Bats have the amazing ability to go from flying with their heads pointing forward to landing with their feet above their head, hanging down from branches and the ceilings of caves. Researchers studying bat flight using both slow-motion video and computer simulations reveal that their weighty wings help them swing their bodies upside down before landing. The findings were published in PLOS Biology last week.
Compared to the wings of birds and insects, bat wings are heavy for their body size: Skin, tendons, multiple independently controlled joints, several dense bones, and relatively large muscles increase their ratio of wing mass to body mass. This seems like it would be detrimental to aerial agility, yet bats reorient from a horizontal flying position to a heels-over-head roosting position with remarkable ease. Exactly how they’re able to generate the aerodynamic forces necessary to perform those maneuvers is a mystery.
So, a Brown University team led by Kenneth Breuer analyzed videos of bats landing in slow motion. They trained bats from two different species – three Seba’s short-tailed bats (Carollia perspicillata) and two Lesser dog-faced fruit bats (Cynopterus brachyotis) – to fly into an enclosure and land on a small piece of mesh attached to the ceiling. Three high-speed cameras captured the subtle wing maneuvers made just fractions of a second before they land – or attempt to land and recover from the subsequent fall.
Even at low speeds – when aerodynamic forces are small – the bats are able to reorient themselves using asymmetric wing beats. As they approach the ceiling, they retract one of their wings slightly toward their bodies, while flapping the other one at full extension. This asymmetric configuration helps them rotate a half turn, positioning their bodies to meet the ceiling feet first. By throwing extra wing weight around in very precise ways, bats generate inertial forces in order to reorient themselves – rather than rely on aerodynamic forces generated by pushing against the air.
The team then used simulations to confirm that the effect they were seeing was in fact due to inertia and not aerodynamics. They recorded the bats’ movements using motion capture, and then replayed the movements through a computer simulation that allowed them to turn the effects of various forces on and off. When they ran the simulation with aerodynamic forces switched off, the virtual bats were still able to recreate the motion of the real ones. But when they ran the simulation with the bat wings reduced to fruit fly proportions, the landing rotation wasn’t possible without aerodynamic forces.
Bats rapidly reorient their bodies during landing. Top row: Selected images from high-speed recordings of C. perspicillata executing a landing maneuver and, upon failing to find a landing site, executing a righting maneuver. Bottom row: Corresponding 3D reconstruction of flight kinematics. 2015 Bergou et al., PLOS Biology