Dolphins are fatheads - and that's how they hear

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DOLPHINS HEAR, despite their lack of external ears, because they are "acoustic fatheads". They use the fat inside their heads to focus sound waves - a skill that makes up for their lack of outer ears.

The discovery was made by Darlene Ketten of the Woods Hole Oceanographic Institution in Massachusetts, who used advanced computer tomography and magnetic resonance imaging systems to build up a picture of how dolphins hear.

Dolphins let out a constant stream of supersonic sounds, such as clicks and high-pitched squeals, which they use in the same way as sonar. The echoes of the sounds are processed by their inner ear - which has twice as many receptors as the human ear - to give them an auditory picture of what objects lie in the ocean around them. But because they often dive at high speed and to great depths, a normal eardrum (such as in bats, which also use echo location methods) would be a liability rather than an asset. Hence dolphins have smooth heads with just pinpricks for ears.

Scientists have known for years that deep inside dolphins' heads are structures called the auditory bulla, containing the dolphin's inner ear. It is encased in a separate bone, connected to the skull by fibrous tissue. The organs in the bulla do the same task as humans' middle and inner ears, where sounds are translated into nerve impulses for the brain.

Scientists also knew that dolphins had an unusual amount of fat in their heads and faces compared with the rest of their bodies, and suspected that it might somehow conduct sound - but were unsure how.

Dr Ketten paid particular attention to the bundles of fat in the dolphins' lower jaws and ear canals - and, by using magnetic resonance imaging scans, discovered that "they have a shape like an ear trumpet".

It is that which helps to channel the huge range of sounds into the bulla, she revealed at a meeting for the Society of Marine Mammalogy last month, reported by New Scientist magazine today.