THERMAL BIOLOGY OF A POPULATION OF Xenosaurus newmanorum (SQUAMATA: XENOSAURIDAE) FROM XILITLA, SAN LUIS POTOSÍ, MEXICO: DO THEY ACTIVELY THERMOREGULATE?

The knob-scaled lizards (family Xenosauridae) comprise a clade of crevice-dwelling North American lizards with a trend towards low body temperature and high rates of water loss, and are often regarded as thermoconformers. However, there are few studies on their thermal ecology, most of them limited to documenting the body temperatures and their relation with environmental temperatures. Here, we documented the field body temperatures (Tb), selected temperatures (Tset), operative temperatures (Te), thermoregulatory effectiveness, and thermal tolerances of 10 individuals of the Newman’s Knob-scaled Lizard, Xenosaurus newmanorum, collected in the fall of 2017 in La Huasteca region of Mexico. Mean Tb was 22.75 °C and mean Tset 25.08 °C (1.64 °C and 3.97 °C above mean Te, respectively). The species had a relatively narrow thermal tolerance range (25.89 °C). Furthermore, based on two different thermoregulatory indexes, we found direct evidence of active thermoregulation in X. newmanorum. This constitutes the first record of active thermoregulation in a xenosaurid from a low-elevation environment and the second record of thermal tolerances. We discuss on the thermal strategies of other species of the genus and pose that active thermoregulation might be more widespread in Xenosaurus than previously thought.