Fig 1.
Apparatus for motion capture of landing bats.
Videos are captured using three high-speed cameras (A) equipped with 50 mm lenses (C), at a frame rate of 1,000 frames per second. A uniform background (B) made of heavy-duty paper is placed behind the bat (D) to enhance contrast and visibility. The landing pad (E) is removed in order to generate a falling-and-recovery maneuver.
Fig 2.
Bats rapidly reorient their bodies during landing.
(Above) 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. (Below) Corresponding 3-D reconstruction of the 52-degree-of-freedom flight kinematics. The images from left to right correspond to t = 0.185 s, 0.26 s, 0.335 s, 0.41 s, and 0.485 s (also, see Fig 3). To give a sense of scale, C. perspicillata have a characteristic tip-to-tip wingspan of approximately 30 cm. Tracked video data available in file tracked_data.zip from the Dryad Digital Repository, http://dx.doi.org/10.5061/dryad.21qs5 [31].
Fig 3.
Changes to a landing bat’s body orientation, roll, ψ, pitch, θ, and yaw, ϕ, are prompted by pronounced changes in its wing kinematics.
The top two frames indicate body orientation and angular velocity (about body-fixed axes). The lower three frames show simplified wing kinematics: instantaneous wing extension, e, wing stroke angle, ϕw, and protraction-retraction angle, θw. The departure from symmetric left-right wing motion coincides with changes in the body orientation. The five moments in time indicated by dashed vertical lines correspond to the images shown in Fig 2.
Table 1.
Morphological and other constants used to simulate simplified dynamics using the minimal model.
Fig 4.
Minimal model of bat dynamics applied to body roll maneuver.
Both wings are fully extended (er = el = 1) at mid-dowstroke, while one wing is fully retracted (er = 0) at mid-upstroke. For morphological parameters matched to those of C. perspicillata (I* = 5), simulations show that this asymmetric wing extension induces body roll and that aerodynamic forces do not influence the motion significantly. The response is insensitive to modest changes in the relative wing inertia (I* = 2), although when the morphological parameters are matched to those of fruit flies (I* = 0.02,C* = 0.05), aerodynamic forces dominate, while inertial forces have minimal effect on the body orientation. MATLAB code available in file minimal_simulation.zip from the Dryad Digital Repository, http://dx.doi.org/10.5061/dryad.21qs5 [31].
Fig 5.
Minimal model of inertial mechanism bats use to adjust body pitch along with schematic of wing positions.
The wings are protracted during the downstroke and retracted to the pitch-neutral position during the upstroke. Three values of the relative wing inertial parameter, I* = 2,5,7, are shown. I* = 5 corresponds to the morphology of C. perspicillata. MATLAB code available in file minimal_simulation.zip from the Dryad Digital Repository, http://dx.doi.org/10.5061/dryad.21qs5 [31].
Fig 6.
Inertial changes due to wing movement are sufficient to explain the complex reorientation of the bat’s body.
Beginning at t = 0.25 sec, we simulate the free motion of a virtual bat due to the inertial effects of the measured wing motion. The simulated posture (dotted line) is compared with the measured posture (solid line). Data available in file EulerAngles.txt from the Dryad Digital Repository, http://dx.doi.org/10.5061/dryad.21qs5 [31].
Fig 7.
Comparison between the simulated and measured Euler angles for 11 flight sequences.
The symbols denote the species (○: C. perspicillata; +: C. brachyotis); the colors identify each flight sequence. For clarity, every fifth data point during each flight sequence is plotted. For all flights, after subtracting the angle at t = 0 so as to remove bias, the correlations between the measured and predicted roll, pitch, and yaw angles is R2 = 0.687, 0.935, and 0.721, respectively. Data available in file EulerAngles.txt from the Dryad Digital Repository, http://dx.doi.org/10.5061/dryad.21qs5 [31].