A new expedition to track down the elusive orang pendek begins today. Richard Freeman describes the tantalising sightings and frustrations of previous expeditionsOn Thursday, Freeman introduced the orang pendek and suggested the creature's possible evolutionary roots

My first expedition to Sumatra took place in 2003. As the zoological director of the Centre for Fortean Zoology I have been all over the world in search of creatures such as the giant anaconda, the almasty and the Mongolian death worm. But it is Sumatra and the orang pendek that keep pulling me back.

In 2003 I accompanied Dr Chris Clark and Jon Hare to Gunung Tujuh, the Lake of Seven Peaks, in Kerinci Seblat National Park. The jungle around the lake is a hotspot for orang pendek sightings. Our guide Sahar Dimus found tracks of a bipedal creature, but they were too rain-damaged to cast.

In the 1980s Sahar's father and a friend had been cutting logs to build a house close to where the village of Polompek now stands. The area has long since been deforested. Both men saw a bipedal ape lifting up cut logs and throwing them about. It was covered in blackish-brown hair and was about 1.5 metres (five feet) tall. The hair on the creature's spine was darker. Its legs were short and its powerful arms were long. The face was broad and black in colour with some pink markings. Both men fled.

We also visited the village of Sungi-Khuning in another part of the park. Here, the year before, a poacher who had set a snare for deer claimed to have caught an orang pendek. The powerfully built ape was struggling with the snare. The poacher tried to jab the creature with his spear but the beast smashed it to matchwood and screamed at him. The poacher fainted and when he awoke the creature had pulled itself free and was walking off into the jungle. It had long, powerful arms and walked erect.

We could not find the man to interview him, but explored the jungles beyond the village.

The following May we returned to explore a remote gorge in the park where, apparently, no westerner had been. We interviewed eyewitnesses who had seen the creature on the semi-cultivated land at the edge of the park known as the Garden. The creatures are said to steal sugar cane and other crops.

One witness was a farmer called Seman who told us he had seen the creature in an area of land adjacent to a river at noon one day in February 2004. Back then the area was overgrown. The creature was only visible from the waist upward and probably over a metre tall. It had short black hair, a broad chest with visible pink skin and a pointed head, possibly indicating a sagittal crest. The ears were long. The creature vanished and Seman said that he had the feeling it had fled to the river and swam across it, though he did not see this. The river was a torrent when we were there but in February it was much lower.

On visiting the area we worked out that the creature had been 22 metres away from the witness. Seman produced a sketch showing a powerfully built, ape-like creature with broad shoulders, long arms and a conical head. At no time did it raise up its arms, as gibbons do on the rare occasions they move about on the ground.

We returned to the same area the next day to interview another witness, called Ata, who claimed to have seen the creature about three weeks after Seman. He heard a strange ooooha! ooooha! cry coming from the same part of the the Garden where Seman had his encounter. Upon investigation Ata found himself only five metres away from the beast, which was a metre tall and had short black hair. Its prominent chest made him think it was female. Its lower half was hidden by vegetation.

He noticed that it had large owl-like eyes, a flat nose, and a large mouth. It seemed aggressive and Ata said he felt the hairs on the back of his hands stand up.

Ata produced a drawing of a muscular, upright creature with large round eyes. It lacked the pointed head of Seman's description.

Our guide Sahar had found an old man called Pak En, the only person who knew the way to the gorge that we had dubbed The Lost Valley. Pak En told us that he had seen an orang pendek in the jungle just above the valley three years ago. He was walking along a jungle trail when he saw it approaching. It was one metre tall, upright, and powerfully built. It had black hair with red tips and a broad mouth. Its prominent breasts made Pak En think it was a female. He noticed that it grasped the vegetation as it moved. It let out an ooooha! ooooha! sound. He watched it move down the trail for two minutes before it saw him. On seeing Pak En it quickly turned about and walked back the way it had come.

After several days' hard trek we descended into the Lost Valley. After a fruitless search we moved down to the Bangko area, a place of lowland jungle inhabited by the Kubu. We met the local chief of the Kubu, a man named Nylam who said he had seen an orang pendek in the area only three months previously. He had been up a tree at the time. The animal was 1.25 metres tall and covered with red-tinted black hair. It had a broad mouth, walked upright and held its arms like a man. It made a weeeehp! weeeehp! noise and looked about itself as if it could smell its observer. Nylam watched it for half an hour.

It would be another five years before I returned to the jungles of Sumatra. Joining Clark and me was Adam Davies (who already had a number of expeditions under his belt) and Dave Archer.

We returned to Bangko to meet a small number of Kubu at a pre-arranged place in the jungle. This group seemed shyer than those we had met in 2004 and the women and children ran away leaving one man. The other men were apparently away hunting in the jungle. The man, who would not give his name, told his story through a translator.

Three years previously he had seen an orang pendek close to the wonderfully named village of Anoolie Pie some 23km away. It was around 1.2 metres tall and covered with black hair. The creature's face reminded the man of a macaque, with a flat nose and broad mouth. It stood and walked on two legs, never once dropping down on all fours. He said it was not a monkey, gibbon or sun bear. The creature seemed afraid of him and walked quickly away while looking from side to side.

He told us that the Kubu thought the creature was half-man half-animal.

That evening we had a visit from an unassuming man called Tarib who was the supreme chief of the Kubu. Most of the Kubu were away hunting but he had made a special effort to visit us and had an amazing tale to tell. Five years ago he had seen an orang pendek as he was walking in the forest. He took the creature by surprise and it became aggressive, raising its arms above its head and charging. He fled and hid behind a tangle of rattan vines. He watched as it looked for him, turning its head from side to side. Finally it moved away.

The next day we rose early for the trek up to Gunung Tuju. After making camp we set up camera traps, splitting into two groups. Dave had brought four camera traps and Chris had a number of sticky boards (cardboard strips coated with a powerful adhesive usually for trapping rats and mice). We would place them on jungle paths, baited with fruit, in the hope an orang pendek would leave some of its hairs stuck in the solution.

The trail Chris and I followed for several miles ran abreast of the lake. We came across some orang pendek tracks – I instantly recognised the narrow, human-like heel and the wider front part of the foot. They were impressed in loam on the forest floor and not good enough to cast. We set up two camera traps in the area and two sticky boards that we baited with fruit.

Upon returning to camp we heard amazing news. Earlier in the day Adam had heard a large animal moving through the forest, while in the distance siamang gibbons were kicking up a fuss. Sahar and Dave crept forward and were greeted with an amazing sight.

Squatted in a tree around 100 feet from them was an orang pendek. They could not see the face clearly as it was pressed against the tree trunk. Dave felt that it was peering at them sideways. He saw the creature's eye rolling in alarm and could see large teeth in the bottom jaw. It had broad shoulders and long powerful arms, and dark brown, almost black fur. The hands and feet were not in view. The hair and shape of the head were reminiscent of a gorilla, but the high forehead was like that of an orang-utan. Dave said he was sure it was not a sun bear or a siamang gibbon.

From his vantage point Dave could not get a good photograph because leaves and branches were in the way. As he moved to get a better view Sahar saw the creature climb down from the tree and walk away on two legs. Adam said that Sahar had wept for 10 minutes because he did not have a camera to get a picture – he has been on the trail of the beast since 1997. Wildlife photographer Jeremy Holden saw the orang pendek briefly in Kerinci National Park and spent the next 15 years fruitlessly trying to get a photograph of it.

Next to the tree was some rattan vine the animal had been chewing. Adam carefully placed this in a specimen tube full of ethanol in the hope that some of the cells from the creature's mouth would have adhered to the plant much like a DNA swab.

In the morning we re-traced our steps to the camera traps. En route we found more orang pendek tracks. They were recognisable as the creature – nothing else in the area makes tracks like them. Despite sceptics' insistence that people mistake sun bear tracks for orang pendek tracks, the two spoors are completely different, the sun bear showing long claws. However, they were not of a good enough quality to cast. Once again the sticky boards and camera traps turned up nothing of interest.

Later we hiked to the area where Dave and Sahar had seen the orang pendek. We heard a "uhhg-uhhg-uhhhhg" sound in the distance briefly. We called out in response but there was no reply. We found nothing on the camera traps or sticky boards so we reset them and returned to camp.

The tracks Adam had found were still visible. The heels looked human but the front part was more ape-like, wide with a well-separated big toe. Unfortunately our supply of plaster of paris had degraded so we could not cast them. We had to make do with taking a number of photographs using our hands as frames of reference.

I am a former zoo keeper and have worked with all the known great apes. I have seen their tracks in just about every medium and I can tell you that the orang pendek's tracks are quite different, with a human-like heel but a well separated big toe that seems less prehensile than that of any other ape.

Checking the traps in the morning we once more came up empty-handed. We decided to split up. Adam and Sahar followed the bed of a stream, Doni and Chris took a path to the right and Dally and I one to the left, while Dave and John took higher ground.

We checked the traps again next morning. The stills cameras had caught nothing except a bird. However, the camera Dave set to film mode had no less than 70 five-second sequences. Unfortunately we would have to wait till we got back to England to watch them.

In the afternoon we decided to cross the lake and search on the far side. Adam had been there only once and the rest of us had never seen the area.

A possible orang pendek footprint found by Adam Davies in Gunung Tujuh in 1999. Photograph: Adam Davies

Only a few yards into the jungle we stumbled across a trackway made by a tiger about a week before. A little further on Adam spotted something that even the guides had missed. Coming up a slope towards the path was a set of orang pendek tracks that were clearer than any we had seen before. The toes were all individually visible. We photographed them extensively and cursed our lack of plaster (ruined by the damp) to cast them.

We sent half of the samples off to Lars Thomas at Copenhagen University, and Adam sent off his half to Professor Todd Disotell of New York University.

Lars studied the structure of the hair and found that it was similar to, but distinct from, orang-utan. He said he was forced to conclude that there was a large, unknown primate in Sumatra. His colleague Dr Tom Gilbert found some DNA that seemed to be human. We think the sample may have been contaminated during collection. Todd was unable to extract any DNA from his sample.

Shortly after my return to England, Dally – the man who had been our "fixer" on this expedition – emailed to tell me of further sightings in Kerinci. On 8 October some birdwatchers from Siulak Mukai village saw an orang pendek near Gunung Tapanggang. They watched it for 10 minutes from a distance of only 10 metres. It had black skin and long arms, and walked like a man.

On 18 October a man called Pak Udin saw an orang pendek in Tandai Forest. The creature was looking for food in a dead tree, possibly insect larvae. It had black and silver hair, long arms and short legs. He watched it for three minutes before it ran away.

On Friday, we embark on the latest Centre for Fortean Zoology expedition to Sumatra. This time we are taking more personnel and more cameras than ever before. The two-week expedition consists of members from both the UK and Australia. In addition to myself, the UK contingent consists of Adam Davies, Dr Chris Clark, Dave Archer, Lisa Malam and Andrew Sanderson. The Australian contingent consists of Rebecca Lang and Mike Williams.

The group will split into two teams. The first will trek into the deep jungle around Gunung Tujuh and the second will concentrate on the "garden" area at the edges of the jungle.

The second team will be using fruit as bait and waiting in hides in the hope of photographing the orang pendek, as it seems attracted to crop fields. With the thinner tree cover and open spaces they stand a good chance of capturing the creature on film if it emerges.

The first team will be setting up a dozen or so camera traps in the deep jungle in the hope that the creature will pass by. They will also be exploring the region on foot armed with both still and video cameras. All team members will be equipped with kits for taking samples of hair and other biological material without contaminating it with human DNA.

We have Professor Bryan Sykes of Oxford University, one of the world's leading geneticists, on board. Together with Lars Tomas, Dr Tom Gilbert and Prof Todd Disotell they will be carrying out independent analysis on any samples we bring back.

This will be my fourth expedition to Sumatra since 2003. Each time I visit I see the rainforest shrinking. Areas that were once verdant jungle are now agricultural monocultures.

Kerinci Seblat National Park is huge, but it cannot hold out against human pressure forever. If we can prove the existence of a new species of ape in these jungles then the eyes of the world will be on Sumatra and pressure will be exerted on the Indonesian government to fight poaching, logging and to stem the tide of agriculture that is eating into the park.

No one knows how many orang pendek are left. It would be an unthinkable crime and a massive loss to science if this animal were allowed to slip into extinction before it is even officially recognised.

Richard Freeman is zoological director of the Centre for Fortean Zoology. On Thursday he introduced the orang pendek and suggested its likely evolutionary origins

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