Primate population studies at Polonnaruwa. III. Somatometric growth in a natural population of toque macaques (Macaca sinica)

Abstract Growth studies of non-human primates in natural populations are rare due to difficulties in obtaining measurements on animals of known age. We report a cross-sectional analysis of growth in the natural population of toque macaques ( Macaca sinica ) from Polonnaruwa, Sri Lanka, including the timing of growth and physical maturation and the developmental basis of sexual dimorphism. Twenty-eight measurements were collected from 274 macaques aged 2 weeks to 34 years. Non-parametric spline curves were fit to the distribution of measurement values over age separately by sex. The spline curves suggest four age intervals in which linear regressions of trait on age and analysis of covariance of measurement value with age, sex, and their interaction, can be used to parametrically test specific hypotheses about growth and sexual dimorphism. Most growth had ceased for both sexes in the adult phase. Prior to this, females had two and males three distict growth phases. There was little sexual dimorphism in growth rates among infants and young juveniles (birth to 2·5 years). The age limits for subsequent growth periods differed by sex and trait. Thus, skeletal limb growth ceased in juvenile females by about 5·5 years, but continued into an additional subadult phase in males reaching completion by about 7·5 years, whereas muscle mass and weight reached maturity by about 8 and 12 years in females and males, respectively. An adolescent growth spurt was evident only for body weight in males, but cannot be ruled out for other traits. These macaques grow for one and a half to twice as long as macaques from other species in laboratory colonies. Much of the sexual dimorphism among adults arises from bimaturism, although sex differences in growth rates among older juveniles are also responsible for a significant portion of adult dimorphism.