Demetriou Georgia
Bio 742
Heraklion 2000
Echoes
When an acoustic wave bounces off an object and returns to its source, that is known as an echo. Echoes enable animals to sense and translate information on their surroundings. The bat emits short, high frequency "chirps" and listens for the echo. A similar process is used by some cave-dwelling birds to navigate in the dark and by dolphins, porpoises, and whales to locate objects under water. People use sonic echoes through special equipment to find and track game fish and submarines, and to map the ocean floor.
Sound
Aside from light, sound is the major source of information available to animals about the location and nature of objects in their environment. The speed of sound in water is much higher than in air.
Sound travels through water a lot differently than it does through air. Because water is relatively denser, sound travels through it very easily. So easily, in fact, that it moves five times faster (at a temp of 20 degrees Celsius, sound travels through ocean water at 1,450 m/s as compared to 334 m/s in the atmosphere). Velocity also increases with increasing salinity and temperature. Although pressure increases steadily from the surface of the ocean to the bottom, the generally dropping temperature below the thermocline (the buffer zone between the upper layer of water and the frigid ocean below) more than offsets this. Because of this, an area of low-velocity sound transmission exists at the base of the thermocline. The refraction of sound waves then causes sound to be trapped in the zone, called a SOFAR (sound fixing and ranging) channel. Animals in the ocean take advantage of this phenomenon to transmit sound. Using the SOFAR channel, animals can transmit sounds for great distances.
Sounds made by dolphins
When we (humans) look at an object, we are seeing light that has been reflected from that object. Given that eyes are often of little use in a watery environment, dolphins have developed a way to "see with their ears" using reflected sound instead of reflected light. Dolphins produce a variety of underwater tonal sounds and sonar pulses. These sounds resemble clicks, trills, moans, grunts, squeaks, and creaking doors. The frequencies of these sounds range from 0.25 to 150 kHz, with those between 0.25 and 50 kHz used for social communication and those between 40 and 150 kHz used mainly for echolocation. Signals are usually generated at intervals between clicks for 20 and 40 microseconds longer than the time for the sound to reach the expected target and back. Each individual click lasts for 50 to 128 microseconds (Sea World, 1996).
Dolphin echolocation
Dolphins can produce clicks. When these clicks hit an object, some of the sound will echo back to the "sender". By listening to the echo and interpreting the time it took before the echo came back, the dolphin can estimate the distance of the object. (That's why sonar is also called echolocation: with information from the echoes, a dolphin can locate an object). In addition, dolphins can determine direction stereophonically and distance and relative motion from the beat frequency caused by interference between the current and previous clicks. Dolphins can regulate their rate of click production to allow the returning "echo" to be heard between outgoing clicks. Depending on the material the object is made of, part of the sound may penetrate into the object and reflect off internal structure. If the object is a fish, some sound will reflect off the skin on the dolphin's side, some of the bones, the internal organs and the skin on the other side. So one click can result in a number of (weaker) echoes. This will give the dolphin some information about the structure and size of the fish. By moving its head (thereby aiming the clicks at other parts of the fish) the dolphin can get more information on other parts of the fish. Dolphins echolocation clicks can penetrate solids in much the same way as our ultrasound devices. It is like a medical ultrasound probe, but the results are far less clear. A medical probe moves back and forth very rapidly, much faster than a dolphin can move its head. Also the frequency of the sounds of the medical probe is much higher than a dolphin's sonar. Therefore the level of detail the echoes can provide is much higher in the medical probe. Many researchers believe that dolphins can interpret some information from echolocation clicks of other dolphins. This idea has even been taken a step further, to a hypothesis where dolphins may communicate directly using their echolocation clicks. Dolphin sonar travelling through the water is made by a series of low and high frequency clicks. The low frequency clicks (low-frequency scans) help a dolphin to get a coarse overall picture of the environment. The high frequency clicks help it gather information at a close range in more detail. Because of their longer wavelength and greater energy, low frequency sounds travel farther. High frequency sounds don't travel far in water. Echolocation is most effective at close to intermediate range, about 5 to 200 m (1 6-656 ft.) for targets 5 to 15 cm (2-6 in.) in length. When any of these clicking sounds hit an object the sound waves they produce bounce back at the dolphin. Dolphins can send as many as 2000 clicks per second.
The production and reception of echolocation sound.
Dolphin sonar is a sophisticated and highly sensitive sensory mechanism that is the result of millions of years of evolutionary refinement, and is superior to man-made sonar in its ability to recognize and classify targets in noisy environments. To echolocate, the dolphin generates sound in the form of clicks. The "clicks" are pulses of ultrasonic sound (sounds repeated as rapidly as 800 times/second) produced in a dolphins nasal sacs (passages), which are located below the blowhole and behind the melon, and focused in a large, lens-shaped organ in the forehead known as the melon. The melon concentrates the sound pulses into a directional beam, which is projected forward into the water in front of the animal. When the outgoing sound waves or "clicks" bounce off objects in their path, a portion of the signal is reflected back to the dolphin. The bony lower jaw of the dolphin receives the incoming sound waves and transmits them to the inner ear where they are converted into nerve impulses and then transmitted to the brain. (The dolphin has two acoustic receptors and also has two separate sound producing organs, which can be used use together as well as independently). In this way, the animal can create an "acoustical picture" of its environment. Dolphins have been found to have optimal sensitivity with frequencies between 40 and 100 kHz, but respond to frequencies as high as 150 kHz. The dolphin midbrain has also adapted to specialize in ultrasonic, ultra brief, fast-rising, closely spaced sounds - like echolocation clicks.
Can dolphins combine information from their sonar with their vision?
The short answer is yes, they can. Just like people can visualize an object by just touching it, dolphins can get an idea of what an object looks like by scanning it with their sonar. They can also identify objects with their sonar that they have only been able to see. If they form a visual picture from the sonar information (visualization) or form an acoustical picture from visual information is still unresolved. This capability is called cross-modal transfer and it has been demonstrated in only a few animal species so far: the bottlenose dolphin and the California sea lion. Despite the effectiveness of echolocation, studies show that a visually deprived dolphin takes more time to echolocate on an object than a dolphin using vision in tandem with echolocation.