When the evenings get particularly thick with mosquitoes where I live, I sometimes sit out in the yard with my daughters and look up at the fading sky. Before too long, a single bat will usually flit out of the nearby trees and start flying circles around the house, scooping up bugs along the way. We can barely make out the bat’s wings as it takes its laps, a flicker of membranes. And so it was a revelation to spend some time earlier this week with two Brown University biologists, Dan Riskin and Sharon Swartz, watching slow-motion movies of bats in flight. There’s a lot going on up there.
Bats evolved about 50 million years ago from squirrel-like ancestors. They probably made their first forays into the air as gliders. Like living gliders, they used flaps of skin to increase their surface area, letting them glide further. Their hands evolved long spindly fingers that were joined by membranes. Some early bat fossils suggest that they may have shifted from gliding to alternating between gliding and bursts of fluttering. Eventually bats evolved sustained powered flight.
Bats evolved a way to take advantage of the same laws of physics birds use to fly. And many scientists who have studied bat flight in the past have basically treated bats like leathery birds. Yet there’s no reason to assume that this should be so. After all, it would not be surprising to find that the way the feathers on a bird’s wing react to air pushing against them are different from the way the stretchy membranes on a bat react. Birds don’t have wing surfaces connecting their front and back legs, like bats do. And while birds only have a couple joints in their wing skeleton, such as at the elbow and wrists, bats have lots of knuckles they could, in theory, bend selectively to alter their wing surface. Bats also have lots of sensitive hair cells on their wings that appear to track the speed and direction of the air flow, and the information they get from the hairs may help them make fine adjustments to their wings many times a second.
And when scientists like Swartz and Riskin study bats, they discover, in fact, that bats are not birds. Bats fly more slowly than birds, but they maneuver more effectively. Bats fly cheap compared to birds. A hovering bat use 60% less energy than a hovering hummingbird. These sorts of discoveries suggest that if you’d like to make an agile, efficient, and tiny flying robot (and who doesn’t?) it might be worth looking for some inspiration in bats.
The problem with looking to bats for inspiration is that scientists are only starting to figure out bat aerodynamics. What’s really challenging to figure out, however, is the difference between the aerodynamics of birds and bats. Riskin and Swartz use lots of tools to find the answer. They paint bright dots on bats and then film the animals as they fly in wind tunnels. The biologists can then use computers to create models of the bat wings and calculate the speed and direction of each dot at each instant of flight. They can spray mist into a tunnel and then film the swirls the bats leave in their wake. From this data on real bats, bat researchers can then test out simulations on computers to see if they produce the same forces and swirls of air as they see in their wind tunnels.
A close look at these movies reveals that bat flight is just too complex for simple labels, like upstroke and downstroke. The shoulder of a bat starts rotating upwards before the wrist, which move up before the fingers. The fingers on each hand don’t move in sync with each other. A joint on the left wing is often out of sync with the corresponding joint on the right wing.
Physicists like to treat wings as rigid surfaces because the math involved causes fewer headaches. But that’s a gross simplification when it comes to bats. The bones in a bat’s hands are surprisingly flexible, and the skin of the bat wing is never fully stretched out during its stroke. In fact, the region of the wing close to its body actually balloons out to double its surface area during each flight stroke. Bats probably use this ever-changing wing surface to control their lift and drag, so that they can make tight maneuvers without stalling.
Bats have clearly evolved a sophisticated flight system, but they face some awkward challenges when they’re not flying. Birds only need two limbs for flying, leaving their remaining two relatively free to land and walk around on the ground. Bats, on the other hand, make their hind legs part of their wings, and so natural selection has to strike a compromise between several different functions. And while birds can stop flying by using their feet to land on the ground, most bats have to use their feet to hang upside down.
To figure out how bats manage this feat, Riskin turned the typical biomechanics lab upside down. Scientists can measure the forces of a running animal by putting a force-sensitive plate on the floor; Riskin put his on the ceiling. In his recordings of landing bats, he’s discovered two strategies. In one species that lives in caves, the bats make an elegant backwards flip combined with an upside-down cartwheel, so that they can land with just two feet.
In a species that hangs from trees branches, the bats use a very different technique. They swoop in without a cartwheel, and bring both feet and both hands upward to grab onto the tree. And they hit the tree hard. The cave bats land with a force that’s twice their body weight; the tree bats generate forces as high as eleven times their body weight.
This discovery (published this week in the Journal of Experimental Biology) illustrates an important fact about bats–a bat is not a bat is not a bat. Bats live in many environments and are adapted to eating many different kinds of food, from moths to fruit to cow blood. They’ve adapted to these different ways of making a living, in part by evolving different ways of moving around. If you’re a bat flying towards a wall of rock, you don’t want to hit it too hard. But if you can grab a branch that can absorb the shock, you can skip the fancy acrobatics.
That same lesson emerges from how bats behave on the ground. With their delicate legs yoked together by their wings, you might expect that bats don’t do very well on the ground. And indeed, most species won’t win any track and field medals. When Riskin puts a typical bat on a treadmill, they stumble around. If the treadmill goes too fast they start to lose all control. It’s likely, then, that the ability to walk efficiently and to run was lost in the early evolution of bats. But millions of years later, that ability evolved once more in at least two species.
One place where bats have taken to the ground again is New Zealand. The remarkable isolation of New Zealand left it without big predators and without any mice or other ground-dwelling mammals. One species, the New Zealand short-tailed bat, has adapted to this niche. While it can still fly, it now moves around comfortably on the ground in search of bugs, nectar, fruit, and pollen.
Riskin found that New Zealand short-tailed bats walk comfortably on a treadmill, using the same pendulum-like movements that other walking mammals use to save on energy. But when other mammals have to move faster, they break into a run so that they can store extra energy in their tendons as they hit the ground. The New Zealand short-tailed bat can’t make the transition from walking to running.
But another species of bat can make that switch. A vampire bat will walk on the ground to sneak up on its victim. If its victim tries to get away, it can scramble in pursuit. Riskin found that if he put vampire bats on treadmills, they can walk like New Zealand short-tailed bats. But when he speeds up the treadmill, they suddenly switch to a bizarre form of running. Instead of pushing off with their hind legs, like a squirrel, they use their long, heavily muscled arms. It’s a mammal version of front-wheel drive versus rear-wheel drive.
The difference between the two species of ground-moving bats is not surprising when you consider where they live. Bats on New Zealand didn’t pay any cost for evoling into slow walkers, because life was pretty easy (at least before humans showed up with their rats and other assorted camp followers). But vampire bats evolved in a more competitive environment where they had to adapt to moving prey.
Once bats evolved flight, in other words, they did not stop evolving. Their movements have been changing in astonishing ways for millions of years, and will continue to change as long as bats fly, walk, or run across the Earth.