Geladas are monkeys that live in the Ethiopian mountains and spend more time on the ground than any other primate (except humans). They only eat grass, and live in small reproductive units within a large group. Within these large groups males must stay on their toes to ensure they pass their genes on to the next generation.
The small mating units consist of one male leader and several females, but these groups are at risk of take-over from single males. Free roaming males are on the lookout for a harem of females of their own, and the quickest way to become surrounded by females is to target the leader of a weak reproductive unit.
To decide which leader males to chase off, single males eavesdrop on fights between others, listen in on vocal contests between other males and hang around unstable groups that have recently been taken over. By watching and listening, unattached geladas can pinpoint weak points in a leader male and swoop in to take his place.
Image from www.flickr.com/photos/gelada