Ecological modelling of convergence patterns between European and African ‘whip’ snakes

The concept of evolutionary convergence in morphological traits among phylogenetically unrelated organisms because of their ecological similarities is one of the most venerable in evolutionary ecology, and indeed has attracted the attention of ecologists for a long time. The ‘whip snakes’ are phylogenetically unrelated, morphologically and ecologically convergent species, which are characterized by slender bodies, long tails, large eyes, alertness, diurnality, saurophagy, oviparity, and rapid movement. It has been hypothesized that their morphology has evolved as a response preying on quick-running diurnal prey, mainly lizards. Here I compared two species of saurophagous whip snakes, the European Coluber (Hierophis) viridiflavus and the African Psammophis phillipsii. I explored whether the proximate factors which influence the presence of the two species in their natural habitat are the same in both Mediterranean central Italy (Coluber viridiflavus) and in tropical Nigeria (Psammophis phillipsii). I also used as control two other species of colubrids, Natrix natrix in Italy and Lamprophis fuliginosus in Nigeria. I selected 11 independent variables that are general for both study systems, and that are easily identified for their presence/absence in/around the site of capture of each snake specimen, and then explored the effects of these variables on the presence of the two species using a robust statistical design, i.e. a forward stepwise logistic regression model. My modelling analyses indicated that two variables influenced positively (i.e. the presence of lizards (LIZ) and of tall grass (GRS)), and one influenced negatively (the presence of water bodies (WBS)), the African species' presence. With regard to the European species, only two variables influenced positively and significantly the snakes presence, i.e. LIZ and GRS. Overall, the comparison of the two models clearly showed that both species' presence was influenced exactly by the same variables, despite the different continents and environmental conditions. The two control species were influenced by entirely different variables. Hence, robust modelling statistics revealed a extreme ecological similarity between the phylogenetically unrelated Psammophis and Coluber, clearly indicating that their peculiar morphologies have evolved to prey a rapid moving type of diurnal prey (lizards) in a specific type of microhabitat (tall grass). As there was no significant correlation between LIZ and GRS, hence the positive influence of GRS on both the snake species' presence cannot be considered as a mere correlate of the LIZ variable, but is likely that the peculiar morphological adaptations of these snakes are particularly efficient in this type of microhabitat compared to others.