You’ve probably gathered that I’m very interested in echolocation, the process by which some organisms use reverberations of emitted sound pulses to generate acoustic images of their environments. Echolocation (also called biological sonar) has been most thoroughly investigated in bats, but dolphins and other cetaceans also have highly sophisticated echolocation systems that are not as well understood.
Dolphins use echolocation for navigation and for the accurate detection, localization and tracking of prey. Like other biological sonar systems, that of the dolphin consists of a transmitter, receiver and processor. Using this system, targets as small as a sardine – which is 9-18 cm in length – can be located at ranges of zero to 100 metres or more. The echolocation system of dolphins far outperforms man-made sonar systems. It even works efficiently in shallow waters, unlike man-made systems, whose signals are confounded by water turbulence, suspended sediment and the increased reverberation of sound waves. A better understanding of biosonar in dolphins could, therefore, lead the development of improved sonar systems.
To this end, Peter Dobbins of SEA Group Ltd., an engineering company based in Bristol, U. K., has developed a model of the echolocation system of the bottlenose dolphin, Turciops truncatus, based on the assumption that at least some of the echolocation signals are received through the teeth. SEA Ltd. has just been awarded a large contract by the British Ministry of Defence to develop advanced sonar systems. Any new technologies developed by SEA would be delivered to the MoD by Lockheed Martin, the world’s largest defence contractor.
What do we know about echolocation in dolphins? The sonar signals of dolphins consist of bursts of clicks with frequencies of up to 160 kiloHertz (kHz). These clicks are produced not in the larynx but in a series of air sacs within the nasal cavities. Associated with the air sacs are valves called bursae (or “monkey lips”) which open into the blowhole passage. The dolphin’s echolocation signals are emitted from an air-filled cavity into water, and there is a mismatch between how sound waves are propagated in the two media. This problem is overcome by a large deposit of fatty tissue, called the melon, located in the forehead. This is a sac-like pouch which consists mostly of fatty tissues and which extends into the nasal sac muscles. It acts to slow the transmission of sound waves as they are emitted from the nasal air sacs, thus making the transfer of the sound waves from one medium to the other smoother.
Dolphins lack a complex external ear through which sound waves can be transmitted to the cochlea; the outer part of the ear consists simply of a small opening covered by a fibrous tissue. It is now widely accepted that the lower jaw is a major component of the echo-receptor in dolphins. It is through the lower jaw and surrounding structures that many of the acoustic signals to which dolphins are sensitive are transmitted to the middle and inner ear. Auditory stimuli delivered to the lower jaw evoke responses in the auditory regions of the brain stem. In behavioural experiments, bottlenose dolphins had hoods placed over their lower jaws, attenuating the reception of acoustic signals; this greatly hindered their ability to echolocate.
The hypothesis that the teeth are a component of dolphins’ echolocation receiver was first proposed by Goodson and Klinowska in 1990, and is based on a number of observations about the arrangement of the teeth in the jaw. Firstly, whereas humans have different types of teeth – incisors for cutting, molars for chewing, etc. – dolphins are homodonts – all of their teeth are of the same type. A dolphin’s teeth are evenly spaced along the jaw, in two two straight lines, which diverge at an angle of 10-20°. Whereas acoustic signals received by the lower jaw are transmitted to the brain via the inner ear, each tooth is innervated by a nerve which projects directly to the brain. Delays in the propagation of nervous impulses in these fibres cause signals from all teeth to arrive simultaneously at the auditory cortex, the region of the brain in which the acoustic signals are processed.
According to this model, the teeth are resonant pressure transducers – they form an array of receivers which vibrate in response to the changes in pressure caused by sound waves as they travel through the water. There is some experimental evidence that dolphins’ teeth might be involved in receiving acoustic signals. One group of researchers recently used a technique called laser Doppler vibrometry to measure resonances in the teeth, and found that they do in fact resonate at frequencies of 115-135 kHz in response to sounds; this frequency range overlaps with that of the sonar signals used by dolphins. Flanking the jaw are structures called mandibular canals, which contain fat-filled channels that could function to impede the propagation of sound waves to a similar extent as sea water. These channels may therefore transmit the resonances of the teeth towards the inner ear, but exactly how signals from the teeth might be transmitted to the channels is as yet unclear.
In developing his model, Dobbins assumed that most of the sounds detected by the dolphin would be coming from in front. He modelled the bottlenose dolphin’s jaws as if they were two straight lines of receivers meeting at an angle of 10-20°, and assumed that sound waves would enter the jaw from the front. This is known as an endfire arrangement; it is used widely in radio and radar, but not in sonar systems, which employ a broadside system, whereby sound waves travel at right angles to the receivers. A drawback of broadside systems is the phenomenon of near-field degradation – that is, at close range, the broadside array cannot determine the direction of incoming sounds.
According to the model developed by Dobbins, endfire systems are less susceptible than broadside systems to near-field degradation, and are therefore far more efficient at close range. Dobbins also used his model to investigate other dolphin species with different arrangements of teeth. In river dolphins, for example, which usually live in shallow murky waters, the teeth near the tip of the jaw are larger and closer together than those nearer the base of the snout. On the basis of his model, Dobbins predicts that this arrangement makes the echolocation less susceptible to the frequency changes produced by the increased reverberations of sound waves in shallow water.
Whitlow Au, chief scientist on the Marine Mammal Research Program at the Hawai’i Institute of Marine Biology, dismisses the idea that teeth are acoustic signal receivers as a “wild hypothesis.” He points out that there are examples of captive dolphins that have lost all their teeth but are still capable of echolocation. Dobbins, however, says this is not important; he agrees that further investigation is needed to determine whether or not he has accurately modelled dolphin biosonar, but says that technologies based on his model could be developed regardless of whether or not dolphins’ teeth are a component of the acoustic signal receiver. His aim is to develop compact, lightweight and high resolution sonar systems capable of operating effectively in shallow waters. These systems could be carried by divers or submersible vehicles during naval mine-clearing operations.
Dobbins, P. (2007). Dolphin sonar – modeling a new receiver concept. Bioinsp. Biomim. 2: 19-29. [Full text – available until 14th April]