Western hoolock gibbon. Adult males have a long, black genital tuft and the white brow may be separated in its middle. Some males have slightly longer black hair under the brow and toward the center of the face, giving the appearance of a separated brow . Adult females of this subspecies have a distinct central part in the head hair. Eastern hoolock gibbon.
Adult males have black body hair with a brown cast, and the hair on the legs is usually black.
Males have a separated brow. There is a white or brown genital tuft, and the chin hair is often grizzled with white-beige hairs. Females' digits may have a trace of black on them. The female's crown hair grows slightly upwards and toward the nape of the neck and is not parted down the middle.