A          Bats have a problem: how to find their way around in the dark. They hunt at night, and cannot 

            Use light to help them find prey and avoid obstacles. You might say that this is a problem of their

            own making, one that they could avoid simply by changing their habits and hunting by day. But

            the daytime economy is already heavily exploited by other creatures such as birds. Given that

            there is a living to be made at night, and given that alternative daytime trades are thoroughly

            occupied, natural selection has favoured bats that make a go of the night-hunting trade. It is 

            probable that the nocturnal trades go way back in the ancestry of all mammals. In the time

            when the dinosaurs dominated the daytime economy, our mammalian ancestors probably only

            managed to survive at all because they found ways of scraping a living at night. Only after the 

            mysterious mass extinction of the dinosaurs about 70 million years ago were out ancestors able

            to emerge into the daylight in any substantial numbers.

B          Bat have an engineering problem: how to find their way and find their prey in the absence of

            light. Bats are not the only creatures to face this difficulty today. Obviously the night-flying

            insects that they prey on must find their way about somehow. Deep-sea fish and whales have

            little or no light by day or by night. Fish and dolphins that live in extremely muddy water. Plenty

            of other modern animals make their living in conditions where seeing is diffcult or impossible.

C          Given the questions of how to manoeuvre in teh dark, what solutions might an engineer

            consider? The first one that might occur to him is to manufacture light, to use a lantern or a 

            searchlight. Firefiles and some fish (usually with the help of bacteria) have the power to 

            manufacture their own light, but the process seems to consume a large amount of energy.

            a male's tiny pinprick of light can be seen by a female from some distance on a dark night, since

            her eyes are exposed directly to the light source itself. However, using light to find one's own 

            way around requires vastly more energy, since the eyes have to detect the tiny fraction of the 

            light that bounces off each part of the scene. The light source must therefore be immensely

            brighter if it is to be used as a headlight to illuminate the path, than if it is to be used as a signal

            to others, In any event, whether or not the reason ts the energy expense, it seem to be the

            case that, with the possible exception of some weird deep-sea fish, no animal apart from man

            uses manufactured light to find its way about.

D          What else might the engineer think of? Well, blind humans sometimes seem to have an uncanny

            sense of obstacles in their path. It has been given the name 'facial version', because blinde people

            have reported that it feels a bit like the sende of touch, on the face. One report tells of a totally

            blind boy who could ride his tricycle at good speed round the block mear his home, using facial

            vision. Experiments showed that, in fact, facial vision is nothing to do with touch or the front of

            the face, although the sensation may be referred to the front of the face, like the referred pain

            in a phantom limb. The sensation of facial vision, it turns out, really goes in through the ears.

            Blind people, without even being aware of the fact, are actually using echoes of their own

            footsteps and of other sounds, to sense the presence of obstacles. Before this was discovered,

            engineers had already built instruments to exploit the principle, for example to measure the

            depth of the sea under a ship. After this technique had been invented, it was only a matter of

            time before weapons designers adapted it for the detedtion of submarines. Both sides in the 

            Second World War relied heavily on these devices, under such codenames as Asdic (British) and

            Sonar (American), as well as Radar (American) or RDF (British), which user radio echoes rather

            than sound echoes.

E          The Sonar and Radar pioneers didn't know it then, but all the world now knows that bats, or

            rather natural selection working on bat, had perfected the system tens of millions of years

            earlier, and their radar' achieves feats of detection and navigation that would strike an engineer

            dumb with admiration. It is technically incorrect to talk about bat 'rader' , since they do not use

            radio waves. It is sonar. But the underlying mathematical theories of rader and sonar are very

            similar, and much of our scientific understanding of the details of what bats are doing has come

            from applying rader theorhy to them. The American zoologist Donald Griffin, who was largely

            responsible for the discovery of sonar in bats, coined the term 'echolocation' to cover both

            sonar and radar, whether used by animals or by human instruments

how early mammals avoided dying out