Bone histology of a lowland and montane species from the family Cordylidae (Reptilia : Squamata)

[摘要] ENGLISH ABSTRACT:Ecological adaptations, size and longevity, are known to be reflected as histological variationswithin vertebrate skeletons. To obtain baseline information on bone histology in the lizardfamily Cordylidae, two species, which differ considerably in general morphology and biology,were selected as representatives of the family. These were Cordy/us cataphractus, a lowlandlizard found along the western districts of South Africa, and Pseudocordylus capensis, amontane lizard occurring in the Cape Fold Mountains of South Africa. Cross-sections of thefemora from an ontogenetic range of individuals were examined for each species. In thesmallest juveniles of both species, the compacta consisted of typical embryonic bone, while inadults 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 adultsand juveniles of C. cataphractus, but was limited in P. capensis.Bone remodeling, which incorporates the removal of old bone as new bone is added, produceddistinctive featiires in both species. Endosteal resorption was intensive but irregular in bothspecies. In C. cataphractus, however, resorption was more widespread along the medullarycavity margin, while in P. capensis, it was more localized. This observation was supported bythe presence of embryonic bone remnants in most juveniles and some adults of P. capensis, aswell 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 beenresorped, 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 bonethickness (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 variationsobserved between these two species. P. capensis bone morphology, resulting in a lighterskeleton, could possibly be due to the need for speed and agility. C. cataphractus, on theother hand, is a heavily armoured, sluggish lizard with a lower basal metabolic rate than anyother cordylid species studied to date. A heavier skeleton would, therefore, be an advantagein supporting a heavily armoured body. It is suggested that the intensive, widespreadresorption in C. cataphractus could be due to food shortages as a result of group-livingbehaviour 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 linewas visible in only one C. cataphractus juvenile. LAGs varied in distinctness and spatialarrangement within and between the two species, possibly due to a variety of factors, includingaltitudinal differences, climatic conditions and individual variation. C. cataphractus possessedmore double LAGs than P. capensis, a feature usually related to cold climates and highaltitudes. This phenomenon is suggested to be mainly due to food shortages rather thanclimatic conditions, as C. cataphractus occurs in a warm temperate climate.Skeletochronology was used to provide information on age, longevity, growth rates and age atsexua! maturity for both species. Individual ages were estimated by calculating the number ofLAGs resorped and adding them to the number of visible LA Gs. The maximum estimated agewas 13 years in C. cataphractus and 11 years in P. capensis. Sexual maturity, assessedthrough 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 withmacroscopic and microscopic assessments of the gonads. Log-transformed variablesregressed against log number of LA Gs, were used to assess growth rates and sexualdimorphism. In general, there was considerable variation in all variables within age classesamong 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 ofLAGs (age). In P. capensis, sexual dimorphism in growth rate was only observed betweenBW A and age. Evidence suggests that both species have the potential for long life-spans.

[发布机构] Stellenbosch University