This paper presents the results of a taphonomic research programme conducted over the past 5 years. The main objective of the programme is to investigate differential survivorship in guanaco (Lama guanicoe) bones, taking into account the ontogenetic development of this species and the densitometric characteristics of its skeletal parts. First, density analysis was carried out on selected bones of modern individuals corresponding to different age classes. Second, two experimental designs were conducted to explore the response of animal bone of different ages to weathering and fluvial transport. The first is a long term experiment that examines the deterioration of skeletal elements from a newborn, a juvenile and an adult modern guanaco exposed to subaerial weathering under controlled conditions. The second experiment examines the hydrodynamic sorting of dry and wet skeletal elements from a newborn, a juvenile, and an adult modern guanaco in an artificial flume under controlled current velocities (15 and 30 cm/s). The main results of the research programme indicate that immature bones have higher hydric transport potential and weather at a faster rate. We propose that this differential bone behaviour is partially related to structural density, as demonstrated by density analysis. These results show age-related biases in zooarchaeological assemblages affected by taphonomic processes. Copyright © 2011 John Wiley & Sons, Ltd.