The ability of bats to land upside down has been shown to be due to their heavy wings, according to research conducted at Brown University in the US using high-speed imaging.
‘Bats land in a unique way,’ said Sharon Swartz, a biologist at Brown University who was a senior author of the research along with Kenny Breuer from Brown's School of Engineering. ‘They have to go from flying with their heads forward to executing an acrobatic manoeuvre that puts them head down and feet up. No other flying animal lands the same way as bats do.’
But exactly how they are able to generate the forces necessary to perform those manoeuvres hadn't been clear.
‘When they come in to land they're not moving very fast, which makes it hard to generate the aerodynamic forces needed to reorient themselves,’ Breuer said. ‘So the question is, how do bats get themselves in position to land?’
Using a special flight enclosure, high-speed cameras and some sophisticated computer modelling, the researchers showed that it has a lot to do with wing mass and inertia.
Credit: Breuer Lab/Brown University
Bats' wings are heavy, hand-like assemblages of bone, muscles, joints, tendons and skin. By throwing that extra wing weight around in very precise ways, bats generate inertial forces in order to reorient themselves, rather than relying on the aerodynamic forces generated by pushing against the air. It's similar, Breuer said, to the way high divers shift their weight to perform flips and twists, or the way cats reorient themselves to land feet-down when they fall.
The findings are published in the journal PLOS Biology.
For the study, led by Attila Bergou, a former postdoctoral researcher in Breuer's lab, the researchers trained bats from two different species to fly into an enclosure and land on a small piece of mesh fixed to the ceiling. High-speed cameras were able to catch a subtle wing manoeuvre that the bats make in the fraction of a second before they land.
The videos showed that as the bats approach the ceiling, they retract one of their wings ever so slightly toward their bodies, while flapping the other at full extension. With each wing beat in that asymmetric configuration, the bats rotate a half turn, helping to put them in position to meet the mesh feet first.
The researchers then used computer simulations to confirm that the effect they were seeing was due to inertia rather than aerodynamics. They used motion capture to record the bats' movements and then replayed those movements through a computer simulation in which the effects of different forces could be switched on and off. When the simulation was run with aerodynamic forces turned off, the virtual bats were still able to recreate the motion of the real ones.
‘What this tells us is that in bats, with their heavy wings, it's the inertial forces that are more important relative to aerodynamics,’ Breuer said. ‘That's a bit of a counterintuitive conclusion. Normally you'd think that an animal would not want to have such massive wings. But here, it turns out that the mass can be used to some benefit.’
Further information:
Falling with Style: Bats Perform Complex Aerial Rotations by Adjusting Wing Inertia
