Morphological and ecological convergence in two natricine snakes

Abstract

Similar morphologies between species may derive due to shared ancestry, convergent evolution, or chance. I studied morphological similarity, trait evolution, and functional significance of ecologically relevant traits in two stream-dwelling natricine snakes, Thamnophis rufipunctatus and Nerodia harteri. Both species live in shallow riffles in streams and forage visually for fish. Phylogenetic analyses of mitochondrial DNA sequences for the ND4 gene from these species and four other natricines provided an independent phylogenetic framework for character mapping of traits and statistical analyses of the influence of phylogeny on the relationships between morphology, performance, and trait function. Character mapping of snout length and head width indicated the long snout morphology evolved independently in T. rufipunctatus and N. harteri. Head morphology was not correlated with locomotor performance (e.g., swimming in a current) in these snakes. However the long snout resulted in reduced hydrodynamic drag on the snakes' heads when in a swift current, compared to other species lacking the long snout trait. The long snout morphology also resulted in reduced degree of binocular vision field in these snakes. Independent contrast analysis showed the correlations between snout length and hydrodynamic drag and binocular vision field were not explained by relatedness. Reduced binocular vision field was contrary to the hypothesis that visually oriented snakes should exhibit relatively greater binocular vision. Reduced binocular vision field should be either neutral, or effect negatively, a visually oriented predator. I suggest that reduced binocular vision field is neither an advantage nor disadvantage to these snakes and is merely a consequence of the narrow head and long snout morphology. Conversely, reduced hydrodynamic drag is an ecologically relevant function of head shape in these snakes that forage in swift water and strike for prey. The independent, repeated, evolution of the long snout in T. rufipunctatus and N. harteri and its resulting function of reducing hydrodynamic drag is consistent with the hypothesis that the long snout is an adaptation for reduced hydrodynamic drag when foraging for prey in a swift current.