Habitat Use by the Rock-Dwelling Karoo Dwarf Tortoise, Chersobius boulengeri

Abstract

The global population status of many reptiles, particularly tortoises as one of the most threatened tetrapod groups, necessitates conservation efforts and study. Baseline information about habitat use can help formulate conservation strategies and improve effectiveness of surveys. I studied habitat use by Karoo Dwarf Tortoises (Chersobius boulengeri), an endangered South African endemic with only one population known to contain males, females, and juveniles, to aid conservationists in assessing which nature reserves (with incidental records) and potential new reserves may sustain populations, and to inform surveys for additional populations. Available microhabitats and rock types were mapped using an unmanned aerial vehicle. I analyzed the tortoise distribution and determined home range sizes (minimum convex polygons) and minimum daily movement distances of nine males and 16 females using radio transmitters. All fieldwork was conducted in spring 2018 and 2019, and in summer 2018, 2019, and 2020. Relatively high numbers of tortoises inhabited sills (i.e., exposed dolerite intrusions between sandstone) that contained ample retreats among boulders and rocks, whereas relatively few lived on level plateaus that might expose them to predatory birds. Despite their apparent preference of sills, tortoises were able to survive in home ranges that contained little sill microhabitat. Tortoises were rather sedentary, having small sexually and seasonally similar home ranges (1.10–1.14 ha) and movement distances (27.6–28.4 m/day), although their home ranges were larger than those of the related Speckled Dwarf Tortoise (Chersobius signatus), possibly as a result of different habitat productivities. Conservationists and surveyors should consider that sills appear important for Karoo Dwarf Tortoises, but generic rocky slopes may also harbor individuals. Future studies could address aspects that underlie microhabitat use, such as availability of retreats and vegetation, and integrate available information in a spatial model.