An investigation of the palaeoecology and past distribution of tortoises (chelonians) in the arid interior of South Africa: a tool to aid present day conservation

Abstract

This dissertation focuses on the past and present ecology of land tortoises (Testudinidae) from the arid interior of South Africa, with special emphasis on the leopard tortoise (Stigmochelys pardalis). Aside from an Introduction and Conclusion, it comprises four chapters, each a stand-alone paper of which two have already been published and two others are being prepared for submission, dealing with different aspects of tortoise ecology and palaeoecology. It addresses an important anthropogenic threat to persistence of tortoise populations, and then investigates human-tortoise interactions that occurred in past Quaternary environments. This research also led to the creation of an extensive modern osteological tortoise collection which can be used as a comparative and reference collection for future researchers. The first paper focuses on mortality profiles of tortoises that were collected along electrified farm fences. We have examined and modelled the impact that these fences have on a leopard tortoise population. Previous studies only reported on deaths due to these fences, but so far the long-term consequences for populations have not been investigated. Results show that fence-related mortalities are biased towards larger, breeding-age individuals, which in turn has significant negative effects on population projections and extinction risks. With the growing popularity of electrified fences for the protection of livestock and game from unwanted predators and theft, the tortoise populations are being affected negatively and this could lead to their demise in the near future if nothing is done about the situation. The remaining three papers focus on human-tortoise interactions in the past, using material from the archaeological site of Wonderwerk Cave (Northern Cape Province) as a case study. In the second paper of the dissertation, bone mineral density (BMD) values were calculated for leopard tortoise limb bones using a densitometer equipped with software for small 2019 Holt PhD animals. Values obtained were compared with published values for leporids (rabbits/hares), canids (dogs/wolves etc.) and marmots. Since the shape of tortoise bones differ from those of mammals, new scan sites had to be defined as this was the first time a reptile has been scanned. The third paper reports the results of a second round of scanning that was undertaken also with the bone densitometer, but this time for the shell (carapace and plastron). As no tortoise shell has ever been scanned before, there were no values to compare our findings with. To overcome this, computed tomography (CT) scans on three different species of South African tortoise were compared with one another. BMD values obtained from extant tortoises were then used to investigate survivorship of bones in an archaeological context, the Later Stone Age tortoise remains from Wonderwerk Cave, Northern Cape Province (~15,000 years BP to present). The results showed that bone density can be a key taphonomic agent in archaeological and palaeontological assemblages, as the denser parts of the bones will survive better than those that are less dense, and so can help predict which elements of a tortoise should preserve the best over time. In the fourth paper, the results of analysis of the Earlier Stone Age (ESA) tortoise remains from Wonderwerk Cave were reported, spanning the period ~2.0 million years to ~0.5 million years BP. These remains were studied taxonomically, enabling identification of the tortoise remains as belonging to the leopard tortoise, making this assemblage the oldest occurrence of this species in an archaeological site in southern Africa. In addition, Geographic Information Systems (GIS) were used to plot the spatial distribution of the tortoise remains from all the ESA strata. Results showed that finds from the three strata with the largest samples were concentrated in squares implying that they represented bones from the same animal. Taphonomic analysis of the ESA remains, compared to data for other 2019 Holt PhD Early- and early Mid-Pleistocene sites in Africa, suggest that the Wonderwerk Cave assemblage is primarily anthropogenic in origin with some evidence for carnivore activity. This dissertation has contributed new and valuable information specifically on the threat faced by leopard tortoise populations due to electrified fences, and highlights the need for urgent attention. It has also provided the first bone mineral density values for tortoises, and information on species biogeography in the Pleistocene. The results presented herein open new opportunities for investigating tortoise (palaeo)-biogeography in southern Africa, and the value of such research to understand environmental change impacts on this animal group.