Wild monkeys use loud calls to assess the relative strength of rivals
Gelada males - monkeys that are related to baboons - listen to the loud calls of rivals to gain information about their strength.
The monkeys use these calls to discern the strength of other males relative to them, and whether or not it's worth fighting them.
Males who lead large harems of females engage in the loud calls to deter challenges from bachelor males, who compete with leader males to gain access to females.
Geladas, also known as 'bleeding heart baboons,' are a species of monkey found only in Ethiopia.
They have a red patch of skin and their chest and neck - something that no other primate has.
Researchers based at the University of Michigan, Georgia State University and Princeton University have discovered that gelada males decide to escalate contests with opponents based on their condition relative to to the condition of their opponent.
They do this by interpreting the acoustic quality of the loud calls of their rivals, which are long distance vocalizations that carry honest information about the fighting ability of the caller.
The researchers conducted a study on populations of wild geladas living in the Simien Mountains National Park, Ethiopia.
The University of Michigan Gelada Research project has been collecting long-term data on this population of gelada's since January 2006.
To analyze how male geladas interpret calls, the researchers conducted experiments where they observed male geladas responses to other male's calls, both in natural observations and with playback experiments.
For the playback experiments, the researchers recorded loud calls during signalling contests using directional microphones.
They only used complete calls that didn't contain any background noise and interruptions.
Then, the researchers played back the calls to each of 60 adult geladas (20 females, 20 leader males and 20 bachelor males).
Each gelada's response was filmed, and the results were analyzed.
The researchers found that, supporting a mutual assessment strategy, gelada males responded to loud calls of different quality according to their own attributes and the attributes of their opponents.
'Previous studies in wild primates have shown that they use mutual assessment, but this was between animals that knew one other,' said Dr Jacinta Beehner, Univesity of Michigan associate professor of psychology and anthropology and co-author of the study.