Some really interesting research into how orangutans build their sleeping nests, something they do daily in the wild, hints that they may be smarter than we tend to think—and that's from an intelligence baseline which we already know is high.
Scientists from the University of Manchester in the UK looked at which branches orangutans choose and how they put them together. Dr Roland Ennos, lead research for the paper published in Proceedings of the National Academy of Sciences:
We found that the orangutans chose strong, rigid tree branches for the structural parts of the nest that supported their weight, and weaker more flexible branches for the nest's linings, suggesting that the apes' choice of branch for different parts of the nests was dictated by the branches' diameter and rigidity. Further, branches chose for the nests' structural framework were fractured differently from those chosen for the lining: Whereas structural branches were broken halfway across, leaving them attached, branches used for lining were completely severed, suggesting that orangutans might use knowledge of the different ways in which branches break to build to strong and comfortable nests.
They seem to have learnt about the mechanical properties of wood, and use this knowledge in a clever way. (Science Daily)