Among the reptile order Squamata (lizards and snakes), the loss of limbs to give a snake-like morphology is likely the most dramatic evolutionary change that has occurred. It is often associated with the acquisition of an underground, burrowing life-style, nocturnality and a preference for relatively low temperatures. Nonetheless, how such an interesting evolutionary transition took place remains poorly understood. The authors examined this process in ten, closely-related species of gymnophthalmid lizards (spectacled lizards) from the Brazilian Caatinga (desert scrubland), representing one full transition from typical lizard species to burrowing snake-like ones. Some of the species studied have typical lizard morphology, while others have a burrowing, snake-like morphology. Species of both forms live together in sandy soil regions of the Brazilian Caatingas and burrow to some extent.
The study used automatic temperature data loggers and X-ray images to study evolutionary relationships between morphology, burrowing performance, exposure to extreme temperatures and the evolution of thermal physiology in those lizards. The results show that the evolution of a snake-like morphology allows a better burrowing performance in our studied species. An improved burrowing performance allows those species to reach thermally safe (cooler) areas and also seems to favour the evolution of lower preferred temperatures. At the study sites, snake-like lizards not only can avoid diurnal extreme temperatures at the soil’s surface, but also access their preferred temperatures within the sand until late night.
In addition, it was found that snake-like lizards active at cool hours of the day have lower critical thermal limits. Using the obtained evidence, the authors propose a sequential explanation for the evolution of the snake-like, burrowing syndrome in lizards that can be tested in other lineages.