The work could help researchers improve robotic control algorithms and prosthetics for humans, and it also provides insight into how bats evolved to maneuver so well in the air. Beyond this, the findings offer fresh appreciation for how specialized bat legs and wings are, and how they work in concert to enable this unique landing strategy.
The research is published in the Journal of the Royal Society Interface.
"Landing is a really difficult problem in general, especially when an animal has wings instead of legs, but bats have evolved a really elegant way of managing it," said Kenneth Breuer, assistant professor of engineering and co-corresponding author of the paper.
Landing is particularly tricky for bats because their flight is highly maneuverable. They can perform tight turns and rapid accelerations, but to pull off such moves, they must store a lot of energy while flying, typically in the form of kinetic energy. When landing, they have to somehow rapidly dissipate that energy, all while touching down softly enough to avoid injury.
"If a bat comes in for a landing with a lot of energy, and doesn't have some way to lose it, it could smash itself," said Breuer.
To observe bats performing their landings in lab conditions, the team built a custom flight arena. The arena is an enclosure about 2.5 meters long, with walls, floor and ceiling lined with motion capture cameras. When a bat enters the arena, the cameras capture its movements, providing data that the team uses to calculate the animal's body orientation, wing and joint angles, and linear and angular velocities at different time points.
The team worked with two big brown bats, Eptesicus fuscus. Brown bats are fairly common in North America and Europe, hunting insects and roosting in caves, buildings and trees. The researchers trained the bats to enter the flight arena and land on a landing pad equipped with a force sensor that measured the impact force of landings.
The results show that bats make ground contact with their ankles, and that they dissipate their flight energy by absorbing the landing force in their leg joints and by slowing their wing flaps. The wings actually never make contact with the ground.
"Their wings are so delicate and thin, making contact would likely damage them," said Breuer.
Instead, bats use their wings to slow their fall prior to landing and control the orientation of their bodies. Once their ankles touch down, the bats then absorb the remaining energy by bending their legs.
The findings help explain why bat legs and wings are so different from those of other mammals. Their legs, with long shin bones and short feet, are well-suited for absorbing shock, and lack features like claws that might get in the way of executing their landing maneuver. Their wings, on the other hand, are incredibly thin, with reduced muscle mass and bone density, which reduces their weight but also makes them more susceptible to damage. To compensate for this fragility, the bats have evolved to avoid ground contact with their wings by expertly absorbing impact forces with their legs.
The researchers believe the detailed account of bat landings is a good basis for future work aimed at improving robotic control algorithms for landing vehicles and prostheses. Understanding the mechanics of the bat landing might also help researchers understand how bats perform other behaviors, like hanging upside down from walls or trees.
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