Dolphin Echolocation -by Steve Bebington
While dolphins do have good eyesight, this is of minor use to them in an aquatic environment where vision may be limited by various factors such as light penetration or sediment in the water. As such, their primary sensory awareness relates to sound in what is known as echolocation or bi-sonar, which effectively uses sound to simulate a visual environment.
The brain of a dolphin, second only to that of man in its complexity, has a different design in order to accommodate its dependence on sound over sight. The part of the brain that processes imaging is a tenth of that in humans, while that dedicated to acoustical imaging is about ten times that in the human brain.
So, how does echolocation work? First of all, the dolphin is able to generate sound waves, which are thought to originate from the sinus area below the blowhole. If one looks at the forehead of dolphins, you will notice that it is very rounded. This is to accommodate an organ known as the melon. The melon is a fatty deposit in the forehead, the shape of which can be manipulated to focus the sound waves passing through at either a wide or narrow angle, much the way glass lenses of different shape will disperse light differently. These sound waves then travel out in front of the animal until the hit an object.
The sound wave then rebounds off the object and travels back to the dolphin where it is picked up by nerve endings in the lower jaw. It has been speculated that the teeth in the lower jaw may also assist in ‘triangulating’ what direction the sound came from, while the interval from transmission to reception will help determine distance. Within the lower jaw there is a narrow channel which travels its length and feeds into a reservoir, all of which is filled with an oily substance through which the sound vibrations travel. The vibrations within the reservoir are then picked up by the auditory nerve which transmits them to the brain where they are then deciphered to give the dolphin an acoustical image of the object in its path.
Echolocation has numerous uses for the dolphin. Primary amongst these is navigation and the dolphin will use it to determine distance from objects and the depth of the water. It will also use it for finding food. As the dolphin travels along, it will likely be transmitting sound waves at a wide angle and low frequency, in order to scan the area ahead of it for obstacles, threats and food. Should it receive a return signal that is of interest, such as a fish, the dolphin would then narrow the beam being transmitted and adjust the frequency higher for closer analysis. By narrowing the beam, the dolphin then follows the sound returns until it is close enough to inspect what they have found. It has further been suggested that should a dolphin find something of interest or of use to the rest of the pod, it is able to replicate the sound signature it has received and deciphered and then transmit it to the rest of the group, much like you would share a picture on facebook.
Additionally, as the sound waves are able to penetrate tissue, the dolphin is able to determine what species of fish they have found (by scanning the air-filled swim bladder which has a unique shape in each fish), or what gender another dolphin is and if it is a female, whether it is a viable mate.
Our understanding of echolocation and its various uses is still very limited. What we do know is that it is extremely sensitive. In experiments, a captive dolphin was able to differentiate a copper cube and copper ball, each only 1mm across. With this sensitivity, it is also likely affected by ambient noise and therefore it is thought that dolphins are negatively affected by noise pollution as produced by seismic testing, sonar in ships and submarines, noise from marine craft and other man-made sources; with some attributing mass strandings of dolphins and other toothed whales that have sonar abilities, to these factors.