Borneo Orangutan Survival Foundation (BOSF) has successfully released 46 orangutans back into the wild. These orangutans had been rescued from starvation, injury, illness or captivity by BOSF's Nyaru Menteng Rescue and Introduction Project, and have now been returned to a tropical rainforest in the remote heart of Borneo, where they should be safe from human persecution - and the devastation of their habitat by the palm oil industry.

The Nyaru Menteng Rescue and Reintroduction Centre is the largest primate rescue project in the world. Founded in 1999 by Lone Drӧscher Nielsen, it has rescued over 1,000 orangutans. Around 600 of these are young orphans whose mothers were killed when their rainforest habitat was destroyed to make way for oil palm plantations. These orphans will not be released until they've been completely rehabilitated - a process which can take up to 10 or 12 years to complete. The 46 which have just been released were not at Nyaru Menteng for rehabilitation - they were there to recuperate, and to undergo medical treatment and observation until they were deemed fit enough to live in the wild again. Finding a safe place to release them has, however, been difficult.

'Our job is only complete when we return orangutans to the forest,' says Lone, 'but nearly all suitable habitat for orangutans in Borneo is being illegally logged, converted to plantations or is already full with wild orangutans. There were many times when I wondered if we would ever find a suitable location.'

After poring over maps, painstaking ground surveys and helicopter flyovers, in 2007 Lone and her team finally found the place they were looking for - many hours from Nyaru Menteng, in the heart of the island of Borneo.

'Here we found several hundred thousand hectares of pristine forest, in lush green valleys and mountain ridges, divided by large rivers flowing through gorges and over rapids. It's the perfect location,' Lone says.

Simon Husson, conservation biologist, expert in orangutan ecology and the scientific adviser of Nyaru Menteng's reintroduction program, is in full agreement.

'The area is clearly full of food for orangutans, yet doesn't have a large wild orangutan population, probably because the wide rivers and mountain ridges have acted as barriers against the species' dispersal. Of course, the remoteness that makes the area so suitable is also an obstacle to actually releasing the apes here. It's been a difficult operation to carry out, but so incredibly rewarding when you see the orangutans swing away into their new home.'

The release process started back in the Centre, firstly identifying those amongst the rescued adult and sub-adult animals which were sufficiently independent to be released, and then making sure that they were healthy and not carrying any disease that could spread through the new population. Those orangutans were then flown to a holding camp near to the release site, ready for their final journey back to the wild.

BOS veterinarian, Dr Agus Irwanto, and paramedic, Bram Sumantri, were on hand throughout the entire operation, making sure that the apes didn't suffer too much from the stresses of the journey.

'Transporting the orangutans in small cages and under anaesthetic is, of course, stressful for them,' says Bram, 'but it's nothing compared with what they went through when they lost their home and were first captured.'

For all of the orangutans released this week, they started this journey when their forest was cleared for oil palm plantations. There are around 35,000 orangutans remaining in the wild in Borneo, and all are in danger from illegal logging, wildfires and hunting, but the biggest threat by far today is the conversion of forest to oil palm plantations. Palm oil is used in a huge range of products, including foods, soaps, cosmetics and machine lubricants, and - more recently - as an alternative source of fuel. Increased demand for this kind of 'biofuel' has led to a huge expansion of oil palm plantations across Borneo, and thus widespread clearance of tropical rainforest,

home to thousands of orangutans.

Most orangutans that lose their habitat die of starvation, or get shot when they're forced to raid the newly-planted palm trees, or are captured when they have no option but to venture into villages to find food. But those who are fortunate enough to be rescued are taken to the Nyaru Menteng Rescue and Reintroduction Centre, where they are cared for and brought back to health by the Centre's dedicated staff, before being released back to the wild. For 46 of them, this moment came in the last week of August.

Among them was Gatuso, a large 25-year old male who was captured in lowland forest - the best habitat for an orangutan. It used to cover large areas of southern Borneo, but has since been largely cleared for plantations. When Gatuso's forest was devastated by a palm-oil company, he was captured by plantation workers who were scared of him. Thankfully the workers called in the Nyaru Menteng rescue team, who saved Gatuso from the certain death which has befallen many hundreds of other orangutans in similar circumstances.

Also released last week were a female orangutan and her baby, who had been rescued from a plantation as well. They had survived for a long time in the cleared land, finding whatever food they could in the barren landscape, including nibbling the shoots from newly planted oil palms, but the mother became weak and the pair were found in a state of desperately ill health. When they arrived at Nyaru Menteng, the baby, christened Sensi, had to be taken away and nursed, because her mother was too weak to produce milk. Four months later, mother and daughter were happily reunited, and now, within a year, they have found themselves together, high over Borneo, flying to a strange, but safe, forest.

Helicopters were used for the final stage of the release. The orangutans were sedated and placed in individual cages, then flown - six to a sling, suspended 100 feet below the helicopter - to the release points.

By this time the release teams were all in position. The field team, led by Field Co-ordinator, Ahmat Suyoko, camped out in the forest, awaiting the orangutans' arrival, and the staging camp team, led by Communication Co-ordinator, May Sumarnae, co-ordinated the orangutans' arrival from Nyaru Menteng and their subsequent departure in slings, making sure that whilst they were in the holding cages, they were well fed and healthy.

'It's a very complicated operation.' says Simon. 'Opening the cage doors is the final step, but to get to that point you need to coordinate a team of technicians, vets, helicopter pilots, forestry officials and boatmen; ensure that everyone has the right equipment and is well fed; that all the boats, helicopters, planes and cars that you need can arrive and depart on schedule ' and do all that from a dense rainforest with intermittent radio communications. It was tough, but Nyaru Menteng has an excellent team, with a very focused goal.'

On the morning of Monday August 17th, the field team set off from the jungle camp to the first release point, an hour upriver by boat, then a four-hour walk to a clearing in the forest. Two helicopter drops took place on the first day, with 13 orangutans set free to climb into the trees once again. For Gatuso, it was the first time in a long while that he had experienced swinging between branches and lianas, but as he disappeared down the hill slope, it was as if he had never been away.

Later that afternoon, as the sun slowly set, it was Sensi's turn. Her mother, still drowsy from the anaesthetic, made a nest to rest in, but Sensi was happy to play nearby. When she was rescued she was too small to have left her mother's body, but now she was experiencing the forest where she would spend the rest of her life and raise her own offspring.

The team made it back to camp after dark, tired but delighted. Fittingly, the first orangutans to become independent once more had been released on the day that the Indonesian nation celebrated its own independence.

Six more orangutans were released on the Tuesday, and another six on the Wednesday. In this latter group was Karen, the oldest orangutan to be released that week. For over 40 years she had lived in her native forest before it was cleared, and she was forced into a neighbouring plantation, to find whatever food she could. When Karen was caught she had been beaten, and her arm was broken. Now fully healed and in the canopy once more, Karen watched the departing helicopter - a sight which will, we hope, be her last encounter with humans.

Twenty-one more orangutans were released on the last two days of the operation, into a spectacular valley surrounded by high hills, with the sound of an unseen river running over waterfalls far below.

For Skipio, this vast landscape was in stark contrast to the tiny cage in which she was found when rescued by Lone - a box so small that she couldn't even turn around in it. With the release of Skipio and the other 45 orangutans, Lone and her team have now released 107 orangutans into this forest - a major achievement, but one that comes with the understanding that there is so much more to do.

'For each orangutan we've released here in the last two years, we've rescued two more from elsewhere in Borneo. We have 200 individuals ready to return to the wild, and to do that we need more resources to pay for the helicopters, ensure long-term security of the site, and provide care and monitoring for the released orangutans.'

But for Gatuso, Sensi and her mother, Karen and Michael, they are now back where they belong.