Forelimb motion and reciprocation mediate aerodynamic control in a gliding lizard

The origin of the flight stroke in vertebrate flight evolution remains obscure. However, using forelimbs to control aerodynamic forces while gliding provides a possible exaptation from which wingless taxa evolved incipient wing flapping and powered flight. We used flat-tailed house geckos (Hemidactylus platyurus) to model the possible dynamics of those gliding taxa ancestral to vertebrate flyers, and characterized their limb and body kinematics while gliding in a vertical wind tunnel, so as to determine biomechanical consequences of forelimb movements during controlled aerial behavior.

Geckos mostly assumed a stereotypical skydiving posture but intermittently would flex the body ventrally as the forelimbs were retracted posteriorly. Shoulder retraction, spinal column flexion, and subsequent translational velocity in the vertical and cranial directions were positively correlated; such alteration of body posture with simultaneous forelimb displacement thus modulates the directions and magnitudes of aerodynamic forces, including horizontal thrust production. Independent of shoulder retraction and body bend, body pitch correlated positively with vertical acceleration and negatively with horizontal acceleration.

Gliding geckos actively use their forelimbs to alter body speed and to generate thrust, suggesting aerodynamic function for limb displacement and reciprocation in the absence of wings. Prior to the origin of the flapping of winglike structures, analogous forelimb motions (including symmetric reciprocation) may have thus provided biomechanical advantage in the evolution of volant vertebrates.