Hydrodynamics of the frontal strike in aquatic snakes: drag, added mass and the possible consequences for prey capture success.

Abstract

Natural selection favors organisms that are the most successful in fitness-related behaviors such as foraging. Secondary adaptations pose the problem of re-adapting an already optimized phenotype to new constraints. When animals forage underwater, they face strong physical constraints, particularly when capturing prey. Successful prey capture requires a predator to be fast and to generate a high acceleration. This involves two main constraints due to the surrounding fluid: drag and added mass. Both constraints are related to the shape of the animal. We experimentally explore the relationship between shape and performance in the context of an aquatic snake strike. As a model, we use two different 3D- printed snake heads representing typical shapes of aquatically-foraging and non-aquatically- foraging snakes, and frontal strike kinematics based on in vivo observations. By using direct force measurements, we compare the drag and added mass faced by the aquatic and non- aquatic snake models during a strike. Our results show that both drag and added mass are optimized in aquatic snakes. Using flow field measurements with particle image velocimetry, we examine the fluid dynamical mechanisms that could be behind the reduction of hydrodynamic constraints observed for the aquatic snake head shape, which makes it well suited to capture prey under water.\n\nSummary statementThe present work explores the functional implications of head shape 15 in a group of aquatic predators using a fluid mechanics approach.