Can a tropical lizard (Xenosaurus platyceps) survive freezing temperatures?

Thermoregulation is crucial for lizards, which can employ stenothermic or eurythermic strategies and active or passive thermoregulation. Some organisms that live at low temperatures use physiological freeze avoidance (supercooling) or freeze tolerance mechanisms, often involving glucose as a cryoprotectant. Lizards from the ancient family Xenosauridae are typically thermoconformers, with some species exhibiting remarkably low body temperatures. This study investigated the thermal ecology of Xenosaurus platyceps in a low-elevation population (390 m) throughout the year. We examined thermoregulatory behaviour, cold tolerance, and blood glucose maintenance, as well as seasonal variation in thermoregulation patterns, supercooling points, and glucose concentrations in this tropical species. This study demonstrates that the micro-endemic lizard Xenosaurus platyceps can survive experimental freezing, marking the first such report for the Xenosauridae. Body temperature correlated significantly with environmental temperature, particularly with substrate temperature. The lizards showed stenothermic behaviour with body temperatures ranging from 20.8 to 30.8 °C, with significant differences amongst seasons. The species actively thermoregulated during spring and winter, exhibited thermoconformer behaviour during autumn, and avoided extremely warm microhabitats during summer. Remarkably, lizards had crystallisation points as low as -7.01 °C during spring, with a moderate increase in blood glucose after freezing that likely contributed to freeze tolerance. The study demonstrates that X. platyceps has remarkable seasonal thermal adaptability and resistance to sub-zero temperatures.