Browsing by Author "Curtin, Amanda"

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    Bone histology of a lowland and montane species from the family Cordylidae (Reptilia : Squamata)
    (Stellenbosch : Stellenbosch University, 1999) Curtin, Amanda; Mouton, P. Le F. N.; Chinsamy-Turan, A.; Van den Heever, J. A.; Stellenbosch University. Faculty of Science. Department of Botany and Zoology.
    ENGLISH ABSTRACT: Ecological adaptations, size and longevity, are known to be reflected as histological variations within vertebrate skeletons. To obtain baseline information on bone histology in the lizard family Cordylidae, two species, which differ considerably in general morphology and biology, were selected as representatives of the family. These were Cordy/us cataphractus, a lowland lizard found along the western districts of South Africa, and Pseudocordylus capensis, a montane lizard occurring in the Cape Fold Mountains of South Africa. Cross-sections of the femora from an ontogenetic range of individuals were examined for each species. In the smallest juveniles of both species, the compacta consisted of typical embryonic bone, while in adults it consisted of parallel-fibred bone. In general, the compacta was poorly vascularized. Endosteally formed lamellar and parallel-fibred bone, lined the medullary cavity in most adults and juveniles of C. cataphractus, but was limited in P. capensis. Bone remodeling, which incorporates the removal of old bone as new bone is added, produced distinctive featiires in both species. Endosteal resorption was intensive but irregular in both species. In C. cataphractus, however, resorption was more widespread along the medullary cavity margin, while in P. capensis, it was more localized. This observation was supported by the presence of embryonic bone remnants in most juveniles and some adults of P. capensis, as well as the shape of the medullary cavity area and number of lines of arrested growth (LAGs) resorped in both species. In C. cataphractus, up to four LAGs were estimated to have been resorped, and in P. capensis, up to three LAGs were estimated to have been resorped. Endosteal bone was present in 80 % of the C. cataphractus sample, but in only 68 % of the P. capensis sample. When it occurred in P. capensis individuals, it was thinner than the endosteal deposits of C. cataphractus individuals. In general, therefore, the relative bone thickness (RBT) of P. capensis was thin (9 - 39 %), compared to the RBT of C. cataphractus (18 - 49 %). Genetic and epigenetic processes can explain the differences and variations observed between these two species. P. capensis bone morphology, resulting in a lighter skeleton, could possibly be due to the need for speed and agility. C. cataphractus, on the other hand, is a heavily armoured, sluggish lizard with a lower basal metabolic rate than any other cordylid species studied to date. A heavier skeleton would, therefore, be an advantage in supporting a heavily armoured body. It is suggested that the intensive, widespread resorption in C. cataphractus could be due to food shortages as a result of group-living behaviour and apparent lack of active foraging. Growth was periodic in both species and growth marks were represented by zones and LAGs. Most P. capensis juveniles and even some adults had a clearly defined birth line. A birth line was visible in only one C. cataphractus juvenile. LAGs varied in distinctness and spatial arrangement within and between the two species, possibly due to a variety of factors, including altitudinal differences, climatic conditions and individual variation. C. cataphractus possessed more double LAGs than P. capensis, a feature usually related to cold climates and high altitudes. This phenomenon is suggested to be mainly due to food shortages rather than climatic conditions, as C. cataphractus occurs in a warm temperate climate. Skeletochronology was used to provide information on age, longevity, growth rates and age at sexua! maturity for both species. Individual ages were estimated by calculating the number of LAGs resorped and adding them to the number of visible LA Gs. The maximum estimated age was 13 years in C. cataphractus and 11 years in P. capensis. Sexual maturity, assessed through the analysis of femoral microstructure, occurred around six to seven years in C. cataphractus and four to six years in P. capensis. This estimate corresponded favourably with macroscopic and microscopic assessments of the gonads. Log-transformed variables regressed against log number of LA Gs, were used to assess growth rates and sexual dimorphism. In general, there was considerable variation in all variables within age classes among the juvenile cohort, but less variation was noted among adults of both species. In C. cataphractus, males and females exhibited different growth rates when snout-vent length (SVL), femoral length (FL) and bone wall area (BWA) were regresses against the number of LAGs (age). In P. capensis, sexual dimorphism in growth rate was only observed between BW A and age. Evidence suggests that both species have the potential for long life-spans.