Profitable Wonders
Besides elegantly wielding his bat at the crease, former England Captain David Gower is a long-standing admirer of the other, flying version.
He has been championing the bat’s cause, countering the reputational damage from recent events in Wuhan. ‘One cannot help but be fascinated by these gorgeous, delicate creatures,’ he wrote in the Spectator. He described a foray at dusk on the banks of the River Test, ‘just about following their captivating flight’, as they swooped over the water, hoovering up mosquitoes and other pestiferous insects. ‘Bats are indeed our friends,’ he wrote – and on several counts, too.
Every evening, as the sun sets, a long, black column of millions of free-tailed bats emerges from the dark mouth of the Bracken Cave in Texas, spinning like a tornado across the sky. They are heading for their hunting grounds, 10,000 feet up in the stratosphere, where they will spend the night pursuing and consuming legions of migrating, egg-bearing moths – 150 tons of them – that would otherwise decimate the cotton, corn and wheat fields of the North American plains.
Meanwhile, several hundred miles to the south, nectar-loving bats are performing the similarly meritorious service of pollinating night-flowering desert plants. ‘At one in the morning, I heard the familiar wingbeat of an approaching long-nosed bat,’ recalls bat biologist Merlin Tuttle of one nocturnal vigil. Sweeping up from below, it thrust its long snout deep into a funnel-shaped cordon flower, and then three more in quick succession – by when its head, dusted with luminous pollen, ‘looked as if it had been dunked in a bag of flour’.
Around the world, numerous flowering species rely on bats for their successful propagation – bananas, guavas, mangoes, the tequila-yielding agave plant, durian fruit trees in SouthEast Asia and the African ‘Tree of Life’, the baobab.
So bats are very useful – but strange and mysterious, too, possessed it seems of unearthly powers that can be described but only dimly understood. One might reasonably wonder why or how, 60 million or so years ago, a small, terrestrial shrew-like creature took to the air to become the only mammal capable of powered flight.
One might wonder but cannot know, as there are no intermediary transitional forms, the earliest, complete, fossilised bat being instantly recognisable for what it is.
Still it is possible to infer the many anatomical adaptations required by standing with arms raised to shoulder height, hands open and palms forward. In bats, the fifth fingers (or little fingers) are enormously elongated, reaching almost as far as the ankle. The fourth and third are almost as long. The second finger forms the top surface of the wing, with a residual, clawed thumb pointing upwards. An ultra-thin membrane of living skin, criss-crossed with nerves, blood vessels, sensory cells and elastin fibres, stretches across the intervening space.
These wings, powered by a unique array of chest and back muscles, push against the air, rowing the bat forward and generating two sets of vortices at their tips that provide both lift and thrust.
The practicalities of how the bat controls the complicated aerodynamics of flight remain largely uncharted territory, though recent experiments have demonstrated how neural signals generated by tiny hairs covering the membrane transmit information about wind speed and direction.
Bats’ further impressive attribute is, of course, their ability to ‘see’ with their ears in the pitch-black dark. Several other species have this facility of ultrasonic echolocation, but none as sophisticated or as discriminatory, capable of detecting a moth sitting quietly on a leaf and flexing its wings.
Two details merit emphasis. The first is the inverse square law, whereby the ultrasonic sounds emitted through the larynx are attenuated, both when radiating outwards and on their return, losing – over a distance of 30 feet, for example – 96 per cent of their power.
Hence the bat’s echolocating signals, though silent to us, must be deafeningly loud, comparable to a smoke alarm. But the bat is not deafened because, a millisecond before the signal is emitted, muscles disconnect the three minuscule bones in the middle ear that transmit sound.
Secondly, with bat colonies in the millions, there is the obvious danger that the cacophony of echolocating signals might become confused. But no, each bat has its own frequency and combination of pulse types which it alone recognises, discounting those of others. Astonishing but true.