Cooler performance breadth in a viviparous skink relative to its oviparous congener

Susceptibility of species to climate change varies depending on many biological and environmental traits, such as reproductive mode and climatic exposure. For example, wider thermal tolerance breadths are associated with more climatically variable habitats and viviparity could be associated with greater vulnerability relative to oviparity. However, few examples exist detailing how such physiological and environmental traits together might shape species thermal performance. In this study we compared the thermal tolerance and performance of two sympatric skink congeners in Hong Kong that differ in habitat use and reproductive mode. The viviparous Sphenomorphus indicus lives on the forest floor while the oviparous Sphenomorphus incognitus occupies stream edges. We quantified the thermal environments in each of these habitats to compare climatic exposure and then calculated thermal safety margins, potential daily activity times within each species' thermal optimal range, and possible climate change vulnerability. Although we did not detect any differences in thermal tolerance range or thermal environments across habitats, we found cooler performance in S. indicus relative to S. incognitus. Moreover, while optimal activity time increases for both skinks under a warming scenario, we project that the thermal safety margin of S. indicus would narrow to nearly zero, thus losing its buffering capacity to potential extreme climate events in the future. This research is thus consistent with recent studies emphasizing the vulnerability of viviparous reptiles to a warming climate. The results together furthermore highlight the complexity in how environmental and physiological traits at multiple spatial scales structure climate change vulnerability of ectothermic species.