Bergman, an expert in primate vocalization and cognition, has been recording the leader males' triumphant triple "Yeow" to see how the yell degrades over time, signaling weakness to bachelors.
A deposed leader may be allowed to stay with a harem, where he cares for the young but loses mating rights, taking on a sort of avuncular role. Within a few months of being dethroned, the flaming redness of his chest patch subsides to an anemic pink.
The gelada is the only species remaining from a lineage of grazing primates once more common than baboons, says Robin Dunbar, who studied geladas in the 1970s. Its predecessors began disappearing a million years ago when the climate warmed. Palatable grasses began growing only at much higher altitudes, the monkeys shifted their range or starved, and now, Dunbar says, "we have only this one species on the tops of the mountains."
Today, with Ethiopia's 1974 to 1991 civil war concluded and the government stabilized, the northern mountains are accessible to researchers once again. And the local economy is picking up. Goats, cows and sheep compete with the monkeys for grass in alpine pastures, and farmers sometimes kill geladas that plunder barley crops. It's not clear how many geladas there are. Dunbar's estimates from the 1970s put the population at 100,000 to 200,000, but much land has been converted to farms since then. Roaming herds and rugged terrain make counting difficult, but Beehner, who has done surveys of her own, worries that the current figure is much lower—perhaps as few as 20,000.
Beehner and Bergman are also studying herd structure. Although the monkeys spend hours socializing within their harems—especially while sunbathing in the morning—they tend not to know their neighbors in the herd. The bachelor gangs notwithstanding, Beehner says, "it's a little like humans in the suburbs."
Abigail Tucker is a staff writer for Smithsonian.
Anup Shah and Fiona Rogers live in Kenya and London; Shah's photographs appeared in a 2006 Smithsonian story about wildebeests in the Serengeti.