Aggregating has been widely studied in a variety of animals and found to have important benefits in terms of sociality, courtship, predator avoidance and physiology. Several species of nocturnal geckos form diurnal aggregations; however, little is known about the benefits of these groupings. We conducted a series of experiments to determine the benefit of aggregation for the desert-dwelling western banded gecko, Coleonyx variegatus. We found that banded geckos benefit from aggregation by a reduction in evaporative water loss (EWL). No social or mating benefits were detected, and geckos did not group to avoid predators. Geckos did not select diurnal retreat sites based solely on the scent of conspecifics, although they aggregated readily when conspecifics were present. Thus, C. variegatus appear to achieve physiological but not social benefits from grouping. Banded geckos belong to an ancestrally tropical lineage whose descendants invaded present-day North American deserts at a time when these regions were more mesic. This may explain their relatively high rate of EWL. Aggregating seems to be a solution to this physiological handicap. Our study also suggests a path for the evolution of social behaviour: as animals aggregate for physiological benefits, the stage is set for the evolution of more complex social interactions.