Scaling of the locomotory apparatus and paddling rhythm in swimming mallard ducklings (Anas platyrhynchos): Test of a resonance model

It was hypothesized that the constant propulsive frequencies observed in several paddling animals as they swim at different speeds reflects "resonance-like" swimming mechanics. This behavior may result if the hydrodynamic load on the paddle is dominated by acceleration-dependent, rather than velocity-dependent, terms, and the musculoskeletal system operates with nearly constant stiffness over a large fraction of the propulsive cycle. A model was developed that uses the form of the paddle and the architecture of a representative limb muscle (lateral gastrocnemius) to predict a value proportional to propulsive frequency in mallard ducklings of different size. Fifteen ducklings were videotaped as they swam in a water channel, and the measured frequencies were compared to model predictions. Although the predicted and measured frequencies were highly correlated (r = 0.926), the 95% confidence interval estimated for the intercept of the reduced major axis fit to the points did not span the origin. This lack of proportionality means that even though it could account far much of the observed variation in paddling frequencies, the present model of resonance-like swimming is not supported. (C) 1994 Wiley-Liss, Inc.