The gelada is an Old World monkey, not a baboon despite previous naming conventions. It is the only living member of the once widespread genus Theropithecus and is only found in the highlands of Ethiopia. The present day distribution of the gelada is limited to the steep escarpments and gorges that border the eastern side of the central highlands and the northwestern highlands of Ethiopia. The gelada feeds predominantly on fresh shoots of grass, and to a lesser extent on grass roots and seeds. The gelada is also called the bleeding heart baboon as a result of the distinctive, bright red, heart-shaped patch on its chest. The gelada social system consists of a hierarchy of social groupings. The basic group is a reproductive unit of the breeding males (1-4) and females (1-10) and their dependent young.
The females tend to be closely related and have strong social ties and stay in their band all their lives. A band of gelada shares a common foraging and sleeping area and may contain 2-10 reproductive units, as well as 1-3 all male groups (non-breeding males of a young age, who remain in these groups for 2-4 years before trying to enter a reproductive unit). The ranging areas of different bands overlap, and can mix easily for a short period, without any aggression, to form very large gelada ‘communities’ or herds. These herds can be up to 1,000 strong – geladas can associate in one of the largest groups of any primate on earth.
It is estimated that approximately 2,500 geladas live in the park with a further 2,000 on the surrounding Simien massif. The average band size is 200 geladas. There is a higher density of geladas in Sankaber and a lower density towards the cliffs of Gich and Chennek.