Orangutans are native to the tree-lined jungles of Borneo and Sumatra, where their heart melting, expressive faces and high levels of animal intelligence fascinate all who are lucky enough to see them.

This species of red ape is thought to number approximately 14,000 in the wild, but further work is needed to safeguard their survival. However, what of their private lives? Orang-utans are not the easiest of animal species to investigate and study, due to their solitary nature and preference for spending time in the treetops. This means that until recently, not a great deal has been known about the behaviour of these mysterious and secretive apes.

Patiently, the wildlife investigators have tracked down a large number of orangutans to the islands off the coast of Borneo and Sumatra. On one of these green, leafy islands, scientists at a research station sit under canopies and observe the impressive animals as they wander around, eat and interact as part of their daily routine. One scientist, Cheryl Knott, works at the Gunung Palung research station in Borneo. She decided to focus the investigation on how food supplies affected female orang-utan hormones - and whether this influenced the animals' reproduction cycle.

The question is an interesting one, not least because adult female orangutans give birth only once every six to nine years. Scientists have also noted a natural fluctuation in food cycles, with trees producing peak amounts of fruit every four years or so. These increases seem to coincide with raised levels of reproductive hormones in the urine of female orang-utans. Fascinatingly, this testing is quite simple - the animals are so similar to humans that standard pharmacy pregnancy test kits can be used on the females' urine samples to give accurate results. Orang-utans also appear to put on weight when fruit is plentiful but lose kilogrammes when supplies are scarce. Investigators are considering using drones to investigate these intriguing patterns.

Certainly, this fruit-loving species can often be seen feasting on jungle food, with attentive mother orangutans teaching their young how to find and eat the most nutritious tree crops. Babies and offspring stay with their mothers for up to ten years, learning to plan their movements with surprising precision and developing resourceful behaviour. They are known to engage in group rituals and rites of passage with distinctive facial markings that indicate their adolescence at around twelve years of age. Other orang-utan behaviour such as nest building, leaf wiping and unique feeding habits captivate the researchers' attention. It is even thought at the species has begun cultural development; this may help humans to understand the origins and development of our own species.

Even though more orang utan colonies have been discovered over the past two decades, conservationists are still concerned that the total population has fallen by around four-fifths during the last seventy to eighty years. The logging industry, oil palm plantations, hydropower schemes and the forest fires in Borneo in late 2015 have all affected the animals' habitat. Illegal hunting to defend agricultural crops or for bush meat and the pet trade are also problems; hunters kill ferociously protective mothers to trap their young offspring. Animal rescue organisations try to help, though returning raised orangutans to the wild is an unproven challenge.