How do dolphins use sound?

In 1953 the underwater explorer Jacques Coustea (1910-1997) observed that when his boat was heading along a deep channel in murky water, porpoises would follow. If he steered off course, the porpoises also steered off course but then quickly went back to following the channel. He thought that the porpoises could detect the channel (the best route to take) without being able to see it. Other scientists had also come up with this theory. 

In 1960 Kenneth Norris (1924-1998) showed that this theory was correct. He put suction cups over the eyes of a dolphin and found that it could still find its way through a maze. Since then, many scientists have become interested in the different high-frequency ‘clicks’ and lower-frequency ‘whistles’ that these creatures make. 

Today we know that dolphins produce clicks using nasal sacs. The sound reflect off objects and are picked up again in the dolphin’s lower jaw. The reflected sound waves travel through a channel filled with fat and then into the ear. Like light waves, sound waves can be focused, and dolphins have a ‘lens’ made of fat that they use to focus sound waves. 

Sound waves that have a frequency above 20000 Hz (the upper limit of human hearing) are called ultrasound. Ultrasound waves are used in hospitals to look inside people – a so-called ultrasound scan, which most pregnant women have. Some dolphins seem to be able to find fish that are well-buried in sand and this has led some scientists to suggest that dolphins can also use ultrasound to see things in the same way that an ultrasound scanner does. However, dolphins are only known to emit sound waves with frequencies up to about 150 kHz whereas ultrasound machines use of waves of 1-20 MHz. 

In 2006, a team from St Andrew’s University recorded the whistles made by dolphins and played them back to the dolphins. They discovered that each dolphin had its own ‘signature’ whistle by which other dolphins recognized them. Whether dolphins actually have a language is the topic of a lot of research. Liz Hawkins, of the Whale Research Centre in Australia, spent three years listening to bottlenose dolphins and by 2007 had identified 186 different whistle types.