Echolocation

The echolocation (from the prefix eco-, from the Latin echo, and this from the Greek ἠχώ ēchṓ 'echo', and the Latin locatĭō, 'position'), is the ability of some animals to know their environment through the emission of sounds and the interpretation of the echo that the objects around them produce due to them. "Echolocation" is a term coined in 1938 by Donald Griffin, who was the first to conclusively demonstrate its existence in bats.

Several mammals have this ability: bats (order Chiroptera —although not all species of the order use it—), dolphins (family Delphinidae) and the sperm whale (Physeter macrocephalus). The birds that use this system to navigate in caves without visibility are the guácharo (Steatornis caripensis) and swifts and swiftlets (family Apodidae), especially the Papuan swiftlet (Aerodramus papuensis), of the Collocaliini tribe. The sonar of ships and submarines is based on this principle. Recently, studies have come out that talk about the ability of echolocation in humans, but these studies lack scientific foundation.

History of echolocation

Research on the sonar of animals is attributed to the Italian scientist Lazzaro Spallanzani in 1793, who proposed that bats could "see with their ears", to reach this conclusion he locked an owl and a bat in a room in which he placed a series of threads crossed from one side to the other from which bells hung; which would sound in case the animals collided with them, in the light both animals were able to fly, but when it became totally dark in the room, he observed that the owl became disoriented and collided with the bells while the bat kept intact his ability to fly, so he realized that the bat had an additional ability which did not depend on light.

In order to find said ability Spallanzani proceeded to burn the eyes of the bats to blind them completely and released them into the room; With this he verified that bats had the same facility to fly and to hunt insects as those who still had the sense of sight. He communicated his results to the Swiss zoologist Charles Jurine who 5 years later realized that it was impossible for bats to avoid objects if their ears were covered, despite this, the knowledge was not sufficient to formulate a theory about the echolocation, so their opinions were rejected by the scientific community, which continued to abide by the explanation given by the French naturalist Georges Cuvier: bats used their sense of touch, feeling the objects in their environment with their wings.

In 1912 the British-American engineer Hiram Maxim wrote in Scientific American magazine: “bats detect obstacles by listening for reflections of low-frequency sounds produced by their wings (at about 15 Hz)., and the ships could avoid collisions with icebergs or other ships by installing a device that emits high-power sounds and a receiver that listens to the echoes of the return» this was said after the sinking of the Titanic , although Maxim he was wrong his idea came true with the development of sonar by the French physicist Paul Langevin.

In 1938 Robert Galambos and Donald Griffin used an ultrasound detector developed by William Pierce to demonstrate that bats echolocated by emitting ultrasound and receiving echoes. Griffin later realized that bats could fly in total darkness without colliding and coined the term echolocation to describe this phenomenon in 1944. It was soon discovered that other animals such as whales and dolphins were also endowed with this ability to echolocate.

Principle for echolocation

Echolocation resembles the operation of active sonar; the animal emits a sound that bounces when it encounters an obstacle and analyzes the echo received. Thus, it manages to know the distance to the object or objects, measuring the delay time between the signal it has emitted and the one it has received.

However, sonar relies on a narrow beam to locate its target, and animal echolocation relies on multiple receivers. These animals have two ears placed at a certain distance from each other, the bounced sound arrives with differences in intensity, time and frequency to each of the ears depending on the spatial position of the object that generated it. This difference between both ears allows the animal to recreate the spatial position of the object, including its distance, size and characteristics.

Types of pulses in echolocation

Echolocation sounds can be classified into three types which can be identified based on their visual characteristics on a spectrogram as:

- Frequency Modulated (FM): These types produce a spectral signature that aids in determining the shape of an object and discriminating between object types. Bats that use these types of calls live in environments with dense vegetation. Generally FM calls start with very high frequencies and descend in a very short period quickly. This type of sound encodes information for the detection and classification of vegetation during spatial orientation.

- Constant Frequency (FC): Used by bats that forage in environments with few or no obstacles; in these, the frequency remains unchanged, which prevents it from providing detailed information but allows it to detect prey and its location, in addition to facilitating the detection of Doppler shifts, which is why they are more suitable for detection in motion.

- Combined pulses (FC-FM and FM-FC): they are a combination of both which allows a good detection thanks to the FC and detailed information on the FM side. This signal gives an advantage in dense vegetation where it is necessary to identify if the prey is moving or not and if what is detected is prey or not.

On bats

Microchiroptera, hunting in the dark.

Contrary to popular belief, bats are not blind, as many bats use sight for different activities in addition to their sonar system. Unlike microbats (suborder Microchiroptera), megabats (suborder Megachiroptera) use vision to orient themselves and locate their prey (a single species in this suborder has evolved an echolocation mechanism that it uses only when flying in total darkness).

The eyes of megabats are more developed than those of microbats, and in general, no bat is completely blind; even microbats can use highly visible objects on the ground as signals during flight to return to their refuge.

Microbats use it to navigate and hunt, often in total darkness. They usually emerge from their burrows and hunt for insects at night. Echolocation allows them to find places where there are usually many insects, little competition for food, and few predators for them. They generate the ultrasound in the larynx and emit it through the nose or through the open mouth. The bat call uses a frequency range between 14,000 and 100,000 Hz, most of which are frequencies above the hearing capacity of the human ear (20 to 20,000 Hz).

There are specific species of bats that use specific frequency ranges to adapt to their environment or for their hunting techniques. This has sometimes been used by researchers to identify the type of bats in an area by recording their calls with ultrasonic recorders, also known as bat detectors. However, echolocating calls are not specific to each species, so there are bats that overlap their call types. For this reason these recordings do not serve to identify all types of bat. In recent years developers in different countries have developed a bat call library, which contains reference recordings of the calls of local species to help with identification.^{[citation required]}

Since the 1970s there has been a growing controversy about whether bats use a form of radar processing known as coherent phase correlation. Coherence means that bats use the phase of echolocation signals, while phase correlation implies that the emitted signal is compared to the received signal in a continuous process. Most—not all—researchers claim that the bat uses a type of phase correlation, but in an incoherent way, similar to a fixed receiver filter bank.

When they hunt they produce sounds at a very low frequency (10-20 Hz). During the search phase, the sound emitted is synchronous with breathing and with the frequency of flapping. It does this to conserve energy. After detecting their prey, the microbats increase the frequency of the pulses, also called Hunting Buzz, ending the final buzz at frequencies above 200 Hz. During the approach to the target, the duration and energy of the sound are decreasing.

Bats catch their prey from a group of short echolocation pulses called Hunting Buzz, which are rapidly emitted and are produced by bats before making physical contact with their bats. dams. Once the bat detects its prey, it approaches it and the emission of these pulses decreases in order to reduce the return time of the echo, thus it receives more detailed information on the trajectory of the prey in order to follow it. and intercept it.

In cetaceans

Ecolocalization process in a dolphin: in green the sounds generated by the dolphin, in red those from the fish.

Generation, propagation and reception in a dolphin. The emitted sounds are represented in cian and, the sounds captured, in green.

Before the echolocation abilities of cetaceans were officially discovered, Jacques-Yves Cousteau suggested their existence. In his first book, The Silent World (1953, pp. 206-207), he reported that in the course of an investigation he was heading to the Strait of Gibraltar and noticed a group of porpoises followed them. Cousteau observed the changing course of porpoises to make the most of seaworthiness in the strait, concluding that cetaceans had something like sonar, which was a relatively new feature in submarines.

The sides of the dolphin's head and lower jaw, which contain oily fat, are the areas that receive the echo. When a dolphin travels, it usually moves its head slowly from side to side, up and down. This movement is a kind of global exploration, allowing the dolphin to see a wider path in front of it.

Toothed cetaceans (suborder Odontoceti) form one of the two large groups of cetaceans, which includes dolphins, porpoises, river dolphins, orcas, and sperm whales, use biosonar because they live in an aquatic habitat that has favorable acoustic characteristics for the phenomenon and where vision is extremely limited due to absorption or haze.

Echolocation involves the dolphin making a wide range of sounds in the form of short bursts of sound impulses called clicks, and obtaining information about the environment by analyzing the returning echoes. This ability to use a full range of both high and low frequency sound emissions, combined with highly sensitive directional hearing thanks to the asymmetry of the skull, facilitates extremely accurate echolocation and gives these animals a unique sensory system in the sea..

Sound is generated by passing air from the nasal cavity through the phonic lips. These sounds are reflected by the dense concave bone of the dolphin's skull and the air sac at its base. The focused beam is modulated by a large fatty organ known as the melon. It acts as an acoustic lens due to its lipid composition of different densities. Many toothed cetaceans use a consecutive series of clicks or a train of pulses; however, the sperm whale can produce individual clicks. The whistles they produce seem not to be used in echolocation, but in communication.

Varying the frequency of the clicks in the pulse train generate the familiar squeals and grunts of the dolphin. A train of pulses with a frequency of about 600 Hz is called a burst pulse. In bottlenose dolphins, the auditory brain response can analyze each click independently up to 600 Hz, having a gradual response for higher frequencies. The echo is received through the lower jaw. Furthermore, the placement of the teeth in the jaw of a bottlenose dolphin, for example, is not symmetrical in the vertical plane, and this asymmetry could possibly be an aid to the dolphin, which detects by far if the signal arrives from one or other side of the jaw. Lateral sound is received through lobes that surround the eyes with a density very similar to bone.

Many researchers believe that when this animal approaches the object of its interest, it protects itself against the high level of echo by decreasing the sound emitted. In bats the effect is known, but in this case the sensitivity of the ear also worsens near the target.