Many desert organisms cope with extreme and variable conditions by retreating to sub-surface refugia, yet little is known of the patterns of refuge use by most desert inhabitants. We investigated shelter use by the Gila Monster, Heloderma suspectum, an ectotherm closely tied to sub-surface refugia in a strongly seasonal desert environment. We addressed hypotheses that ectotherms may use habitats based on availability of shelters, select shelters based on particular cues, respond to seasonal variation in refuge characteristics, and show fidelity to specific retreats. Using radiotelemetry, we monitored microhabitat use by eight to ten Gila Monsters for six years, recording timing, frequency, and duration of visitation to over 250 specific shelters. We used transects to assess shelter availability, recorded structural features of all shelters, and used dataloggers to monitor seasonal changes in microenvironments within subsets of shelters for periods up to two years. Shelters, and the habitats where they occurred, were not chosen by Gila Monsters at random. Heloderma spent more time in areas where a higher density of potential shelters was available, and selected shelters based on rockiness, slope and entrance aspect, depth, humidity, and temperature. Gila Monsters showed longer residence times and greater fidelity to shelters used during extreme periods (e.g., winter) and these patterns were paralleled by seasonal changes in the characteristics of shelters chosen. Winter shelters tended to be south-facing, rockier, deeper, and warmer than those used in other seasons, whereas dry-summer shelters were more soil-like in composition, cooler, and more humid. Gila Monsters showed strong fidelity to specific shelters, some of which were used by two or more lizards, sometimes concurrently. Seasonal variation in use of “social” shelters coincided with annual cycles of intraspecific behaviors and reproduction. Our results underscore the importance of sub-surface refugia in the ecology of a sedentary desert ectotherm. Because many other ectotherms also spend significant periods sequestered in below-ground retreats, it is surprising that ecologists have not more extensively investigated this phenomenon. An understanding of the cues used by desert ectotherms to choose refuge-sites, the spatial and temporal variability in refuge characteristics, and the fidelity shown to particular retreats, may help better explain patterns of habitat selection, behavior, distribution, and abundance.
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