The prolongation of postnatal life among primates affects all life periods, including infantile, juvenile, adult, and senescent. Although humans are the longest-lived members of the order, the potential life span of the chimpanzee has been estimated at 60 years, and orangutans occasionally achieve this in captivity. The life span of a lemur, on the other hand, is about 15 years and a monkey’s 25–30 years.
The characteristic growth spurts of human infants in weight and height also occur in nonhuman primates but start earlier in the postnatal period and are of shorter duration. Primates differ from most nonprimate mammals by virtue of a delayed puberty in both sexes until growth is nearly complete; in humans, the peak of the growth spurt in boys comes slightly after the sexual maturity, whereas in girls the growth spurt precedes menarche. There is some controversy over the very existence of an adolescent growth spurt in nonhuman primates. In some species, males are very much larger than females; this extra growth occurs long after sexual maturity and rather rapidly, so it is possibly equivalent to the human growth spurt. The most remarkable case of such postmature growth is seen in orangutans. A male can mature physically in his early teens, or he can spend as much as 20 years as a subadult and then suddenly, within a year, almost double his weight and develop the cheek flanges characteristic of full maturity. It appears that this is related to social conditions; in proximity to a full-grown, dominant male, a subadult male’s development will remain suppressed, and when the dominant male moves away (or, in a zoo, is removed from the vicinity), the subadult undergoes a flush of testosterone and matures rapidly.
