Skeletochronology of the endangered side-neck turtle, Podocnemis expansa

Introduction Management of endangered species requires knowledge of the age structure of populations. Often this is ascertained by mark–recapture studies. However, such studies usually require an extensive time span for long-lived organisms, whereas the endangered status of many species calls for faster data collection that can be used in risk assessment and, ultimately, decision making. The study reported here examines the potential of using skeletochronology as an alternative approach to mark–recapture studies of the endangered giant Amazonian river turtle, Podocnemis expansa, by examining the microscopic structure of the bones of these turtles recovered from kitchen remains and natural predation events. This method involves the determination of the age of the animal from growth marks evident in the skeleton. In many poikilotherms, skeletal growth is cyclical, that is, in a single year an animal has alternating periods of fast and slowed growth. Bone fluorescent labelling experiments have demonstrated that a period of fast growth, represented by a so-called zone, which is deposited during the favourable growing season, which usually corresponds to spring and summer months or periods of rainfall. The period of slowed growth is termed an annulus, and it is often associated with a line of arrested growth (LAG). In some cases, when periodic interruption is abrupt, only a LAG is formed. Thus, assuming that the growth rings (that is, annuli and/or LAGs) are annual, counting the number of these rings present in the skeleton provides a reasonable estimate of the age of the individual. Skeletochronology has been, and remains largely, used to determine the age structure of amphibian populations, including a threatened species, as well as some reptilian populations. More specifically, the validity and annual growth marks in marine turtles has been validated in Caretta caretta and in Lepidochelys kempii.