Simple Physical Principles and Vertebrate Aquatic Locomotion

SYNOPSIS. Swimming is a common and important component of vertebrate behavior. Therefore the study of swimming form and function is essential to understanding vertebrate biology. An interactive process between biologists and hydrodynamicists has proven extremely successful in providing such understanding. This process starts with a definition of forces that may act on an animal; viscous and pressure drag, acceleration reaction, lift and body inertia, and ground reaction. Dominant force components are selected on the basis of ground contact, Reynolds number, reduced frequency, and shape. Conservation of momentum requires forces and their moments be in balance leading to models describing swimming motions and associated forces. These provide the bridge between the hydrodynamics studies and biological applications. Numerical solutions to models estimate thrust and drag, and rates of working, which are several times greater than expected for manmade non-flexing bodies. These solutions are used in prediction of optimal two-phase swimming behaviors, alternating high resistance swimming with low resistance behavior such as gliding. Qualitative predictions from models define optimal forms for various behaviors and habitats. Optimal forms tend to be mutually exclusive, providing benchmarks for analyses relating form and function in an ecological context. Life history patterns are also affected by locomotor capabilities of various morphologies. Scale effects limit the distribution of propulsors. Numerical solutions to models are probably only good within an order of magnitude because of the assumptions in their formulation, but rankings of mechanisms or organisms are adequate for comparative study. Predictions must be tempered by consideration of non-locomotor function, developmental limitations of structural options, and non-equilibrium community structures.