Interaction of morphology, thermal physiology and burrowing performance during the evolution of fossoriality in Gymnophthalmini lizards

The evolution of fossoriality in Squamata often leads to a phenotypic syndrome involving snake-like body form, relatively low preferred temperature (T-P) and low critical maximum temperatures (CTmax). However, how traits interacted among them and with the environment during the evolution of such syndrome remains poorly understood. We studied this process in ten species of gymnophthalmid lizards from the Brazilian Caatinga representing one full transition from typical lacertoid species to fossorial snake-like (FSL) ones. We tested whether different morphotypes exhibited different burrowing performances in response to heat, and also different T-P and CTmax. Then, we estimated how changes in burrowing performance would relate to thermoregulation costs in terms of the number of daily hours for which preferred temperatures are available and of risk of overheating. Fossorial snake-like species burrowed deeper, exhibited lower T-P but kept very high CTmax, comparable to lacertoid species. A better burrowing performance and lower T-P allowed increasing the daily amount of time during which T-P was accessible within the soil of the study region. In addition, temperatures above CTmax of the studied species were present down to 5cm deep, suggesting that just burrowing does not protect against exposure to extreme temperatures in species that are surface-active during the day (all lacertoids and some FSL). Nonetheless, FSL species active at cool hours of the day exhibited lower CTmax than diurnal and syntopic, lacertoid and FSL species. Based on our data and previous literature, we propose a sequential explanation for the acquisition of the fossorial syndrome in Squamata.