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Deep sea echo-sounding is modelled on bat behaviour

Posted in Animals, Biology, Nature, Science on Friday, 4 January 2013

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This edited article about nature originally appeared in Look and Learn issue number 807 published on 2nd July 1977.

Bats, picture, image, illustration


Cuttlefish, trapdoor spiders, bats and glow-worms are only some of the members of the animal kingdom which “discovered” scientific facts long before Man

Powered by its jet engines, a Concorde sweeps several kilometres high across the Atlantic at twice the speed of sound (about 2,240 km per hour at a height of approximately 10 kilometres above sea level). Concorde is one of the greatest achievements in modern transport, but the principle of the engines that give the aircraft its fantastic speed is older than man himself.

Countless millions of years ago, a group of creatures called cephalopods were jet-propelling themselves through now forgotten seas. The most common cephalopods today are the cuttlefish.

Next time you visit a large aquarium or marine zoo look out for a cuttlefish. You will notice that, though it manoeuvres with its fins, when it is surprised it appears to be jet-propelled and moves through the water in a series of darting motions.

The secret of the cuttlefish’s jet propulsion is that it sucks water into its body through a wide opening in its head, called the mantle. As the water is sucked in, muscles in the mantle contract, so reducing the space for the water.

Contracting the muscles increases the pressure at which the water is held in the mantle. Increasing the pressure on the water causes it to squirt out at high speed through a narrow opening in the mantle. This opening is called the syphon. In fact the action is rather like squirting soda water from a syphon.

The cuttlefish’s motion is based on the same principle as that of rocket propulsion. As expanding gases shoot from the tail of a rocket, the force known as reaction causes the rocket itself to move forward. The cuttlefish’s water jet produces the same effect.

Turbo-jet engines that power aircraft work in exactly the same way. The only difference is that the jet engine uses a stream of hot and expanding gases.

Air is sucked in through the front of the engine and passes to a device called a compressor. The compressor then forces the air at increased pressure into a space called the combustion chamber. In the combustion chamber the air is mixed with a highly inflammable fuel and becomes very hot.

Heating the air and other gases makes them expand. As the gases expand, they struggle for more space in the combustion chamber and try to escape. The only escape route is through a nozzle at the rear of the engine.

Just as the cuttlefish is moved by the jet of high-pressure water forced out of its body, so the aircraft travels forward because of the high-pressure gases streaming from the rear of the engine.

Jet propulsion is just one of scores of devices which we use every day but which were really in use by animals before man ever thought of them.

For example, every time you shut and bolt a door you are doing something that a species of spider has been doing for countless thousands of years. The spider, native to California, digs in the ground a burrow about three and a half centimetres in diameter and some 15 centimetres deep. The whole of the burrow is then lined with silk spun by the spider.

The top edge of the burrow is then carefully smoothed and bevelled so that it has a slight inward slope. Next the spider makes a kind of stopper from earth and silk web. The outer edge of the stopper is then exactly bevelled to fit tightly into the opening to the burrow.

Strands of silk are spun across one side of the stopper and the edge of the burrow to form a hinge. The top of the door or stopper is covered with the same material as the surrounding soil so that when the door is closed it is invisible.

The trap-door spider spends most of its time behind the door of its burrow waiting to detect the vibrations made on the ground by one of the crawling insects on which it feeds. Immediately a victim comes within range, the spider opens its door and seizes its prey. It then pops back into its burrow and closes the door after it.

The chief enemy of the trap-door spider is a species of large wasp. This insect often waits for the spider to come out and seize its prey and then tries to swoop on the spider before it can get behind the safety of its door.

Generally, however, the spider manages to get into its burrow and shut the door before the wasp catches it. Very often the wasp tries to force the door open. But the spider promptly “locks” its door by spinning a web across the door and burrow to serve as a kind of bolt.

Although one of the biggest of its tribe, the trap-door spider is quite harmless. In many parts of the U.S.A. it is kept as a garden pet.

Thanks to a technique called “echo-sounding”, map-makers are able to draw accurate charts showing the various depths of the oceans and the contours of the seabeds they cannot see.

Echo-sounding consists of a device fixed to the bottom of the survey ship on which the chart-makers are working. The device emits sound impulses which are reflected or echoed back by the seabed and the objects on it. It is much the same thing as when you shout into a tunnel and your voice is echoed back.

Scientists know the speed at which an echo travels back through water, and by calculating how long the echo takes to be picked up by a receiver on the ship, they know exactly how far under the water are the seabed and other objects echoing the sound from their transmitter. The echo receiver is connected to a TV-type screen which shows an echo picture of everything beneath the ship.

But the principle of echo-sounding was not “invented” by any scientist. Bats had been using it long before man appeared on earth.

Bats are creatures of the night, flying in search of the insects on which they feed and catching them in flight. For centuries naturalists had been puzzled as to how the bat does this and at the same time avoids any obstacle in its patch. Experiments proved that a bat can fly about in a pitch-dark room across which strings have been fixed from wall to wall without hitting a single strand.

It was the present age of electronics that finally solved the mystery of the bat’s early warning system that prevents it from flying into obstacles it cannot see.

By using delicate recording and measuring instruments able to pick up and register any kind of sound, scientists proved that throughout the time it is flying, a bat emits a continuous and rapid succession of high-pitched squeaks. The squeaks are so high-pitched that they cannot be heard by the human ear.

When flying on a course without anything in front of it, the bat squeaks about 30 times a second. But immediately the little creature approaches anything the number of squeaks increases to as many as 100 a second.

The sound of the squeaks is reflected or echoed back by the object with which they come into contact. The echoes of the squeaks are then picked up by the bat’s large and very sensitive ears. This warns the bat that there is something in front of it.

The time lapse between the animal squeaking and the sound being echoed back to it occupies only a tiny fraction of a second. But that time is enough for the bat to take action and avoid flying into an object.

Not only does the bat’s echo-sounding system prevent it from running into obstacles, but it also enables it to detect the presence of any one of the many insects on which it feeds. That explains why a bat can swoop so quickly in the dark on the insects it catches.

Instruments used to detect and measure the bat’s squeaks prove that the sounds are emitted by the animal as a narrow beam of sound impulses through the bat’s complicated nose structure.

Bats rely entirely on their echo-sounding when landing after a flight. They always “touch-down” head uppermost after skimming to the landing surface.

Long before the first cowboy roped his first steer, the mastaphora spider of America had been lassooing the flies on which it feeds.

The mastaphora begins operations by spinning a long loop of silk from the branch of a tree. Then, hanging from the centre of the loop, it spins a single thread which dangles suspended from its body.

Next it pushes down the thread a blob of the liquid from which the silk is spun. The blob hardens into a ball at the end of the thread.

Holding on to the loop line with its hind legs, and clutching the weighted line with one of its front legs, the mastaphora waits for a fly to approach. Immediately the victim comes within range, the spider throws the weighted thread forward and the line wraps around the fly. A second later, the mastaphora darts forward and gives its catch a paralysing bite. The fly is then bound up with silk thread and carried off to the spider’s larder.

The gaucho, or South American cowboy, got the idea of his bolas from the mastaphora spider. A bolas consists of a hide rope with a stone at each end. In the centre of this rope there is a much longer one.

The gaucho fastens the free end of the long rope to his saddle. When he wants to bring down a steer for roping, the gaucho swings the other end of the rope with its stones joined by the short rope around his head. Then he lets go and the stones joined by the short rope tangle round the legs of the steer and bring it down.

Next time you are on the road at night and see the red tail-lights of a vehicle ahead, think of the humble glow-worm. For it was the glow-worm that invented and first used a tail-light.

The bright, flashing greenish-white light of the glow-worm is produced by a chemical and is emitted from the rear of the creature’s body. Both the male and the female produce the light, but that from the female is much stronger and brighter. The female has no wings and so cannot fly, and she uses her light to attract to her the male, who can fly.

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