Grass is a poor food for a primate: abrasive, low in energy, and demanding a great deal of chewing. The gelada is equipped for it. Its molars have high crowns and complex enamel ridges that resist wear, and its incisors are proportionally small compared with the fruit-eating monkeys around it, since nothing needs biting open.
The hand is the more striking adaptation. Geladas have a short index finger and a relatively long, strongly opposable thumb, giving a fine precision grip well suited to pinching individual grass blades. They feed seated, shuffling forward on their haunches so both hands stay free, a posture almost unique among primates.
The diet shifts seasonally. In the dry season geladas dig for underground rhizomes and take more herbs and seeds, so the often-quoted figure of ninety per cent grass describes the wet season rather than the whole year.