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-- DOLPHINS --

. Anatomy . Senses . Feeding . Behaviour . Reproduction .
. Communication . Marine ecosystem . Dolphines on display . Training .
 
 

DOLPHINS: Communication and echolocation


---Background

Although dolphin vision is well developed, their primary sense is hearing. In dark and turbid waters, sound is an important source of information. The dolphins' world is a world of sound and they are well equipped from that environment. Dolphins communicate mainly with sounds and they also get information from their environment by means of their echolocation. They produce a wide range of sounds, including whistles, pulsed sounds (often referred to as squeaks, squawks, snorts, groans and other descriptive names) and echolocation clicks. All these sounds have been recorded both in the wild and in oceanaria.

Dolphins communicate not only by making sounds, though. They are very sensitive to touch and direct bodily contact can be very important in social communication, both affiliative and aggressive. It is possible, that dolphins also communicate chemically. Although they have no sense of smell, they do have a sense of taste and may be able to "taste" signals from other dolphins in the water.

Dolphins can also communicate by non-vocal sounds, such as slapping the tail on the water (which can be a means of attracting attention as well as a sign of frustration or anger), "jaw-clapping" (a
sign of aggression) and jumping. Jumps can have both a visual and an acoustical function: other dolphins can see the jumps and thus locate group-mates and they can also hear the splash when they
re-enter the water.

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---Types of sounds

Dolphins make 3 different types of sounds:

1. Whistles, which are probably used for communication
2. Pulsed sounds (grunts, groans, squawks etc.) which also play a role in communication
3. Clicks, which are used for echolocation only.

sounds
-- Sounds --


---Communication sounds

The dolphin's larynx differs from our own larynx. It is shaped somewhat like a bird's beak and it protrudes through the esophagus into the air passage in the head. The air passages are completely
separated from the esophagus, where the food passes through. The tip of the larynx has 4 flaps, which may vibrate but probably don't play a major role in sound production. There are no vocal cords.

Where the various sounds are produced is still not complete clear yet. In the dolphin's head there is a system of air-filled sacs (the nasal sacs) which play a major role in sound production. Air can be
moved around in these sacs, so that dolphins can produce sounds underwater without losing any air.

The sounds that dolphins use for communication are reminiscent of whistles and screams and include sounds that are sometimes described as rattles, snores, creaking doors or even motorcycle
sounds.

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---The use of sounds

Dolphins use sounds in a variety of contexts: it can express excitement or agitation, anger or it can warn others of danger. Also, sounds can be used to stay in contact with other members of the
groups, a kind of "I am still here" signal. It is often difficult to determine the function of a sound, but in some cases, the other behaviours that accompany the sounds can shed some light on this. Some sounds, such as "jaw-clapping" are clearly associated with aggression.

Most dolphins have a stereotyped whistle, which is unique for each individual. This is often called the "signature whistle", because it can be used to identify an individual dolphin. This whistle may be a tool for a dolphin to let the rest of the group know where he/she is. The signature whistle develops when the dolphin is still very young, when the dolphin is a few months old (2 months or older). Male dolphins develop signature whistles that resemble the signature whistle of the mother, whereas female dolphins develop a whistle that is quite different from the mother's whistle.

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---Do dolphins understand speech? Do dolphins have their own language?

Several experiments have shown that dolphins can learn to understand a language. Several language types have been used: plain English, whistled language, sign language and language using symbols. They could ultimately understand rather complex constructs like "take the ball on the left and put it in the hoop on the right". They also developed quite a large "vocabulary". Similar experiments have also been done successfully with sea lions, gorillas and chimpanzees. Most of the experiments have been aimed at finding out what dolphins could understand, not at developing a language that dolphins could use.

So far there is no evidence, that dolphins communicate using a structured language like our own. Experiments in which the ability to convey messages was tested so far only revealed that dolphins can get a lot of information by passively listening to sounds other dolphins make.

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---Echolocation sounds

Dolphins can produce clicks in rapid succession. These clicks are very short high frequency sounds. For bottlenose dolphins, the frequency of the click sound is 40-80 kHz and it lasts for only 50-80
sec. There are quite some differences in clicks between different toothed whales species. Some have higher frequency clicks (like the harbour porpoise), others lower (like the killer whale). These clicks
are used for echolocation, to "see with sound" underwater. Other animals also have echolocation capabilities, such as some birds and maybe seals. There is one group of land mammals that have a well developed (and well-studied) echolocation system: the bats.

The clicks are usually emitted in series, called click trains. In such a click train, the clicks are spaced 20-200 msec apart. This succession of clicks is audible to the human ear and it sounds like a
rasping or buzzing sound. The time between clicks shortens as a dolphin approaches its target and as a consequence the frequency of the buzzing goes up as well.

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---Echolocation and its applications


The echolocation capabilities of dolphins were first demonstrated in captive animals. Since then numerous experiments in oceanaria and Navy facilities have been done to reveal how dolphins use
echolocation. Dolphins use their echolocation primarily to locate food (fish), but also for orientation, to locate obstacles such as rock, beaches etc. and to investigate other objects. Dolphin may also use their echolocation to investigate other dolphins.

The high-frequency clicks are produced in the dolphin's air passages in the head. The sound passes through the nasal air sacs, which are connected to the melon, a large fatty structure in the forehead of the dolphin. The melon bends the sound and focuses it into a rather narrow beam of sound, just like a lens can do with light. The melon acts as an acoustical lens.

In sea water (of 15 C) sound travels at a speed of 1,505 m/s (in air, the speed of sound at the same temperature and 1 atm. pressure is 340.5 m/s). When a sound wave travelling through the water meets an object (for instance a fish), part of the sound wave will be reflected back to the sender: the dolphin. This echo travels back to the dolphin at the same speed, but the since only part of the sound is reflected, the amplitude (the loudness) will be lower. The dolphin will pick up this echo with a sensitive area in the lower jaw. The sound travels through the fat in the jaw bone to the middle ear and on to the inner ear. The inner ear is similar to our inner ear, but the basilar membrane in the cochlea has a much large variation in width and stiffness than the human ear, making it much more suitable to hearing a wide range of frequencies, including very high frequencies. The sound is then processed in a similar way as in the human ear: nerve cells in the cochlea get activated by the sound and information is send to the brain.

From the echo the dolphin can some basic information about the object in its path: the time between sending the click and receiving the echo gives an indication of the distance. If that time is for instance 10 ms, it means that the click took 5 ms to get there (and the echo took 5 ms to get back). With a speed of sound of about 1,500 m/s, this means the object must be about 7.5 m away. The loudness of the echo tells the dolphin something about the acoustical reflectiveness of the object. A solid piece of rock will give a louder echo than a fish.

A fish may send back multiple echoes: the first one will be reflected off the skin. Some sound will penetrate the fish and be reflected off the swim bladder. A weak echo may also be reflected off the far side of the swim bladder and off the skin on the other side. The presence of multiple echoes tells something about the structure of the object and the time between these echoes may give some indication about the size of it.

Once the dolphin has received and processed an echo, it will send out another click to get more information about the object ahead. It is possible that the dolphin slightly varies the direction and power of a click based on the information extracted from the previous echo. By sending out series of clicks (click trains) while moving its head left and right, the dolphin can get a pretty good idea of how large the target is and in which direction (if any) it is moving.

Because the dolphin get only a tiny bit of information with each click, it needs to gradually build a picture of the target ahead out of all the tiny bits ("pixels") of information. Dolphins can detect small objects at large distances. Tests have shown that a dolphin can detect a 7.62 cm diameter metal sphere at a range of up to 113 m. The dolphin's eyes were covered during the test.

A lot has already been learned about the echolocation signals themselves, although even on this subject there is still a lot of uncertainty. We are still a long way remove from knowing how the dolphin processes the information it gets from it's "vision with sound".

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