A Container Ship and Whales.
Photo: Chris Johnson
June 28, 2002
"Noise Pollution: Part I"
This is Roger Payne talking to you about noise pollution in the sea and what effect it may have on whales.
All whales spend their lives immersed in sound. Having evolved into ocean dwellers 60 million years ago, it was natural that sound would prove more useful than sight for perceiving things at a distance. As a result, some aspects of their ability to analyze sounds developed way beyond our own. Their ability to echolocate is an example. Today, whales and other marine mammals seem to be beginning to display vulnerabilities that are related to hearing-though we still don't fully understand just what is going on.
For example: There is increasing evidence that some beaked whales may have stranded as a result of loud sounds made by mid-range navy sonars. This has produced a great deal of concern about the possible deleterious effects of noise pollution on marine mammals. The noise level in the oceans is increasing at a substantial rate. It is believed that the amount of noise will only increase in the future as the number, type and level of noise from human sources increases. Shipping traffic is the main source of the increase, although many other sources such as sonar, seismic exploration, explosions, drilling, military maneuvers and overflying aircraft all contribute to noise pollution in the sea at frequencies that may be damaging to a whale's hearing.
Over the past five days, Odyssey had been been tracking a loosely associated group of sperm whales in an area of extensive shipping activity, the crew writes that they wonder how the noise may be affecting these whales. The ear bones of a cetacean (a whale, dolphin or porpoise) are the densest bone on earth. The reason is not fully understood. However, in order to hear, a whale, like every other marine animal, must capture the energy from a sound that would otherwise pass right through the whale's body without losing any significant energy and thereby registering its presence. That is because the bodies of fish and whales are made largely of water, so underwater sounds may travel right through them and out the other side without being significantly absorbed by any structures. In order to register a sound, there must be what is called an impedence mismatch somewhere inside the animal.
That is to say; for any animal to hear any sound, the listener's hearing apparatus must absorb some of the energy of the sound.
In fish this is usually achieved when water borne vibrations compress air in a fish's swim bladder-a kind of air sac.
This stretches the nerve endings attached to the air sac, which triggers nerve impulses in the fish's acoustic nerve.
Fish that live very deep must, and do, produce very high air pressures inside their swimbladders to keep the water pressure around them from collapsing the bladder. But such fish cannot change depth rapidly since to do so would cause fatal bends. Deep sea fish can surface only if they do so very slowly-equalizing the pressure in their swim bladders all the way to the surface and back down again. Since a whale must be able to dive deep and return to the surface rapidly air would be a poor choice for whales for mismatching impedances. However, there are other ways to solve this problem: for example: if part of the whale's hearing mechanism is made out of very dense bone-bone that is less compressible than water. In this case the sound wave compresses the tissue-moves it in relation to the bone.
There is also a possibility that the dense bone acts in the same way that the weight in a seismograph acts: the inertia of the weight holds it still while the earth around the weight vibrates. In the case of the whale
the inertia of its dense ear bone may hold the bone still while the whale vibrates around it. These relative motions would then move the stirrup bone of the middle ear which would stir the fluid in the inner ear, thus activating the whale's hearing receptor neurons to report the presence of a sound. This, I believe, is how a whale may hear sounds in the water.
However it works, it is clear that the cetacean ear, and brain are a triumph of adaptation.
Next time I'll have more to say about the threats to whales from sound now that you've heard a bit about how the ear of a whale works.
This is Roger Payne, wishing you peaceful days and quiet nights both in and out of water.
© 2002 Written by Roger Payne
Research by Genevieve Johnson