Spider monkeys already know: Sometimes you need a hug.
Animals that live in groups often break into smaller groups (fission), and later recongregate (fusion). Fission-fusion behavior is often tied to resource availability, 
but can also be related to social dynamics (competition among group members can lead to conflict, and breaking into smaller groups can alleviate the conflict). While fissioning into subgroups can relieve tension and conflict, bringing the subgroups back together can often be a trigger for aggression.
Spider monkeys live in groups, and break into smaller groups to hunt and eat. It has been noted for some time that when hunting parties return to the larger group, those returning often embrace the waiting spider monkeys.
Scientists from the UK studying two populatations of spider monkeys in the Yucatan have observed that the individuals who engage in this hugging are much less likely to behave in an aggressive fashion, either to each other or to others in the larger group.
Filippo Aureli and Collene Schaffner believe theirs is the first evidence for deliberate conflict management at fusion, and believe it may indicate the reason for similar greetings exhibited by other fission-fusion species such as spotted hyenas, bonobos, and humans.
- Morell V (2007) Hug it out, Monkey. ScienceNOW 21Feb
- Aureli F and Schaffner CM (2007) Aggression and conflict management at fusion in spider monkeys. Biology Letters, Vol 3, no. 2. (published early online)
