the trees. At some research sites such as Bili forest, in Congo, chimpanzees can build a significant proportion of their night nests on the ground. Nests consist of a mattress, supported on a strong foundation, and lined above with soft leaves and twigs. Nests are built in trees which have a minimum diameter of and may be located at a height of . Day and night nests are built. Nests may be located in groups. In orangutans Orangutans build day and night nests. Young orangutans learn by observing their mothers' nest-building behaviour. Nest-building is a leading reason for young orangutans to leave their mother for the first time. Starting at 6 months of age, orangutans practice nest building and gain proficiency by the time they are 3 years old. Initially, a suitable tree is located: orangutans are selective about sites even though many tree species are utilised. The foundation is then built by pulling together branches under them and joining them at a point. After the foundation has been built, they bend smaller, leafy branches onto the foundation; this serves the purpose of and is termed as the "mattress". After this, orangutans braid the tips of branches into the mattress. This increases the stability of the nest and forms the final process of nest building. Orangutans may add additional features such as "pillows", "blankets", "roofs" and "bunk-beds" to their nest. Orangutans make "pillows" by clumping together leafy branches with the leaves in the center and the twig shoots pointed outward. They bite the twigs to blunt sharp ends. Pillows are added to night nests but are usually absent from day nests. A "blanket" consists of large leafy branches with which orangutans cover themselves after lying down. Orangutans may create a waterproof overhead shelter for the nest by braiding together a loose selection of branches. They may also make a "bunk-nest" or "bunk-bed", a few metres above the main nest. In humans Humans build structures that are called constructions, but they can and do serve the same function as non-human primate nests. Modern constructions can be built with a variety of materials, the most common ones being clay, concrete, stone and wood. According to a non-verified hypothesis, archaic humans may have lived in tree houses until about 40,000 years ago. Evolution A study on the phylogeny of primate behaviour has revealed that the use of tree holes or nests are important