Gelada baboons are not actually true baboons, although they look very similar and are easily recognisable by the patches of hairless skin on their chest, which turns crimson when females are in oestrus. They are in fact the last surviving species of a once widespread group of grass-grazing primates. Geladas live in large groups of as many as 600 members.
The relationships of the Geladas are very delicately balanced. To communicate their intentions they have need of a fairly subtle range of signals. They have therefore acquired a great diversity of social behaviour patterns and vocalizations. Greater in fact than any other non-human primate. For examp]e, where the olive baboon has fifteen contact calls, and the colobus six, the gelada makes twenty-seven distinct noises. To hear him speak, is as it were to listen to a foreign language being spoken. The expressions on the face are in fact signals with a distinct meaning: the raising of the eyebrows reveals two red triangles above the eyes - a warning signal; the rolling back of the upper lip in a ghastly smile, a flash of red gums and white teeth, signifies (as perhaps does the human smile) appeasement, and thus avoids possible conflict. Source: selamta.net