The sequence is unusual among mammals. The first change removes the infant's conspicuous pale coat within the first year. The second, at around six to eight years, applies only to females and reverses the direction, producing an adult pair so different that early naturalists described the sexes as separate species.
Both sexes keep a distinctive band of white fur above the eyes, which is far more obvious against the male's black coat and gives the animal its permanently startled expression. Adults weigh six to nine kilograms and, like all gibbons, live in small groups built around a mated pair and their dependent young.
Why the female reverts is not established. Signalling reproductive status to a resident male, or reducing aggression from adults during the period when a maturing female must leave her natal group, are the main proposals, neither of them tested against the alternative.