Gibbons of genus Nomascus have a pronounced sexual
dimorphism which is detectable until after the onset of physical maturity.
Males maintain a black coat with white hair on the cheeks after reaching theirphysical maturity. In contrast females change into cream to golden coloured
coat with black faces and dark or black hair on the head. Infants are born
buff, by six month of age they begin their colour change, reaching a complete
colour change by eighteen month of age. Adult males and immature of both sexes
are thus blackish in colour.
Identical coloration of young animals makes the sex
determination complicated. The situation is further complicated by the fact
that female Nomascus are characterized as having a peniform clitoris, and a
membrane has been shown to occlude the vulva. The clitoris of females is almostas big as the penis and also contains a bacculum compared to genus Symphalangusor Hylobates. Resemblance to the male is enhanced by black pelage in immature
females and therefore it caused confusion in identifying their sex
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