HETEROCHRONY AND SEXUAL DIMORPHISM IN THE PIGTAILED MACAQUE (MACACA-NEMESTRINA)

Somatic growth is not a simple linear process with a constant rate of growth. The most successful attempts to quantify growth as a function of age or size have employed nonlinear techniques. Sexual dimorphism o f primate growth, weight vs, age, was examined using nonlinear models with Sirianni and Swindler's ([1985] Growth and Development of the Pig tailed Macaque, Boca Raton, FL: CRC Press) growth data on the pigtaile d macaque (Macaca nemestrina). The best fit of several exponential gro wth models was the Gompertz curve: Weight = ae(-b*e-K*age). Different multiple phase models were also fit, where each phase represents a di stinct exponential component. The two-phase models proved to be the be st (R(2) = .0.84 for females, 0.91 for males), suggesting that there a re two growth spurts, one in infancy and one at puberty. The timing of the beginning and end of the first spurt is the same in males and fem ales, but the rate, and value of the asymptote for this phase, is grea ter in males. The timing of the second spurt is earlier, and the rate of growth for this spurt is smaller in females than males. The sexual dimorphism in these species is not a simple rate change, but a complex interaction of timing and rate over the entire period of growth. It w ould be impossible to separate these entities with a linear, polynomia l, or single-phase model of the data. While these data and results com plement much of the existing work on adult dimorphism, they also empha size the vital role that ontogenetic data have in elucidating the unde rlying evolutionary mechanisms that generate sexual dimorphism. (C) 19 94 Wiley-Liss, Inc.