by Anders Hedenström
Flight in animals is energetically very costly, but because of its speed it can result in an overall relatively low cost of transport that allows birds, bats and insects to perform seasonal migrations. However, economic transport requires speeds where flight efficiency is high, allowing the wings to generate aerodynamic forces of high lift and low drag. A lot of research related to flight ecology has therefore concentrated on aerodynamic performance by animals, whereby popular models of animal flight often are applied to birds and bats as if they are interchangeable objects, despite their rather distinct flight morphologies (Hedenström et al., 2009).
For the PLOS Biology 15-year anniversary I have chosen to highlight a paper by Bergou and co-workers (Bergou et al., 2015) that focuses on inertial forces deployed by bats during their acrobatic approach to landing head-over-heels as bats usually do. In this study Bergou at al. investigated how bats execute this acrobatic manoeuvre, exploiting asymmetric morphing of the wings to set up a torque that causes the body to rotate. To land on a surface the bat needs to get a grip with their tiny feet, which are interconnected to the wings via the inner wing membrane (the plagiopatagium) and therefore quite immobile. Because the bat has to slow down just before initiating the somersault, this prevents the use of aerodynamic forces – these are small anyway at such low speeds, and the proximity to the landing site prevents vigorous flapping.
Previous observations had showed how bats do it, but to dig further into how inertial forces replace aerodynamic forces, Bergoud et al. also analysed a simple model – a bat with rectangular wings and simplified kinematics, but still capable of generating manoeuvres similar to those seen in the real animal. The model was also extended to a “fully articulated model” with similar results.
The key to the inertia-related manoeuvres in bats is their relatively heavy wings, making it possible to set up a torque through asymmetric wing morphing. This may not possible for other flying animals such as birds and insects, where wings are lighter than in bats. Another reason why inertia-related manoeuvres may not be useful to flies and hummingbirds is that they have also relatively stiff wings that prevent excessive morphing.
This paper shows how complicated tasks such as everyday landing manoeuvres in bats consist of a rapid transition from aerodynamic-based cruising flight to an inertia-based somersault in free-fall under the ceiling. The study may also shed new light on our understanding of the evolution of flight, by investigating how incipient flight manoeuvres in a proto-flyer can be executed by using forelimbs with little or no aerodynamic lift.
Bergou AJ, Swartz SM, Vejdani H, Riskin DK, Reimnitz L, Taubin G, Breuer KS (2015) Falling with style: bats perform complex aerial rotations by adjusting wing inertia. PLOS Biology 13(11): e1002297. https://doi.org/10.1371/journal.pbio.1002297
Hedenström A, Johansson LC, Spedding GR (2009) Bird or bat: comparing airframe design and flight performance. Bioinspiration & Biomimetics DOI:10.1088/1748-3182/4/1/015001 (13pp)
Featured image Credit: Flickr user Shellac
Anders Hedenström image credit: Susanne Åkesson