How Do Bats Fly? Hands Made of Wing and the Physics of a Membrane in the Air
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A bat's wing is a hand. The four fingers are as long as the animal's body, thin as wire, and joined by a membrane of skin two layers thick that runs from the fingertips to the ankles, and the thumb, short and clawed, sticks out at the leading edge to hang from and to crawl with. Birds fly on a stiff arm feathered into an aerofoil; bats fly on a stretched, living sheet that they can change shape at every joint a hundred times a second, and the result is the most manoeuvrable flight of any vertebrate, in the dark, after insects that are trying to escape. A quarter of all mammal species are bats, and they got into the air by a route nothing else has taken.
The wing
The membrane, the patagium, is skin with a layer of elastic fibres and thin muscles embedded in it, supplied with blood vessels and nerves and covered in tiny hairs that sense the airflow; it can be tightened or slackened by the muscles within it and by the fingers, so that the wing's camber, its curvature, changes through each stroke. The fingers spread the membrane and the wrist and elbow fold it, and the whole wing collapses against the body when the bat lands, which it does head down, hanging by the claws of its feet from a locked tendon that needs no muscle to hold. The bones are thin and, unlike a bird's, not hollow; a bat is heavier for its size than a bird, and it flies on more wing area rather than lighter bones.
How the stroke works
A bat flaps with its arms and hands, and the stroke is not a simple up and down:
- •On the downstroke the wing is spread wide and cambered, pushing air down and back to make lift and thrust, as a bird's does
- •On the upstroke the wing folds partly, and in many bats the hand flicks so that the wingtip generates lift on the way up as well, which birds mostly cannot do
- •The membrane billows and stiffens under load, and the fingers adjust its shape moment by moment, so that each stroke is tuned to the speed and the turn
- •The trailing edge and the membrane between the legs act as a rudder and a brake, and the tail membrane scoops insects toward the mouth in flight
Why it is so agile
A bird's wing has a few controllable joints at the shoulder, elbow and wrist and a fixed shape between them; a bat's has those and every finger joint as well, more than two dozen independently controlled joints per wing, and a membrane whose stiffness it can set. That lets a bat turn in less than its own body length, hover, fly backward, and roll and recover in ways that the wind-tunnel studies of the 2000s, using high-speed cameras and smoke, found were producing vortices and forces that no aircraft could manage. The cost is speed and efficiency: a bat cannot glide as far or fly as fast as a bird of its size, and the fastest, the Brazilian free-tailed bat, reaches about 160 kilometres an hour in a dive, which is quick, while a swift cruises at that speed. Most bats do not need speed; they need to catch a moth that is jinking, in the dark, on the sound of their own echoes.
How bats got there
The oldest bat fossils, from Wyoming and Germany about 52 million years ago, are already bats, with full wings, and the earliest, Onychonycteris, had claws on all its fingers and an inner ear that suggests it did not yet echolocate, which means flight came before sonar. What came before the wing is unknown, since no half-bat has been found, and the likeliest ancestor is a small, nocturnal, tree-dwelling insect-eater that glided on skin flaps, as flying squirrels and colugos do today, and whose gliding membrane came to be flapped. The genes that lengthen a bat's fingers are the ones that pattern every mammal's hand, switched on longer, and a bat embryo's hand looks like a mouse's until the fingers keep growing. From that beginning came 1,400 species, from bumblebee bats weighing two grams to flying foxes with a wingspan of 1.7 metres, in every habitat except the poles.
What the flight is for
Bats eat, by one estimate, thousands of tonnes of insects a night across the world, including the moths, beetles and mosquitoes that damage crops and carry disease, and the value of that to farming has been put in the billions; the fruit bats pollinate and spread the seeds of bananas, mangoes, durians and the agave that makes tequila; and the vampire bats, three species in the Americas, drink blood and have given medicine an anticoagulant. The membrane that does all this is also fragile, tearing on wire and healing fast, and it is why bats are studied by the engineers designing drones that must fly through forests and rooms, which a rigid wing cannot do and a hand can.
The takeaway
Bats fly on wings that are elongated hands with a living membrane of skin and muscle stretched between the fingers and down to the ankles, whose shape they adjust at every joint through each stroke, generating lift on the upstroke as well as the down and turning in less than a body length at the cost of speed. They descend from a gliding, tree-dwelling insect-eater and were flying, before they could echolocate, 52 million years ago, and their 1,400 species now eat the insects, pollinate the fruit and inspire the drones of the world.