A recent study of wild siamangs in Way Canguk (Sumatra, Indonesia), by CICASP's Dr. Luca Morino, shows that males in multimale groups mate less frequently and for shorter average durations than males in one-male groups. This is thought to be a way to minimize the risk of detection, harassment, or interruption by other adult male group members, and in a broader perspective promote social tolerance in multimale siamang groups. Long-term investigation of the study population, which is characterized by equal numbers of one-male and multimale siamang groups, allowed for such detailed comparison to be made.
The original article, published in the journal Behavior, can be accessed here.