Furry Logic: The Physics of Animal Life by Matin Durrani & Liz Kalaugher

Author:Matin Durrani & Liz Kalaugher [Durrani, Matin]
Language: eng
Format: azw3
Publisher: Bloomsbury Publishing
Published: 2016-10-06T04:00:00+00:00

Out to bat

Night-time in the parklands of an English stately home. A giant London plane tree towers above the sweeping lawns and the moonlight reflects silver from a river. It’s the kind of place you’d expect to see peacocks, or at the very least a ghost. The evening is mild and the air fresh with the scent of leaves and night-blooming flowers; moths are out in force too. From time to time the fluttering silhouette of a bat breaks the indigo of the sky. It twists and turns, vacuuming up insects.

But it’s dark. How can a fast-flying bat grab a speeding insect on the wing? It’s a feat more impressive than any circus trapeze act, where at least the grabbee wants to be grabbed. This nocturnal activity could be a hangover from a time 220 million years ago when bats’ mouse-like ancestors evolved from reptiles and wanted to avoid being eaten. Living in the dark meant they could keep clear of daytime-active dinosaurs. On the downside, it was hard to see where they were or where they were going. When they first took to the air, some of these early mammals developed a primitive system of firing pulses of ultrasound to learn more about their surroundings.

Not that these early bats knew what they were doing. ‘It probably started with a very basic analysis of echoes coming back, using sounds originally made for a different purpose, such as communication,’ reckons Holger Goerlitz, a bat expert at the Max Planck Institute for Ornithology in Bavaria, Germany. ‘It’s like when you go into a church or a cellar and you talk or make sounds with your feet, you can already hear that it sounds different, that it’s a big or small room.’

Bats create high-frequency pulses of ultrasound just as we talk and sing, vibrating their vocal cords and then ‘shouting’ the sound out through their mouth, or, for some species, their nose. By noting how long it takes the ultrasound to bounce back to them off trees, cave walls and other reflective surfaces, bats can suss out how far away these objects are. A wave that returns fast must have hit something nearby, while one that gets back later must have travelled further. Thanks to this ‘echolocation’, bats can navigate their way past obstacles.

Ultrasonic bat calls are among the loudest sounds in the animal kingdom. To avoid deafening themselves with their own noise, some bats contract muscles in their ears while they shout, to close them up. Others emit sound at a frequency they can’t hear – at least, not until the noise bounces back. For this nifty ear-protection, bats exploit the physics that sees the pitch of an ambulance siren rise as it approaches but fall as the vehicle races away. Known as the Doppler shift after Austrian physicist Christian Doppler (1803–53), the frequency goes up as the ambulance drives nearer because more sound waves must pack into a smaller distance. The frequency drops as the vehicle moves off because the waves are stretched out.

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