Introduction

Individual age is an important parameter in studies of primate sociobiology, ecology, and population genetics. As well as being a major determinant of behavior (Dunbar, 1988), it is critical for the construction of life tables, as a scale against which to plot measures of growth and maturation, and (if translated into date of birth) to provide a timescale for microevolution. Yet only a few exceptionally long and continuous primate field studies document age directly, from individual birth-to-death life histories of substantial numbers of animals. In other cases, individual age must be estimated from unreliable indicators such as external appearance or deportment. However, dental eruption, and subsequent wear of the occlusal surface, displays progressive change extending over most of an animal's lifetime. Where animals can be caught or restrained, dental evidence provides a basis for estimating the age of those whose life history is otherwise undocumented, as long as the timetable of eruption and wear can be reliably calibrated by reference to animals of known birth date. Elsewhere (Phillips-Conroy and Jolly, 1988), we have documented the sequence and timing of dental eruption in wild yellow baboons (Papio hamadryas cynocephalus) and hamadryas baboons (P. h. hamadryas), using animals of known natal age. Here, we develop a method of using the exposure of dentine on the occlusal surface of the molar teeth to estimate age, and apply it to adult individuals. By incorporating data from two populations, we are also able to explore interpopulation differences in dental wear rate.