Sunday, February 18, 2001		Nature

LIKE many species of bats, dolphins and their toothed whale relatives, too locate prey by echolocation. Now it is a established fact that dolphins can not only ‘see’ with sound, but can also kill their prey with it.

By bouncing high-frequency sounds off an underwater target and analysing the signal it gets back, a dolphin can not only pinpoint an object and determine whether and where it is moving, it can also differentiate between various object densities, for instance, fat from bone, and can also make out whether the target is dead or alive. It can stun and sometimes kill it with a high intensity beam of sound.

These sounds which are mostly in the form of clicks and bursts of clicks are produced by pushing air backwards and forwards through a complicated plumbing system at the back of the nostrils. The sounds pass through the fatty, bulbous ‘melon’ in the forehead, where they are focused into a narrow beam of ultrasound. The returning echo is picked up by the teeth of the lower jaw and the signal transmitted through the jaw and up to the brain. Using the system, a trained dolphin can distinguish a tangerine from a small metal ball at a distance of 370ft. In laboratory tests, dolphins have produced such powerful sounds that they are close to the finite limit of sound. Any louder and they would turn to heat. With such a weapon, dolphins are formidable predators.

They first use their sound system to locate schools of fish or squid, and then they spray them with high-intensity sound beams. Striped dolphins have been seen to circle an anchovy school, zap them with sound, and then cut through

The biggest of the toothed whales is the sperm whale, with an omnibus-sized body and an enormous fatty body in its forehead, known as the spermaceti organ. Sperm whales, like dolphins, produce echolocation clicks, and they go hunting in the deep sea. Their favourite prey seems to be giant squid, the world’s largest invertebrate that may grow 70ft long (measured from tentacle tip to body tip) and which have rarely been seen alive. Most specimens have either been washed up dead on the shore or found as pieces in the stomachs of sperm whales caught by whaling ships.

The curious thing, though, is that some whales have been caught that show malformations of the lower jaw, yet their stomachs are filled with squid. The theory is that the squids are zapped with enormous quantities of ultrasound and, thus neutralised, their inert bodies are slurped in by the whale. Tests have shown that squid can be killed with high-intensity blasts of sound, and researchers listening with underwater microphones to hunting sperm whales have reported that they produce very loud, rifle-like cracks that are thought to be salvos of killing sound. The large, saucer-sized, sucker marks on the heads of some sperm whales are witness to the giants who did not succumb to the sound bombardment and tried to fight back.