CURTIN, A.J.; ZUG, G.R.; AVERY, H.W.; SPOTILA, J.R.; Drexel University; Natl Museum Natural History, Smithsonian Institution; Drexel University; Drexel University: Bone Growth Patterns as Indicators of Life History Parameters in Desert Tortoise (Gopherus agassizii) Populations
Ecological constraints influence bone growth strategies, and thus life history strategies, in a wide variety of amphibians and reptiles, however few such studies have been done on tortoises. Gopherus agassizzi are long-lived tortoises, living in a habitat where survival is extremely dependent on temperature and resource availability and quality. We obtained carcasses ranging in size and age from three different geographic areas: Arizona, East and West Mojave Desert. We also sectioned a sample of known-aged tortoises to validate skeletochronological methods used in obtaining age estimates for each population. These populations represent a wide range of inter- and intrapopulational growth strategies in desert tortoises from extreme arid environments. We examined various skeletal elements to investigate growth histories, variation with age and possible population trends. Results showed that bone microstructure and growth ring formation differed considerably within and between individuals from all populations. In general, the smallest juveniles exhibited fast growing embryonic bone. As body size increased, bone tissue varied from parallel-fibered bone (intermediate growth rate) to fibrolamellar bone (fast growth rate). Bone remodeling differed within populations, as did growth ring expression and distinction. Some West Mojave juveniles showed segmented structures in the medullary cavity area, the function of which is unknown. The range in bone microstructure was surprising because such variation is not common within single reptile species. This is the first study attempting to answer questions about tortoise biology across a broad geographic scope and time range with the added advantage of not having to harm, kill or disturb living animals of this threatened species.
