Comparative bone histology of stigmochelys pardalis (leopard tortoise), with specific reference to ontogeny and biomechanics
Botha, Alexander Edward
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Testudines are a group of reptiles characterised by the presence of a shell comprised of bony shields. Stigmochelys pardalis is the most widely distributed terrestrial testudine in southern Africa. Although relatively common with some life history traits (e.g. lifestyle, reproduction, longevity) being well known, the growth of this species has yet to be studied in any detail. This study is the first to use bone histology and microanatomy to examine the growth and biomechanics in an ontogenetic series of S.pardalis. The study also indicates clear short-comings in the determination of lifestyle using a single section on the diaphysis in S. pardalis and possibly in other testudines. The bone microanatomy of this clade differs from that found in other amniotes. In other amniotes, aquatic species tend to display large osteoporotic bone with large infilled medullary cavities and thin cortices. Semi-aquatic species have thick bone walls with small or no medullary cavities whereas terrestrial species tend to have thinner bone walls, open medullary cavities and a sharp transition from cancellous to compact bone. A detailed histological analysis of the limb bones of S.pardalis reveals extensive variation through ontogeny. Cortical bone becomes increasingly thicker through ontogeny and is finally resorbed in the late sub-adult stage, resulting in a thin cortex and a large infilled medullary cavity. The predominant bone tissues are parallel-fibred and lamellar-zonal for forelimb and hind limbs respectively. In certain cases parallel fibred bone tissue transitions to lamellar-zonal bone tissue later in ontogeny. A few older individuals exhibit and External Fundamental System indicating that the growth rate had decreased substantially by this stage. However, these individuals are between 56% and 60% maximum known size indicating that this slow growing species takes many more years to reach its maximum body size. Inter-elemental variability is most prevalent between the forelimb and hind limb. Forelimb elements exhibit characteristics such as faster growing parallel-fibred bone tissue, slightly higher vascularisation and a predominance of annuli over Lines of Arrested Growth compared to the hind limb which exhibits poorly vascularised, slower growing lamellar zonal-bone interrupted by LAGs. These differences indicate that the forelimb grew more rapidly than the hind limb, possibly due to the method of locomotion typical in chelonians. Lifestyle inferences using Bone Profiler indicate an aquatic lifestyle for this species despite it being clearly terrestrial. Sections from individuals of various ontogenetic stages were tested and although the microanatomy of the bone changes dramatically with age, the lifestyle readings remained inaccurate. The extensive bone resorption that occurs from the early sub-adult stage destroys much of the primary cortex, thus destroying the ecological signal. This supports the results from other studies that have found that using bone microanatomy to determine lifestyle in testudines is inaccurate.