Orangutans may be smarter than previously thought if a new study into the sophisticated way they build nests is any indication.

Scientists at The University of Manchester spent a year observing and filming (video footage available) orangutans at a research facility in Indonesia and found they apparently possess complex knowledge of mechanical design and material properties.

The great apes - which only live wild in Sumatra and Borneo and are one of man's closest relatives - build large, oval nests in tree canopies every day where they sleep overnight, possibly for protection from predators and parasites or for warmth during sleep.

Until now, little was known about the nests' mechanical design and material properties but the researchers, led by Dr Roland Ennos, and carried out by PhD student Adam van Casteren, reported that the orangutans used particular branches for different parts of the nest. They also broke the branches in different ways depending on how they would be used.

"We found that the orangutans chose strong, rigid tree branches for the structural parts of the nests 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," said Dr Ennos, based in the University of Manchester's Faculty of Life Sciences.

"Further, branches chosen 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 strong and comfortable nests."

The authors, writing in the journal Proceedings of the National Academy of Sciences, suggest orangutans, like some birds, might possess engineering expertise.

Dr Ennos added: "We witnessed orangutans building safe comfortable nests by half-breaking and weaving thick branches and twisting smaller branches right off to make a sort mattress. They seem to have learnt about the mechanical properties of wood, and use this knowledge in a clever way.

"Our research has implications for the evolution of intelligence and cognition as well as the evolution of tool use in early humans. It provides evidence that the development of all these traits started in apes because of their need to understand their mechanical environment, not just their social environment."

'Nest Building Orangutans Demonstrate Engineering Know-How to Produce Safe, Comfortable Beds,' by Adam van Casteren et al, published in PNAS.