Intrataxonomic variability in metapodial and femur bone density related to age in guanaco (Lama guanicoe). Zooarchaeological and taphonomical implications

The aim of this paper is to present new information about bone mineral density (BMD) variability in guanaco (Lama guanicoe), a South American wild camelid; while taking into account its ontogenic development. The importance of considering intrataxonomic variability in bone density related to age for constructing zooarchaeological indices and applying them to archaeological assemblages is also discussed. The results presented here were developed from a large sample of 46 metacarpals and 45 femurs of modern guanacos, ranging in age from ca. 6 months of gestation to 12 years. Bone structural densities were measured using a biphotonic X-ray densitometer, DEXA method, Norland EXCELL and the bone mineral content (BMC) of each scanned portion or complete element was divided by its volume, which was obtained independently by water displacement. Our results indicate that the bone density of each anatomical unit is highly variable, even among individuals of the same species. According to metacarpals data, age is an important factor affecting BMD. Moreover, our study shows that the density ranges of different skeletal elements can overlap, suggesting that it is not possible to predict that a certain element is always denser than another one. Finally, we present new evidence about the potential age-related taphonomic bias in faunal assemblages, indicating that meanwhile some skeletal elements show clear differences in their density between their fused and unfused state (metacarpal) other do not (femur).