Convergent evolution of tail spines in squamate reptiles driven by microhabitat use

The repeated evolution of convergent or analogous traits is often used as evidence for adaptive evolution. Squamate reptiles show a high degree of convergence in a variety of morphological traits; however, the evolutionary mechanisms driving these patterns are not fully understood. Here, we investigate the evolution of tail spines, a trait that evolved multiple times in evolutionarily independent clades of lizards. Taking a comparative phylogenetic approach, we use 2877 squamate species to demonstrate that the evolution of spiny tails is correlated with microhabitat use, with species that live in rocky habitats significantly more likely to have evolved spiny tails. In the light of previous behavioural observations, our results suggest that spiny-tailed lizards have an advantage in rocky habitats through predation avoidance, where tail spines are used to prevent extraction from rocky crevices. In concordance with previous research on lizard body armour, our results suggest that the evolution of tail spines is coupled to both a rock-dwelling lifestyle and predator avoidance strategies, and highlight a complex interplay between different selective pressures on the evolution of defensive morphologies in reptiles.