Well, they have a degree of engineering expertise, anyway.

Safe, comfortable oval nest structures are built by orangutans in the canopy using thick branches to support their mass and thin ones to line the nest with sophisticated compliant and comfort-giving springiness.

The breaking of branches halfway, then twisting them to enable basket-like weaving is one obvious feature of the engineering task. Rigidity and strength are apparent in the basic oval nest.

They probably use diameter as a measure of a sticks rigidity, implying the evolution of some mathematical "intelligence." The base consists of a mattress or rim for the whole structure, as shown by our gentleman demonstrator in the first video.

"It's clear that orangutans use their knowledge of the mechanics of how different types of branches break to make their nests," as Adam van Casteren from the University of Manchester, UK, puts it. He is the lead author of the research, published in PNAS this week.

Human links are pretty obvious although Manchester are not apparently going to promote their degree for undergraduate orangutans. In Sumatra, Adam was investigating conservation of energy in the bipedal walk of the apes. Instead the intrigue of how the nests are constructed fascinated him. Better sleep and safe conditions obviously encourage many primates to nest in trees.

Humans certainly had ancestors who needed safe trees, caves or whatever to protect them. Because orangutans roam far and wide, new nests are built at least every night, which must cause them to be lighter, easily constructible and to use available timber. Even roofs and pillows are added by certain fastidious apes. Adam adds, "they're solid structures and only take orangutans about 10-15 minutes to build."

Here is our hero, finishing off the master suite:

The obvious argument is that this is analogous to tool-making, even though no tool is used. Similar cognitive abilities are used and the product is equally valuable to the animal. Beavers and birds are next on the agenda to see how they configure their engineering brains.