shifts, which gives a CF-bat additional help in distinguishing a flying target from a stationary one. The horseshoe bats hunt in this way.
Additionally, because the signal energy of a CF call is concentrated into a narrow frequency band, the operational range of the call is much greater than that of an FM signal. This relies on the fact that echoes returning within the narrow frequency band can be summed over the entire length of the call, which maintains a constant frequency for up to 100 milliseconds.