|Range||Horn of Africa and the southwestern tip of the Arabian Peninsula.|
Apart from the striking size difference between the sexes (males are often twice as large as females), which is common to all baboons, this species also shows sexual dimorphism in coloration. The fur of males is silver-white in color and they have a pronounced cape (mane and mantle) which they develop around the age of ten, while the females are capeless and brown. Their faces range in color from red to tan to a dark brown.
Males may have a body measurement of up to 80 cm (31 in) and weigh 20–30 kg (44–66 lb); females weigh 10–15 kg (22–33 lb) and have a body length of 40–45 cm (16–18 in). The tail adds a further 40–60 cm (16–24 in) to the length, and ends in a small tuft. Infants are dark in coloration and lighten after about one year. Hamadryas baboons reach sexual maturity at about 51 mo for females and between 57 and 81 mo for males.
Hamadryas baboons often appear in ancient Egyptian art, as they were considered sacred to Thoth, a major and powerful deity with many roles that included being the scribe of the gods. Astennu, attendant to Thoth, is represented as a hamadryas in his roles as recorder of the result of the Weighing of the Heart and as one of the four hamadryas baboons guarding the lake of fire in Duat, the ancient Egyptian underworld. A pre-dynastic precursor to Astennu was Babi, or "Bull of the Baboons", a bloodthirsty god said to eat the entrails of the unrighteous dead. Babi was also said to give the righteous dead continued virility, and to use his penis as the mast of a boat to convey them to the Egyptian paradise.
Sometimes Thoth himself appears in the form of a hamadryas (often shown carrying the moon on his head), as an alternative to his more common representation as an ibis-headed figure. Hapi, one of the Four Sons of Horus that guarded the organs of the deceased in ancient Egyptian religion, is also represented as hamadryas-headed: Hapi protected the lungs, hence the common sculpting of a stone or clay hamadryas head as the lid of the canopic jar that held the lungs and/or represented the protection of the lungs. Hamadryas baboons were revered because certain behaviors that they perform were seen as worshiping the sun.