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Ecology and Morphology of the Kalahari Tent Tortoise, Psammobates Oculifer

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Ecology and Morphology of the Kalahari Tent Tortoise, Psammobates Oculifer Ecology and morphology of the Kalahari tent tortoise, Psammobates oculifer, in a semi-arid environment. Tobias Keswick A thesis submitted in partial fulfilment of the requirements for the degree of Doctor Philosophiae, in the Department of Biodiversity and Conservation Biology, Faculty of Science, University of the Western Cape Supervisor: Prof. Margaretha D. Hofmeyr March 2012 Ecology and morphology of the Kalahari tent tortoise, Psammobates oculifer, in a semi-arid environment. Tobias Keswick KEYWORDS Behaviour Burrow Growth rings Habitat Home range Psammobates geometricus Refuge Savanna Sexual dimorphism Thermoregulation i ABSTRACT Ecology and morphology of the Kalahari tent tortoise, Psammobates oculifer, in a semi-arid environment. Tobias Keswick Ph.D. thesis, Department of Biodiversity and Conservation Biology, University of the Western Cape. Southern Africa harbours one-third of the world’s Testudinid species, many of which inhabit arid or semi-arid areas, but ecological information on these species is scant. I studied the habitat, morphology and ecology of Kalahari tent tortoises over 13 months in semi-arid Savanna at Benfontein farm, Northern Cape Province, South Africa. In order to allow continuous monitoring of individuals, I attached radiotransmitters to males and females, split equally between two habitats, sites E (east) and W (west), with apparent differences in vegetation structure. Results of the study were based on data obtained from 27 telemetered tortoises and 161 individuals encountered opportunistically. Female Kalahari tent tortoises were larger than males and the sex ratio did not differ from 1:1. Based on person-hours to capture tortoises, the population appeared to have a low density, with more time required to capture a juvenile (35 hours) than an adult (10-11 hours). The frequency distribution of body size ranges was indicative of recruitment. Relative age, based on annuli counts, suggested that males were younger than females, perhaps because males as the smaller sex are more predation-prone than females. Linear relationships between annuli counts and shell volume indicated that, after reaching sexual maturity, female body size increased faster in volume than did male body size, possibly because a larger volume may enhance female reproductive success. Body condition differed between sites, sexes and among seasons. The hot and dry summer may account for low summer body condition, whereas vegetation differences and size effects, respectively, may account for the low body condition of tortoises in site W and in males. Site E was sandy with grasses, particularly Schmidtia pappophoroides, being the prevalent growth form. This habitat resembled a Savanna vegetation type Schmidtia pappophoroides – Acacia erioloba described for a neighbouring reserve. Site W was stonier, dominated by shrubs, and was reminiscent of Northern Upper Karoo vegetation (NKu3). Neither site resembled Kimberley Thornveld (SVk4), the designated vegetation type of the area. Differences in substrate and grazing intensity may have contributed to site vegetation differences. Rainfall had an important influence on seasonal vegetation. Short grass abundance ii correlated with rainfall and annual plants sprouted after spring rain. Refuge use changed according to season and sex. Males selected denser refuges than females did, perhaps because males were smaller and more vulnerable to predation and solar heat. Tortoises selected sparse, short grass as refuges in cool months, probably to maximise basking whilst remaining in protective cover. During hot periods, mammal burrows were preferred to vegetation as refugia. The smaller males spent more time in cover than females, which may be related to predator avoidance or thermoregulation. Females spent more time basking than males, perhaps due to their larger size and to facilitate reproductive processes. Tortoises did not brumate, but through a combination of basking, and orientation relative to the sun in their refuges, managed to attain body temperatures that allowed small bouts of activity. Body temperature for active tortoises was similar among seasons, and was higher for more specialised active behaviours, such as feeding and socialising, than for walking. Increased activity by males in spring could relate to mating behaviour while females were more active in autumn, when they foraged more than males, perhaps due to the high cost of seasonal reproductive requirements. Males displaced further per day than did females, but home range estimates did not differ between sexes. Annual home range estimates varied substantially among individuals: 0.7–306 ha for minimum convex polygons and 0.7– 181 ha for 95% fixed kernel estimates. The ability to cover large areas would assist tortoises in finding resources, e.g., food, in an area where resource distribution may be patchy. Differences among seasonal home ranges and movements probably reflect seasonal climatic change; activity areas shrinking when temperatures were extreme. In order to assess the effects of a semi-arid environment on the morphology of P. oculifer, I compared its morphology to that of its ‘cool-adapted’ sister taxon Psammobates geometricus, using live and museum specimens. Both P. oculifer and P. geometricus are sexually dimorphic and differences between the two species could indicate environmental or sexual selection effects, or a combination of the two. The shorter bridge length, which allowed more leg space, and wider front feet in P. oculifer cohorts probably represent traits for manoeuvring in a sandy habitat, while wider heads in P. oculifer possibly relate to interspecific differences in diet. The flatter shell in female P. oculifer, relative to P. geometricus, may represent a trade-off between space for reproductive structures, e.g., eggs, and the need to fit into small refuges, e.g., mammal burrows. Male P. oculifer had wider shells, more space around their hind legs, and wider hind feet than P. geometricus males had, all characteristics which may assist males to fight and mate in a sandy environment. March 2012 iii DECLARATION I declare that Ecology and morphology of the Kalahari tent tortoise, Psammobates oculifer, in a semi-arid environment is my own work, that it has not been submitted for any degree or examination in any other university, and that all the sources I have used or quoted have been indicated and acknowledged by complete references. …………………………… Tobias Keswick March 2012 iv ACKNOWLEDGEMENTS I would like to thank De Beers Consolidated Mines and the Percy Fitzpatrick Institute for allowing me use Benfontein farm as a study area, and the staff at Benfontein for assisting me in the field and putting up tortoise road signs. At De Beers, Morgan Hauptfleisch was responsible for allowing me to work at Benfontein and subsequently Duncan MacFadyen and Jonathan Denga were of continual support. In particular, I thank Eric Herrmann at Benfontein who never stopped providing me with help and friendship for the duration of my project. Tania Anderson from the Kimberley Museum generously provided me with all botanical back up, including identifying all the plant species I collected. My friends Jan Kamler and Donald Iponga also helped me at Benfontein and I thank them both. From Edward Oppenheimer and Sons, Mark Berry, Gus van Dyk, Dylan Smith and Shari Cade allowed me access to Tswalu and Dronfield game reserves and were helpful and hospitable. I would like to thank Denise Hamerton at the Division of Natural History at The South African Iziko Museum for providing access to the Iziko collection of Psammobates geometricus and P. oculifer specimens. Equally, I thank Lemmy Mashinini and Lauretta Mahlangu at Ditsong (formerly Transvaal) Museum for providing me with the facilities, their time and the access to their collection of Psammobates oculifer, and W.D. Haacke who was responsible for collecting many of the specimens. Ernst Baard and co- workers at Scientific Services, CapeNature, permitted me to accompany them on their Psammobates geometricus surveys, which allowed me to collect valuable morphological data – my sincere thanks to all. The Northern Cape Department of Nature and Environmental Conservation permitted me to collect tortoise and botanical data under permit numbers: 011/2005 015/2006, 0032/06 and 0034/06. I also thank South African Weather Services (SAWS) for providing me with climate data. I received funding from UWC and the National Research Foundation for which I am very grateful. Numerous people took the time and trouble to provide me with information and ‘technical’ support for my project and they include: Timm Hoffman at the Plant Conservation Unit, Simon Todd, Alan Channing (for getting me the original opportunity) and Audrey King and Linda van Heerden at UWC, Alfred Hartemink, Rod Douglas, Tai and Diane Stein, Thomas Slingsby, Ian Little, Jessica da Silva, Brian Henen, Victor Loehr, Bill Clark at Earthpoint, Rosie Stanway and Donovan Kirkwood. v My supervisor, Retha Hofmeyr, could not have been better. She supported me, with the occasional shove, throughout my term as her student and I will always be extremely grateful. Fiona, my wife, has listened to my tales of woe for the last five years and is still here! She read several drafts of chapters, assisted me in the field and was critical, helpful and supportive. My parents are the reason I am here – Cheers Mum and Dad. My brother, Adam, got a few panicked telephone calls too – thanks. vi TABLE OF CONTENT KEYWORDS ................................................................................................................
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