Comparative Forelimb Muscle Function in Turtles: Tests of Environmental Variation and Neuromotor Conservation

Novel locomotor functions in animals may evolve through changes in morphology, muscle activity, or a combination of both. The idea that new functions or behaviors can arise solely through changes in structure, without concurrent changes in the patterns of muscle activity that control movement of those structures, has been formalized as the `neuromotor conservation hypothesis\u27. In vertebrate locomotor systems, evidence for neuromotor conservation is found across transitions in terrestrial species and into fliers, but transitions in aquatic species have received little comparable study to determine if changes in morphology and muscle function were coordinated through the evolution of new locomotor behaviors. Understanding how animals move has long been an important component of integrative comparative biology and biomechanics. This topic can be divided into two components, the motion of the limbs, and the muscles that move them. Variation in these two parameters of movement is typically examined at three levels, intraspecfic studies of different behaviors, and interspecific studies on either the same or different behaviors. My dissertation is a compilation of four studies that examined forelimb kinematics and motor control across locomotor modes in freshwater and marine turtles to determine how muscle function is modulated in the evolution of new locomotor styles. First, I described patterns of forelimb motion and associated patterns of muscle activation during swimming and walking in a generalized freshwater turtle species (Trachemys scripta) to show how muscle function is modulated to accommodate the different performance demands imposed by water and land. Second, I examined whether differences in muscle function are correlated with changes in limb morphology and locomotor style by comparing forelimb kinematics and motor patterns of swimming from rowing Trachemys scripta to those of flapping sea turtles (Caretta caretta). Next, I quantified forelimb kinematics of swimming in the freshwater turtle species Carettochelys insculpta, describing how it uses synchronous forelimb movements to swim and whether these motions are actually similar to the flapping kinematics of sea turtles (Caretta caretta) or if they more closely resemble the kinematics of freshwater species with which they are more phylogenetically similar. I also compared the kinematics of rowing in Trachemys scripta and the highly aquatic Florida softshell turtle (Apalone ferox). Finally, I compared patterns of forelimb muscle activation for four species of turtles to determine whether the chelonian lineage shows evidence of neuromotor conservation across the evolution of different locomotor modes. Data from these studies help improve our understanding of how new forms of quadrupedal locomotion have evolved