Meeting Abstract
Locomotion in tetrapods involves coordinated efforts between appendicular and axial systems, as limbs generate ground reaction forces (GRFs) that are transmitted to the axial system. In the absence of limbs, snakes experience locomotor GRFs in fundamentally different ways than their limbed-lizard ancestors. Without GRFs from limbs, the epaxials of early snakes were no longer required to stabilize against torsional GRFs and were modified to generate propulsive forces. These forces must be transmitted into the environment and since snakes locomote on their bellies, their ribs must play an inherent role in GRF transmission. Snakes locomote with at least two kinematic styles: (1) with no static contact points, i.e. lateral undulation, or (2) with static contact points, i.e. concertina, rectilinear, climbing, and sidewinding. Rib motions are crucial to this locomotor versatility: our XROMM work and previous studies show that ribs change cross-sectional body shape, deform to environmental irregularities, provide synergistic stabilization for other muscles, and differentially exert and transmit forces to control propulsion. Ribs are also used for lung ventilation in snakes, and we found that snakes are able to independently and regionally control ventilatory motions. We suggest that snakes co-opted the rib motions of ventilation to dynamically and passively participate in their diverse locomotor modes. Hence, removal of the constraints of limbed locomotion and new mechanics of limblessness may have influenced snake evolution and enabled snakes to modify rib motions to contribute to locomotion in innovative ways. Future comparisons with other limbless lizard taxa are necessary to tease apart the mechanics and mechanisms that produced the locomotor versatility observed within Serpentes.
