Combined effects of body posture and three-dimensional wing shape enable efficient gliding in flying lizards

Abstract

Gliding animals change their body shape and posture while producing and modulating aerodynamic forces during flight. However, the combined effect of these different factors on aerodynamic force production, and ultimately the animal's gliding ability, remains uncertain. Here, we quantified the time-varying morphology and aerodynamics of complete, voluntary glides performed by a population of wild gliding lizards (Draco dussumieri) in a seven-camera motion capture arena constructed in their natural environment. Our findings, in conjunction with previous airfoil models, highlight how three-dimensional (3D) wing shape including camber, planform, and aspect ratio enables gliding flight and effective aerodynamic performance by the lizard up to and over an angle of attack (AoA) of 55° without catastrophic loss of lift. Furthermore, the lizards maintained a near maximal lift-to-drag ratio throughout their mid-glide by changing body pitch to control AoA, while simultaneously modulating airfoil camber to alter the magnitude of aerodynamic forces. This strategy allows an optimal aerodynamic configuration for horizontal transport while ensuring adaptability to real-world flight conditions and behavioral requirements. Overall, we empirically show that the aerodynamics of biological airfoils coupled with the animal's ability to control posture and their 3D wing shape enable efficient gliding and adaptive flight control in the natural habitat.

Figure 1

Illustration of the body posture and forces acting during a glide and the field motion capture arena. (a) Free body diagram showing the forces experienced by a gliding animal at various stages of the complete glide as well as the change in body orientation. The glide is further divided into the three distinct glide phases of takeoff, mid-glide, and landing. (b) A scaled illustration of the motion capture arena showing all 24 smoothed glides (see SI-3.2) and the seven camera positions used to collect 3D kinematic data. The seven cameras are divided into three groups (color coded) based on the part of the glide they record. The takeoff cameras are marked by purple discs, glide cameras with orange, and landing cameras with red.

Figure 2

An illustration of the tracked body points and the metrics calculated for each glide. (a) Illustration showing the five body points tracked per frame for each glide. (b) The three axes about which the lizard can reorient in flight. Longitudinal axis (lo) about which the lizard rolls. Transverse axis (ta) about which the lizard changes pitch. Vertical axis (ve) about which the lizard changes yaw. (c) Illustration showing the airfoil area used to calculate the aerodynamic force coefficients. The area was calculated by fitting a plane to the 17 3D points tracked around the lizard in a single frame during the glide when the patagium was completely stretched open. (d) Yaw angle is the rotation about the (ve) from the X–Z plane, positive yaw is to the lizard’s left. (e) Pitch angle is the rotation about the (ta) from the X–Y plane, positive pitch raises the head upward. (f) Roll angle is the rotation about the (lo) from the Z–Y plane, positive roll raises the left-wing tip. (g) Angle of attack (AoA) is the angle made by the airfoil surface relative to the direction of motion (airflow), positive AoA is counterclockwise. (h) Camber is the convexity (concave down) of the airfoil from the leading to the trailing edge. The formula provided shows the calculation for % camber. (i) Dihedral angle is the average of the upward angle made by either side of the wing with the X–Y plane.

Figure 3

Average glide mechanics across all 14 individuals in the field motion capture arena. The solid black line shows the average value, and the shaded area shows ± 1 standard deviation. The vertical dashed line denotes the start of the landing phase calculated as the average of start of landing phase for all glides. Overall, the analysis covers ~ 82% of the complete glide. Panels (a–c) show the overall variation in pitch, AoA, and camber. Note the steady increase in pitch in the mid-glide phase and a mostly constant AoA of approximately 25° with varying percentage camber. Panels (d–f) show the change in average force coefficients and their ratio with glide progression. Note the steep drop in lift-to-drag ratio during the landing phase.

Figure 4

Aerodynamic performance of Draco gliding lizards recorded in the field motion capture arena. (a) A drag polar plot of the aerodynamic force coefficients; (b) their variation with the angle of attack (AoA). Note the expected gradual increase in each of the force coefficients with the increase in AoA up to a value of ~ 55°. (c) The lift-to-drag ratio shows a non-linear negative trend with respect to AoA. Because the underlying data are binned by C_D, AoA (b,c) is not strictly increasing with respect to C_D or C_L/C_D due to camber and inter-individual variation.