thumb|upright=1.8|A depiction of the ultrasound signals emitted by a bat, and the echo from a nearby object
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 (FM, varying in pitch during the call) or constant frequency (CF). 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 (toothed whales) 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<!--perhaps as early as 1938--> 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 (using different species of bat), and concluded in 1798 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 (toothed whales) 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 (azimuth) 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 (multibeam sonar), animal echolocation has only one transmitter and two receivers (the ears) 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 (FM) sweeps, and constant frequency (CF) 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. 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 (a call being a single continuous trace on a sound spectrogram, and a series of calls comprising a sequence or pass) 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 (QCF) bats such as other Vespertilionidae, Emballonuridae, and Molossidae; and between 11 milliseconds (Hipposideridae) and 52 milliseconds (Rhinolophidae) 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.
Tradeoff between FM and CF
FM signal advantages
thumb|Echolocation call produced by [[Pipistrellus pipistrellus, an FM bat. The ultrasonic call has been "heterodyned" – multiplied by a constant frequency to produce frequency subtraction, and thus an audible sound – by a bat detector. A key feature of the recording is the increase in the repetition rate of the call as the bat nears its target – this is called the "terminal buzz".]]
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.
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. It evolved repeatedly, an example of convergent evolution.
Echolocating bats generate ultrasound via the larynx and emit the sound through the open mouth or, much more rarely, the nose. The latter is most pronounced in the horseshoe bats (Rhinolophus spp.). Bat echolocation 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). Bats may estimate the elevation of targets by interpreting the interference patterns caused by the echoes reflecting from the tragus, a flap of skin in the external ear.
Individual bat species echolocate within specific frequency ranges that suit their environment and prey types. This has 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 always 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. Researchers in several countries have developed "bat call libraries" that contain "reference call" recordings of local bat species to assist with identification.
When searching for prey they produce sounds at a low rate (10–20 clicks/second). During the search phase the sound emission is coupled to respiration, which is again coupled to the wingbeat. This coupling appears to dramatically conserve energy as there is little to no additional energetic cost of echolocation to flying bats. After detecting a potential prey item, echolocating bats increase the rate of pulses, ending with the terminal buzz, at rates as high as 200 clicks/second. During approach to a detected target, the duration of the sounds is gradually decreased, as is the energy of the sound.
Bat evolution
Bats evolved at the start of the Eocene epoch, around 64 mya. The Yangochiroptera appeared some 55 mya, and the Rhinolophoidea some 52 mya.
There are two hypotheses about the evolution of echolocation in bats. The first suggests that laryngeal echolocation evolved twice, or more, in Chiroptera, at least once in the Yangochiroptera and at least once in the horseshoe bats (Rhinolophidae):
The second proposes that laryngeal echolocation had a single origin in Chiroptera, i.e. that it was basal to the group, and was subsequently lost in the family Pteropodidae. Later, the genus Rousettus in the Pteropodidae family evolved a different mechanism of echolocation using a system of tongue-clicking:
Calls and ecology
Echolocating bats occupy a diverse set of ecological conditions; they can be found living in environments as different as Europe and Madagascar, and hunting for food sources as different as insects, frogs, nectar, fruit, and blood. The characteristics of an echolocation call are adapted to the particular environment, hunting behavior, and food source of the particular bat. The adaptation of echolocation calls to ecological factors is constrained by the phylogenetic relationship of the bats, leading to a process known as descent with modification, and resulting in the diversity of the Chiroptera today.<!----><!-- Bats can inadvertently jam each other, and in some situations they may stop calling to avoid jamming.
Flying insects are a common source of food for echolocating bats and some insects (moths in particular) can hear the calls of predatory bats. However the evolution of hearing organs in moths predates the origins of bats, so while many moths do listen for approaching bat echolocation their ears did not originally evolve in response to selective pressures from bats. 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".
Neural mechanisms
Because bats use echolocation to orient themselves and to locate objects, their auditory systems are adapted for this purpose, highly specialized for sensing and interpreting the stereotyped echolocation calls characteristic of their own species. This specialization is evident from the inner ear up to the highest levels of information processing in the auditory cortex.
Inner ear and primary sensory neurons
Both CF and FM bats have specialized inner ears which allow them to hear sounds in the ultrasonic range, far outside the range of human hearing. Although in most other aspects, the bat's auditory organs are similar to those of most other mammals, certain bats (horseshoe bats, Rhinolophus spp. and the moustached bat, Pteronotus parnelii) with a constant frequency (CF) component to their call (known as high duty cycle bats) do have a few additional adaptations for detecting the predominant frequency (and harmonics) of the CF vocalization. These include a narrow frequency "tuning" of the inner ear organs, with an especially large area responding to the frequency of the bat's returning echoes.
Echolocating bats have cochlear hairs that are especially resistant to intense noise. Cochlear hair cells are essential for hearing sensitivity, and can be damaged by intense noise. As bats are regularly exposed to intense noise through echolocation, resistance to degradation by intense noise is necessary.
Further along the auditory pathway, the movement of the basilar membrane results in the stimulation of primary auditory neurons. Many of these neurons are specifically "tuned" (respond most strongly) to the narrow frequency range of returning echoes of CF calls. Because of the large size of the acoustic fovea, the number of neurons responding to this region, and thus to the echo frequency, is especially high.
Inferior colliculus
In the Inferior colliculus, a structure in the bat's midbrain, information from lower in the auditory processing pathway is integrated and sent on to the auditory cortex. As George Pollak and others showed in a series of papers in 1977, the interneurons in this region have a very high level of sensitivity to time differences, since the time delay between a call and the returning echo tells the bat its distance from the target object. While most neurons respond more quickly to stronger stimuli, collicular neurons maintain their timing accuracy even as signal intensity changes.
Auditory cortex
The auditory cortex in bats is quite large in comparison with other mammals. Various characteristics of sound are processed by different regions of the cortex, each providing different information about the location or movement of a target object. Most of the existing studies on information processing in the auditory cortex of the bat have been done by Nobuo Suga on the mustached bat, Pteronotus parnellii. This bat's call has both CF tone and FM sweep components.
- CF-CF area: Another kind of combination-sensitive neuron is the CF-CF neuron. These respond best to the combination of a CF call containing two given frequencies – a call at 30 kHz (CF1) and one of its additional harmonics around 60 or 90 kHz (CF2 or CF3) – and the corresponding echoes. Thus, within the CF-CF region, the changes in echo frequency caused by the Doppler shift can be compared to the frequency of the original call to calculate the bat's velocity relative to its target object. As in the FM-FM area, information is encoded by its location within the map-like organization of the region. The CF-CF area is first split into the distinct CF1-CF2 and CF1-CF3 areas. Within each area, the CF1 frequency is organized on an axis, perpendicular to the CF2 or CF3 frequency axis. In the resulting grid, each neuron codes for a certain combination of frequencies that is indicative of a specific velocity
- Doppler shifted constant frequency (DSCF) area: This large section of the cortex is a map of the acoustic fovea, organized by frequency and by amplitude. Neurons in this region respond to CF signals that have been Doppler shifted (in other words, echoes only) and are within the same narrow frequency range to which the acoustic fovea responds. For Pteronotus, this is around 61 kHz. This area is organized into columns, which are arranged radially based on frequency. Within a column, each neuron responds to a specific combination of frequency and amplitude. This brain region is necessary for frequency discrimination. Odontocetes are generally able to hear sounds at ultrasonic frequencies while mysticetes hear sounds within the infrasonic frequency regime.
Whale evolution
Cetacean evolution consisted of three main radiations. Throughout the middle and late Eocene periods (49–31.5 million years ago), archaeocetes, primitive toothed Cetacea that arose from terrestrial mammals, were the only cetaceans. They did not echolocate, but had slightly adapted underwater hearing. By the late middle Eocene, acoustically isolated ear bones had evolved to give basilosaurid archaeocetes directional underwater hearing at low to mid frequencies. With the extinction of archaeocetes at the onset of the Oligocene (33.9–23 million years ago), two new lineages evolved in a second radiation. Early mysticetes (baleen whales) and odontocetes appeared in the middle Oligocene in New Zealand.