When the mammalian ancestors of whales began to move from land and back into the sea they were faced with huge problems. A major difficulty was that they had lost the evolutionary advantages necessary for a successful aquatic life; underwater vision was difficult, they were poor swimmers, and they were easy prey for the huge sharks that inhabited the ocean from 50-75 million years ago. In fact, sharks may have been their greatest problem since sharks had already perfected incredible senses of smell, sound detection, and the ability to receive the invisible electrical fields produced by other animals. Not only were the sharks more efficient fishermen but large sharks could detect, out-swim, and then capture ancient whales.
Many whales solved this problem with the help of evolution by becoming so large that even large sharks couldn’t eat them. They also stopped competing with sharks for food by changing to a plankton diet. Their descendants are the baleen whales of today: examples are the Right Whale, Gray Whale, and Humpback Whale.
Other whales evolved to compete with the sharks. They retained their teeth and fish diet but became faster and developed an incredible sense of echolocation which allowed them to ‘hear’ the approach of huge sharks long before they could be seen.
Because sound travels through water better than light does, the ability to make a sound and then interpret the ‘meaning’ of its echo, allowed the toothed whales to find food when sharks couldn’t and avoid predators before they could get too close. Whale echolocation today may be the most sophisticated sensory system in the entire animal kingdom. Unlike our own vision, echolocation for whales carries three dimensional information. Toothed whales can ‘see’ inside and through many objects and reflected sounds seem to allow them to ‘see’ around or behind things.
The information that can come back by echo depends on the frequency of the sound. Low frequency travels long distances and has less detail while high frequency is shorter range with high definition. To get the whole picture some whales ‘sweep’ the frequency range between high and low frequencies. When they home in on prey these ‘sweeps’ sound like a continuous creaking sound.
When humans send out radio signals for locating things (radar), we focus the signal into a beam using a specially shaped antenna. Toothed whales do the same with sound using special fat deposits in the top of their heads and in their jaws. This fat is different from the other fat deposits in the whale’s body and fits into specially shaped areas in the jaw and skull. We use facial muscles to frown or smile but whales seem to use their muscles to adjust the shape of the fat deposit and focus the sound beam.
These special fat deposits are most remarkable in the sperm whale where they may weigh several tonnes. In dolphins the deposit looks like a rounded lobe on the front of the head and is called the melon.
Kind of Whale
|
Sound
|
Broadband level (dB)
|
Sperm whale
|
clicks
|
163–223
|
Beluga whale
|
echolocation clicks
|
206–225
|
White-beaked dolphin
|
echolocation clicks
|
194–219
|
Spinner dolphin
|
pulse bursts
|
108–115
|
Bottlenose dolphin
|
whistles
|
125–173
|
We still don’t know much about all this whale engineering but it seems to work like this:
- A powerful sound is generated by ‘vocal cords’ (phonic lips) within the whale
- As the sound radiates out, the melon focuses it like a lens focuses a beam of light
- The sound beam hits an object in the sea and is reflected back
- The teeth in the lower jaw act as ‘antennas’ collecting the echoes
- Fat deposits in the lower jaw carry the sound to the inner ears
- The complex brain interprets the echoes and constructs a ‘picture’ of the object
In this way a whale may see a picture with sound similar to the picture we see with light. It is clear though that the picture is only as good as the information processor that untangles the complex echoes coming back to the whale. To see with sound, whales have also evolved very large brains.
Many people believe that the large brain means that dolphins in particular are the philosophers of the sea. The truth is that a large part of that brain seems to be used for processing and remembering echo information necessary for feeding and navigation and relating to other whales with their own acoustic information.
The ancestors of whales had ears for hearing on land. Whales still have ear openings that lead to the inner ear. In baleen whales these openings are filled with a hard wax but in the toothed whales the hole is open. There is evidence that dolphins can hear in air and water with these openings but they probably play a very small part in echolocation.
The baleen whales are not hunters like the toothed whales and have not developed the elaborate clicks and whistles to ‘sweep’ their prey. It has been suggested that baleen whales may use their stored echo-soundings to create complex three-dimensional maps of the ocean floor thousands of meters below for use in their long annual migrations covering tens of thousands of kilometers.
Illustrations are from Wiki-commons.
