Orangutans
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Introduction | Habitat | Behaviour | Development

Daily Activity
Orangutans are active only during the day (diurnal). Of their daytime activities 95% is spend on feeding, resting and moving between feeding and resting sites. The daily activity pattern of an average orangutan shows two peaks. After awakening in the morning, orangutans spend on average two to three hours vigorously feeding, after which they rest (midday). They subsequently travel during the late afternoon and in the early evening the orangutans prepare themselves for the night. They do this by constructing a "nest" in the tree branches in which they will sleep. These nests are made out of leaves and branches. Each individual makes its own nest, but mothers share their nest with their nursing offspring. The nest may additionally have leaves as a roof against the rain, leaves placed beneath their head as a pillow, leaves placed on top of their bodies, as a blanket, or flattened branches placed inside, as a mattress. Orangutans also make day nests, to either rest or nap during day time, copulate, eat comfortably, or for social play.


Locomotion
Orangutans travel on average between 90 and 3050 m a day, with an average of 790 m. Males generally travel further than females. Travel distance per day is directly proportional to home range size; orangutans with larger home ranges travel more per day than those with smaller home ranges.
In the peat and lowland swamp forests of Borneo, where faunal diversity is great, home ranges for females are between 3.5 and 6 km2. On Sumatra, where orangutans inhabit higher elevations and swamp forests with less diversity, average female home ranges tend to be larger, closer to 8.5 km2. There is little data on male home ranges because they are likely larger than any study sites, but it is inferred that male home ranges are several times larger than those of females, and as such are neither exclusive nor particularly stable.
Orangutans usually move by swinging from one branch to another, called brachiating. They use both hands and feet to hold onto the branches. Their fingers and toes (especially the thumbs and big toes) act like hooks, so that they can hold on to the branches easily and have a firm grip while moving horizontally through the forest canopy. When they move along the ground, orangutans walk quadrupedally on their fists, not their knuckles as is seen in the other great apes. They are also occasionally seen moving bipedally, like humans.


Feeding behaviour
Orangutans are mainly frugivores, that is, fruits are their favorite food (61% of their diet). Depending on which fruit it is, they may eat the skin, flesh, juice, and/or the seeds of a fruit. Fruit is always preferentially eaten, but when fruits are in short supply, orangutans forage opportunistically and depend more heavily on other plant foods such as leaves and bark. Sumatran orangutans also eat figs, however these are largely unavailable to lowland ranging Bornean orangutans.
Part of orangutan’s diet also consists of insects (especially termites and ants). In some regions, orangutans also have been seen to eat small animals (like slow lorises), or even occasionally soil. Caterpillars and honey might also be part of an orangutan’s diet.
Most water is consumed via the plants the orangutans eat, or by drinking water from tree holes. Orangutans therefore don’t even have to leave their tree branches to drink. Depending on the predator risk however, they sometimes do.


Sociality
The semi-solitary social organization and behavior of orangutans is highly unusual among the great apes, though the degree of sociality among orangutans varies quite a lot per site, sex and age. Especially food availability is an important factor, explaining whether or not individual-based fission-fusion groups may be formed in which animals associate in small parties. In this way feeding aggregations or travel bands (usually consortship groups) can be formed. Feeding aggregations are seen when resident males and females and non-resident subadults gather and feed in large fruiting trees with abundant fruit crops. Because of the large quantity of fruit available in a single location, competition for resources is decreased and individuals may arrive and leave independently. This is also the time when travel bands are formed. Here a few individuals coordinate travel between food sources for a few days at a time. The consortship groups that have occasionally been seen, usually consist of an adult breeding pair (and the adult female's infant or juvenile offspring) and may last from days to even months after copulation. The only long-lasting orangutan social group is a mother with her offspring, who live together for about 7 years.

Subadults of both sexes and some adult males are transient and range broadly. They may travel either alone or in groups of a few individuals. When they become adults they try to become resident. Females usually don’t really disperse. They have discrete home ranges that overlap with other adult females (that may be their mothers or sisters). Males disperse over long distances from the home ranges of their mothers and have a seemingly nomadic phase until they can secure a home range, displacing the dominant, resident adult male. A resident male has a home range that envelops the ranges of one to several adult females.

The overlap of the home ranges of males and females means that orangutans of both sexes might encounter each other while travelling and feeding, and may have brief social interactions. Adult females that encounter one another can be aggressive, mutually avoid each other, or have seemingly affable relationships. However there is little to no evidence of female bonding through affiliation, grooming, or agonistic support. Adult males also have overlap in their ranges and when they meet, encounters between them are usually characterized by agonistic displays. This can be easily explained as male-male competition for mates. Dominance hierarchies are formed and maintained between adult males that regularly encounter one another and the more dominant males are usually the largest and have the best body condition. Moreover adult males are always dominant over subadult males.


Calling behaviour
Due to their semi-solitary life orangutans have been traditionally assumed to have a very small repertoire of calls. However, recent research has demonstrated that this is not the case, and in fact orangutans may possess one of the richess and varied call repertoires among great apes. Orangutans may produce loud calls that can be heard up to a couple of kilometers away or soft calls that are only possible to identify with difficulty in the wild or very close to an orangutan in captivity. Orangutans may combine several different calls to make sequences and they may use hands or tools in front of their lips during the production of a call in order to modify the calls' sound. Some calls are only produced by infants, adult males or adult females, but some other calls are produced by all sex-age classes of orangutans such as their alarm call, which sounds as a kiss. Because of this richness and variety and their close relatedness to humans, orangutans represent a good model species for the study of language and speech evolution. If you would like to hear some of the calls orangutans produce in the wild, click here.


Orangutan Cultures
Culture represented for centuries the hallmark of mankind, separating us from the other animals. Recently it has been shown that afterall wild orangutans, together with wild chimpanzees, also have cultures, despite being much less complex than human cultures. Orangutan infants and naive individuals learn particular behaviours from their mothers and social peers, but once a slight adjustment or modification to the behaviour is made by a certain individual and learned subsequently by others, behaviours may start to differ between the populations. What at first starts as a slight difference in performing a certain behaviour, becomes two distinct behaviours through the gradual accumulation of "errors" as individuals continue to learn the behaviours between them across generations at each separate population. It is said that the populations possess then different traditions and several behavioural traditions constitute different cultures. As with humans, the behaviours that an orangutan knows and performs depends on the place it was born and on the behaviours of its social community. Some examples of orangutan cultures is the use of leaves as gloves for crossing trees with spiky trunk, the use of sticks for extracting inaccessible seeds, to enlace tree branches between trees as a bridge to facilitate crossing and the use of different calls during nest construction at different populations.