What is it?

Echolocation is how whales use sounds to locate objects (such as prey) and to orient themselves in the vast and three-dimensional ocean realm. Earlier, we learned that water is an excellent sound transmitter. Well, whales have capitalized on this characteristic of water to compensate for reduced vision in the sometimes cloudy water of the sea. When sound is reflected after it strikes an object, it creates an echo. So, whales emit short sounds (called clicks) and locate objects by listening for the reflected echo. The animal can tell how far away the object is by measuring the time it takes for the echo to return to them. 

For example, in the above diagram, there is 6 seconds between an outgoing click and an incoming echo. It took half that time for the click sound to reach the object. So, the object is 3 seconds away. As sound travels fairly constantly at 1500m/s, the object is 4500m away. (3 seconds times 1500metres/second = 4500m).

Therefore, echolocation necessitates that whales have both superb sound production and reception systems. The sound reception systems in cetaceans is so advanced that from the returning echoes direction and time, the animal can sense the object's shape and material!!! Cetaceans can resolve sound down to a couple of degrees (directionally) similar to humans. But, they can also resolve sound down to a tenth of a millisecond, an order of magnitude greater than humans!

Sound production:
The click generating site is in the nasal passage system just inside the blowhole. The sound generator is the "monkey lips"/dorsal bursa (abbreviated MLDB) complex . There are two complexes on each side of the head just below the air sacs inside the blowhole. A MLDB consists of:
o two fatty-filled bursae, one in front of the nasal passage (the anterior bursa) and one behind (the posterior bursa).
o The monkey lips: a slit-like opening
o Cartilage, the blowhole ligaments, muscles and air spaces

Sound is produced when air passes through the monkey lips and the MLDB begins to vibrate. Remember that every sound wave originates at a vibrating source. So, the monkey lips opening and closing creates a clack, causing the anterior and posterior bursae to vibrate. These vibrations pass the sound wave to the melon, the sound propagation organ. The melon is filled with low density fatty liquid and acts like an acoustic lens. The sound wave bounces around in the melon and is focussed and directed at the target object and emitted from the front of the forehead. Abracabadra! A click is produced! By opening and closing the monkey lips the flow of air is interrupted and the click repetition rate is altered. Timing is also important in echolocation. If the clicks are not timed right, there will be interference between clicks and echoes and acoustic chaos will ensue. Cetaceans therefore make sure the echo of one click returns before the next click is produced (see diagram of clicks above). The animals are also constantly adjusting the frequency of their clicks to avoid interference and to maximize the information they receive from the object's echo. Different frequencies will give you different resolution. Generally speaking, there are two kinds of clicks: Discrimination clicks - high frequencies and high repetition rate to classify objects as to shape, size and material. Orientation clicks - low frequencies and low repetition rate for detection and orientation.  

Sound reception:
Sound reception is just as important as sound production in echolocation. Odontocete whales have a very unique ear structure. Because of the extreme pressure changes involved in deep diving, their ears cannot be directly open to the air, (otherwise, changes in air pressure during dives might break their ear drums). Therefore, their ears are surrounded by dense bones and separated from adjacent bones by an insulating layer of oil and air. Their external ears are plugged, and it's thought that they are largely nonfunctional. Instead, cetaceans have developed a unique sound reception pathway – the jaw!! The lower jaw (or mandible) is designed to conduct sound to the middle ear. The insulating layer around the ear prevents sound reception from other angles, reducing the intake of ambient noise. Sound can only enter the ear via the jaw. The back of the mandible is wide, thin and filled with a fat body that is liquid oil at body temperature. Like the melon, this fatty body acts as a sound conductor and channels sounds directly to the bulla of the middle ear. Ligaments in the middle ear make the ear bones stiff and sensitive to high frequency sounds. Whale ears also have a higher number of nerve cells per hair cell, again making it more sensitive to high frequency, low level noises.

Continue on to Acoustic Accomplishments

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