Insular geckos provide experimental evidence on refuge selection priorities by ectotherms

In small sedentary ectotherms, patterns of spatial use result from the interplay between multiple, often conflicting factors, including abiotic and biotic interactions. Evaluating the costs and benefits of these pressures is crucial to make correct behavioural decisions in terms of fitness. The insular Sao Vicente's wall gecko Tarentola substituta provides a relatively simple model system to study these questions as it inhabits arid rocky habitats where refuges are limited, density of conspecifics is high, and terrestrial predators are almost absent. In the field, adults tend to find diurnal shelter under mid-sized rocks, frequently in male-female couples, while juveniles occupy small rocks which are thermally suboptimal. A lab experiment was conducted to determine the roles of ecological (shelter size and temperature) and social (conspecifics) factors in refuge selection. Single and pair combinations of geckos of different age and sex classes were allowed to select among four refuges: cold small, hot small, cold large, or hot large rock. Based on previous studies, we hypothesized that larger and thermally buffered rocks would be the preferred refuges, and that adult male-female pairs under the same rock would be more frequent than other combinations.Geckos primarily selected larger shelters, trading off the presence of conspecifics against thermal quality. In social terms, sex, adult condition and size-related disparity shaped the patterns of aggregation, resulting in lower aggregation frequencies between adults and juveniles and even between juveniles of different sizes. These results reasonably match field observations suggesting selection of rocks as diurnal retreats according to their thermal properties, and social aggregations mainly involving adult males and females but not juveniles. Overall, this combined evidence provides insights on the spatial ecology of geckos, and likely other ectotherms, under conditions of low predation, limited resources and high intraspecific competition, such as those prevailing on island systems.