  For nearly a century, researchers have tried to understand the swimming of
aquatic animals in terms of a balance between the forward thrust from swimming
movements and drag on the body. Prior approaches have failed to provide a
separation of these two forces for undulatory swimmers such as lamprey and
eels, where most parts of the body are simultaneously generating drag and
thrust. We nonetheless show that this separation is possible, and delineate its
fundamental basis in undulatory swimmers. Our approach unifies a vast diversity
of undulatory aquatic animals (anguilliform, sub-carangiform, gymnotiform, bal-
istiform, rajiform) and provides design principles for highly agile bioinspired
underwater vehicles. This approach has practical utility within biology as well
as engineering. It is a predictive tool for use in understanding the role of
the mechanics of movement in the evolutionary emergence of morphological
features relating to locomotion. For example, we demonstrate that the
drag-thrust separation framework helps to predict the observed height of the
ribbon fin of electric knifefish, a diverse group of neotropical fishes which
are an important model system in sensory neurobiology. We also show how
drag-thrust separation leads to models that can predict the swimming velocity
of an organism or a robotic vehicle.
