Hans Kummer, who has studied them extensively, found that the young are born black and remain in that pelage for the first six months. But at their moult of around six months of age, they turn a dirty brown color, and it is as if a social door had slammed shut. They become social competitors. Similarly, a human grandfather's attitude towards a toddler who spills his best tobacco is mild and protective, but a pimpled adolescent who steals it can expect a much harsher reprimand. Both male and female juvenile and adolescent Hamadryas baboons are brown.
Females tend to lighten slightly with age but remain brown.
Males on the other hand continue to lighten so that the old male Hamadryas baboons are almost pure white. Mature males are grizzled gray, and as they age the cheek whiskers lighten first, then the mantle, until finally the only brown remaining is a spot on the lower back, and in very old males even that disappears. The amount of hair-graying is highly correlated with rank. A quick glance gives one a ready estimate of his fellow's position in the age spectrum and hence his social rank. George Schaller found a similar phenomenon among the mountain gorillas.