Substrate Temperature Preference in Pygmy Short-Horned Lizards (Phrynosoma douglasii)

Lizards, as ectotherms, rely on external heat sources such as solar radiation, substrate temperature, and air temperature to regulate body temperature using common behaviors such as shuttling, basking, and burrowing into the substrate. Few studies have been done to observe thermoregulation in the pygmy short-horned lizard (Phrynosoma douglasii), which inhabits eastern Washington State. During this study, lizards were captured from the Quilomene Recreation Area near Vantage, Washington. To observe thermoregulatory responses we placed the lizards in an artificial enclosure (i.e., the ectothermatron), which contains a sandy substrate, wooden shelters, and food and water. The ectothermatron contains heating elements and cooling coils to create a thermal gradient from approximately ~11 to 50°C and timed lighting to simulate a natural photoperiod. We measured body-surface and substrate temperatures with an infrared thermometer every hour between 0900 h and 1300 h from mid-April to mid-May, 2014. We hypothesized that P. douglasii would: (1) select relatively high temperatures; (2) prefer lower temperatures at night; and (3) show more burrowing behavior into substrate than use of shelters. Phrynosoma douglasii actively thermoregulated, maintaining, for example, an average body temperature of 35.8 ± 0.8°C at 1400 h but only 22.5 ± 1.4°C at 2100 h. Substrate usage also differed throughout the 24-hour period with lizards on the surface for 83 percent at 1400 h but only 6.3 percent at 2100 h. We conclude that P. douglasii actively thermoregulates, preferring higher substrate temperatures during the day than at night, and frequently burrows at night. Future studies could investigate whether photoperiod or temperature factors influence burrowing behavior in P. douglasii