Variation in Cold Tolerance among Three House Geckos with Tropical Origins at the Northernmost Range Limitations in the Ryukyu Archipelago, Japan

For tropical ectotherms invading temperate regions, cold stress imposes significant negative impacts on their activity and survival. A previous study demonstrated that the recent northward range expansion of Hemidactylus frenatus to Amamioshima Island, Japan, after 2000 was driven by climate change rather than acquisition of stronger cold tolerance: the ambient temperature on Amamioshima after 2000 has increased compared to that on Okinawajima Island prior to 1955, where H. frenatus has long persisted. Two other gecko species of tropical or subtropical origin, H. bowringii and Gehyra mutilata, established populations on Amamioshima around the 1950s–1960s, suggesting they possess stronger cold tolerance compared to H. frenatus. To test this hypothesis, we compared cold tolerance among the Amamioshima populations of these three species, representing the current northernmost limits of their distributions, through laboratory experiments. Results showed significant differences in critical thermal minimum (CTmin) among the three species: G. mutilata exhibited the highest CTmin (11.35±0.34°C), whereas H. bowringii showed the lowest CTmin (8.58±0.40°C) and H. frenatus exhibited intermediate values (10.36±0.55°C). These findings suggest that G. mutilata persists in Amamioshima via behavioral thermoregulation, most likely relying on anthropogenic heat sources to survive the cold winter there. In contrast, H. bowringii may have stronger cold tolerance inherently or acquired locally, allowing this species to spread across Amamioshima in the 1960s. Rising ambient temperatures with climate change after 2000 enabled H. frenatus to establish a stable population on this northernmost island.