• Findings of the present study suggest that gibbons do not exhibit cooperative behaviours among each other
• Behavioural observations revealed that the gibbons spent significantly more time alone than with their conspecifics and were very little involved in social interactions
• As mentioned above, gibbons are socially monogamous and live in small groups
  --> Support of the social brain hypothesis

• Cooperative behaviours have not only been invested in apes but also in several monkey species:

• Based on the findings of previous research and the present study, occurrences of cooperative behaviours appear to have evolved in two lineages:
  • Platyrrhini (i.e. Callitrichidae and Cebidae)
  • Catarrhini (i.e. Hominidae)

• Suggestion: This development is a result of convergent evolution
  • The gap between families that do and do not exhibit the corresponding behaviours seems to be too large in order to consider this trait to be a homology
• More likely: Similar selective pressures and adaptations to comparable circumstances have independently caused the development of cooperative behaviours
• All the mentioned mammal species that were successful in the cooperative problem-solving tasks have in common that they are not socially monogamous but live in groups with fluctuating social structures and flexible breeding compositions

Cooperation test with siamangs (Symphalangus syndactulus) --> Will they be more successful?

• Live sympatrically with H. lar
• More cohesive groups than H. lar
• More socially involved than H. lar