UC Berkeley

The evolution of aerial locomotion has allowed organisms to occupy habitats and exploit resources that were previously unavailable to them, providing a selective advantage. Aerial locomotion has evolved repeatedly, most commonly in arboreal lineages as gliding – i.e., descending aerial locomotion with lift-derived horizontal components of displacement, and including maneuvers controlled via axial and appendicular motions. There is strong evidence that gliding led to the evolution of powered flight in birds through a progressive improvement in the ability to manipulate aerodynamic forces to control and prolong a gliding trajectory. However, few studies regarding active limb and body use for controlling airspeed and trajectory while gliding in organisms that had primitive wings, or lacked wings altogether have been conducted. I used extant organisms and a robot to model the gliding abilities and forelimb dynamics of gliders in order to explore how gliding may have led to the evolution of the flight stroke in birds.

In the first chapter, I document gliding ability in multiple semi-arboreal and arboreal species of Anolis lizards that vary in habitat and morphology but possess no specialized morphological adaptations for gliding. I demonstrate that all species tested exhibited equivalent wing loadings (i.e., the ratio of mass to airfoil surface area) and that wing loading is directly related to glide angle in the species tested. I found performance and kinematic variation in gliding among species and ecomorph groups. The results from this study show that animals that lack specialized aerodynamic structures are capable of gliding and support the hypothesis that controlled aerial behaviors precede specific morphological adaptations to gliding.

In the second chapter, I describe detailed limb and body kinematics of gliding flat-tailed house geckos (Hemidactylus platyurus), a species of gecko that is highly arboreal yet lack well-developed airfoils. I discovered distinctive forelimb retraction patterns that are related to body flexion and horizontal velocity. I characterized the classical skydiving posture in three-dimensions as well as a second posture in which the gecko flexed its body ventrally and retracted its forelimbs posteriorly. I suggest that switching between body postures and activating the forelimbs are used by the gecko to increase horizontal velocity by aerodynamic means. Further studies are needed to detail the functional consequences of variation within the forelimb retraction behavior. In addition to the symmetry inherent to the stable skydiving posture that is ubiquitous among gliding taxa, the symmetrical forelimb movements described in this study suggest a plausible origin of aerial forelimb motions that preceded the evolution of the flapping stroke through progressive increases in stroke amplitude and consequent improvements in controlling aerodynamic forces.

In the third chapter, I used a robotic ornithopter to model the biomechanics of paravian fossils that are phylogenetically intermediate to theropod dinosaurs and modern birds. I evaluated aerial performance of the robotic paravian model with two interchangeable pairs of wings that were either held laterally or activated at different combinations of wingbeat frequency and stroke amplitude. Results from this study demonstrate that, wing length increases gliding performance and, when the wings are sufficiently long, activate the positive relationships between flapping kinematics and gliding performance through interaction effects. Rudimentary wing flapping at low frequencies and amplitudes thus improves aerodynamic performance for gliders at the physical scales relevant to vertebrate flight origins.

The results from these studies show that gliding is common among arboreal animals, that wingless animals actively use their forelimbs to manipulate aerodynamic forces and shallow glide angle, and that incipient flapping of rudimentary, wing-like structures improve gliding performance. Collectively, these chapters establish an understanding of how body form and dynamic function are used to produce and control useful aerodynamic forces during aerial locomotion.