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Three-dimensional launch kinematics in leaping, parachuting and gliding squirrels

Richard L. Essner, Jr

Department of Biological Sciences, Ohio University, Athens, OH 45701, USA

View larger version (14K): [in a new window]   Fig. 1. Defining sciurid arboreal locomotion. Discrete airborne postures are used to define locomotor mode. (A) Chipmunks are relatively unspecialized semiarboreal leapers that exhibit an adducted limb posture in mid-air. (B) Red squirrels are arboreal parachuters that exhibit a flattened posture in mid-air characterized by abduction of the proximal limb elements and flexion of the distal limb elements. (C) Flying squirrels are arboreal gliders that exhibit an abducted posture in mid-air with extension of the distal limb elements. (D) Phylogenetic relationships of the sciurid taxa included in this study (taken from data in Hight et al., 1974; Oshida et al., 1996; Roth, 1996).

View larger version (18K): [in a new window]   Fig. 2. Landmarks used to describe limb, body and tail movements in squirrels during the launch phase: 1, occiput; 2, shoulder over the glenoid fossa; 3, point slightly above wrist; 4, wrist; 5, base of the fifth phalanx of the manus; 6, hip over the greater trochanter; 7, point slightly above ankle; 8, ankle at the lateral malleolus; 9, base of the fifth phalanx of the pes; 10, base of the tail; 11, point slightly above tail base; 12, tip of the tail.

View larger version (165K): [in a new window]   Fig. 3. Representative video frames portraying a single take-off sequence during a launch in the flying squirrel. Three distinct phases were identified: (i) the preparatory phase is characterized by a stereotyped preliminary hop that transports the hindlimbs forward to the edge of the platform; (ii) the countermovement (CM) phase is characterized by flexion of the knee and ankle, producing a countermovement that increases take-off velocity; (iii) the propulsive phase immediately follows the countermovement phase and is characterized by rapid extension of the knee and ankle until the animal loses contact with the platform. Note that, during the propulsive phase, the tail is dorsiflexed and the forelimbs are protracted. Landmarks (cotton pom-poms)=5 mm.

View larger version (34K): [in a new window]   Fig. 4. Mean kinematic profiles of the hindlimb and tail in chipmunks (triangles), red squirrels (squares) and flying squirrels (circles). All three species exhibit similar hindlimb kinematics and share a preparatory phase with a preliminary hop. The foot is initially extended during the preliminary hop, then flexed during transport and extended once again as it is set back down. Toe-down (TD) marks the beginning of the countermovement (CM) phase characterized by flexion of the knee and ankle. This is followed by extension during the propulsive phase. Tail kinematic profiles indicate a divergence among the three species, with chipmunks exhibiting the greatest amount of dorsiflexion, followed by flying squirrels and red squirrels. Values are means ± S.E.M., N=5 trials per species.

View larger version (19K): [in a new window]   Fig. 5. Mean kinematic profiles of the forelimb in chipmunks (triangles), red squirrels (squares) and flying squirrels (circles). Protraction brings the forelimbs closer to the head and is indicated by an increasing angle. A protraction angle of 90° indicates that the forelimbs have been brought forward to the level of the occiput. All three species exhibit similar values for forelimb protraction, bringing the forelimbs from approximately 45° at the onset of hand-off to approximately 65° at toe-off. Forelimb abduction moves the forelimbs away from the midline of the body and is indicated by an increasing angle. Forelimb adduction moves the forelimbs closer to the midline of the body and is indicated by a decreasing angle. An abduction angle of 90° indicates that the forelimbs are fully abducted to the level of the occiput, while an angle of 0° indicates that the forelimbs are fully adducted to the midline. Flying squirrels abduct to approximately 72° before losing contact with the platform. The other two species show a decrease in the abduction angle, indicating that they are adducting the forelimbs during the propulsive phase. Values are means ± S.E.M., N=5 trials per species.