The origin of evolutionary innovations: locomotor consequences of tail shape in aquatic snakes

1. Phylogenetic shifts in habitat use often impose strong selective forces on locomotor systems; for example, the transition from terrestrial to aquatic existence has stimulated the evolution of laterally compressed 'paddles' on the tails, feet or fins of a diverse array of vertebrate taxa. 2. Under the traditional gradualist model of evolutionary change, even a small paddle is predicted to enhance aquatic locomotion but impose a cost in terrestrial performance. However, direct evaluation of those early evolutionary stages is impossible from modern-day aquatic species, because the initial steps have been obscured by complex subsequent adaptations of morphology, physiology and behaviour. 3. Unlike most major features of locomotor-system morphology (e.g. leg length, muscle mass), the caudal paddles of aquatic snakes are morphologically so simple that they can be recreated experimentally. We attached artificial paddles to the tails of juvenile tigersnakes (Notechis scutatus) to assess the effect of tail shape on locomotor performances. 4. The presence of a small paddle on the tail greatly increased swimming speeds (by 25%) but decreased crawling speeds on land (by 17%). A small paddle (35% of tail length) was more effective for aquatic locomotion than a larger paddle (84% of tail length). 5. Presumably, larger paddles are effective only after adaptive modification of musculoskeletal propulsive systems. Our experimental manipulations thus provide unusually direct evidence of a functional advantage to modest lateral flattening of the tail in the earliest aquatic snakes, mediated via enhanced locomotor speeds in water.