Gimme Shelter: Thermally-mediated refuge site selection by Gila monsters (HELODERMA SUSPECTUM)

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Austin Peay State University
The Gila monster (HELODERMA SUSPECTUM) is a desert lizard which experiences strong above-ground thermal constraints throughout its range and copes with suboptimal environmental temperatures by retreating to sub-surface refuge sites. We addressed the hypothesis that refuge sites selected by Gila monsters would have higher thermal quality and provide more thermally stable regimes than sites not used as refuge. We measured the thermal properties of 48 selected shelters, each matched with two control shelters; a potential shelter having similar physical characteristics to the selected shelter but not observed to be used as refuge, and a random shelter, in which physical attributes were not controlled for, but still having the size and depth to be used as sub-surface refuges. We found that overall mean temperature did not differ among the three shelter types. In addition, random shelters had more time within the preferred thermoregulatory range of the Gila monster (Tset) when compared to potential shelters but shelters that were selected by Gila monsters did not differ from the other shelter types in the total amount of time spent within Tset. Over the course of the activity season, selected shelters deviated less from Tset and were more thermally stable than potential or random shelters. Our results also indicate a temporal shift in thermally-mediated shelter selection. Early in the activity season (April), selected refuge sites had temperatures within Tset for longer when compared to potential shelters. Later in the activity season (June and July) when above-ground temperatures were higher and potentially lethal, selected shelters had higher thermal stability and temperatures that were closer to Tset than other shelter types. Overall, our results indicate that shelter-selection in Gila monsters is thermally-mediated in ways that change over the course of the activity season, and that using biologically informative metrics is important in measuring thermal suitability of refuge sites in the field.
Gila monster, Habitat selection