Vertebrate Locomotion

Tortoises develop and overcome position biases in a reversal learning task

The capability of animals to alter their behaviour in response to novel or familiar stimuli, or behavioural flexibility, is strongly associated with their ability to learn in novel environments. Reptiles are capable of learning complex tasks and offer a unique opportunity to study the relationship between visual proficiency and behavioural flexibility. The focus of this study was to investigate the behavioural flexibility of red-footed tortoises and their ability to perform reversal learning. Reversal learning involves learning a particular discrimination task, after which the previously rewarded cue is reversed and then subjects perform the task with new reward contingencies. Red-footed tortoises were required to learn to recognise and approach visual cues within a Y-maze. Once subjects learned the visual discrimination, tortoises were required to successfully learn four reversals. Tortoises required significantly more trials to reach criterion (80% correct) in the first reversal, indicating the difficulty of unlearning the positive stimulus presented during training. Nevertheless, subsequent reversals required a similar number of sessions to the training stage, demonstrating that reversal learning improved up to a point. All subjects tested developed a position bias within the Y-maze that was absent prior to training, but most were able to exhibit reversal learning. Red-footed tortoises primarily adopted a win-stay choice strategy while learning the discrimination without much evidence for a lose-shift choice strategy, which may explain limits to their behavioural flexibility. However, improving performance across reversals while simultaneously overcoming a position bias provides insights into the cognitive abilities of tortoises.

Quadrupedal coordination of bipedal gait: implications for movement disorders

During recent years, evidence has come up that bipedal locomotion is based on a quadrupedal limb coordination. A task-dependent neuronal coupling of upper and lower limbs allows one to involve the arms during gait but to uncouple this connection during voluntarily guided arm/hand movements. Hence, despite the evolution of a strong cortico-spinal control of hand/arm movements in humans, a quadrupedal limb coordination persists during locomotion. This has consequences for the limb coordination in movement disorders such as in Parkinson's disease (PD) and after stroke. In patients suffering PD, the quadrupedal coordination of gait is basically preserved. The activation of upper limb muscles during locomotion is strong, similar as in age-matched healthy subjects although arm swing is reduced. This suggests a contribution of biomechanical constraints to immobility. In post-stroke subjects a close interactions between unaffected and affected sides with an impaired processing of afferent input takes place. An afferent volley applied to a leg nerve of the unaffected leg leads to a normal reflex activation of proximal arm muscles of both sides. In contrast, when the nerve of the affected leg was stimulated, neither on the affected nor in the unaffected arm muscles EMG responses appear. Muscle activation on the affected arm becomes normalized by influences of the unaffected side during locomotion. These observations have consequences for the rehabilitation of patients suffering movement disorders.

Flight respiration and energetics

We use a comparative approach to examine some of the physiological traits that make flight possible. Comparisons of related fliers and runners suggest that fliers generally have higher aerobic metabolic capacities than runners but that the difference is highly dependent on the taxa studied. The high metabolic rates of fliers relative to runners, especially in insects, are correlated with high locomotory muscle cycle frequencies and low efficiencies of conversion of metabolic power to mechanical power. We examine some factors that produce variation in flight respiration and energetics. Air temperature strongly affects the flight metabolic rate of some insects and birds. Flight speed interacts with flier mass, so that small fliers tend to exhibit a J-shaped power curve and larger fliers a U-shaped power curve. As body size increases, mass-specific aerobic flight metabolism decreases in most studies, but mass-specific power output is constant or increases, leading to an increase in efficiency with size. Intraspecific studies have revealed specific genetically based effects on flight metabolism and power output and multiple ecological correlates of flight capabilities.

How animals move: an integrative view

Recent advances in integrative studies of locomotion have revealed several general principles. Energy storage and exchange mechanisms discovered in walking and running bipeds apply to multilegged locomotion and even to flying and swimming. Nonpropulsive lateral forces can be sizable, but they may benefit stability, maneuverability, or other criteria that become apparent in natural environments. Locomotor control systems combine rapid mechanical preflexes with multimodal sensory feedback and feedforward commands. Muscles have a surprising variety of functions in locomotion, serving as motors, brakes, springs, and struts. Integrative approaches reveal not only how each component within a locomotor system operates but how they function as a collective whole.