Deep in the Congolese jungle is a band of apes that, according to local legend, kill lions, catch fish and even howl at the moon. Local hunters speak of massive creatures that seem to be some sort of hybrid between a chimp and a gorilla.

Their location at the centre of one of the bloodiest conflicts on the planet, the civil war in the Democratic Republic of Congo, has meant that the mystery apes have been little studied by western scientists. Reaching the region means negotiating the shifting fortunes of warring rebel factions, and the heart of the animals' range is deep in impenetrable forest.

But despite the difficulties, a handful of scientists have succeeded in studying the animals. Early speculation that the apes may be some yeti-like new species or a chimp/gorilla hybrid proved unfounded, but the truth has turned out to be in many ways even more fascinating. They are actually a population of super-sized chimps with a unique culture - and it seems, a taste for big cat flesh.

The most detailed and recent data comes from Cleve Hicks, at the University of Amsterdam, who has spent 18 months in the field watching the Bili apes - named after a local town - since 2004. His team's most striking find came after one of his trackers heard chimps calling for several days from the same spot.

When he investigated he came across a chimp feasting on the carcass of a leopard. Mr Hicks cannot be sure the animal was killed by the chimp, but the find lends credence to the apes' lion-eating reputation.

"What we have found is this completely new chimpanzee culture," said Mr Hicks. Previously, researchers had only managed to snatch glimpses of the animals or take photos of them using camera traps. But Mr Hicks used local knowledge to get closer to them and photograph them.

"We were told of this sort of fabled land out west by one of our trackers who goes out there to fish," said Mr Hicks whose project is supported by the Wasmoeth Wildlife Foundation. "I call it the magic forest. It is a very special place."

Getting there means a gruelling 40km (25-mile) trek through the jungle, from the nearest road, not to mention navigating croc-infested rivers. But when he arrived he found apes without their normal fear of humans. Chimps near the road flee immediately at the sight of people because they know the consequences of a hunter's rifle, but these animals were happy to approach him. "The further away from the road the more fearless the chimps got," he added.

Mr Hicks reports that he found a unique chimp culture. For example, unlike their cousins in other parts of Africa the chimps regularly bed down for the night in nests on the ground. Around a fifth of the nests he found were there rather than in the trees.

"How can they get away with sleeping on the ground when there are lions, leopards, golden cats around as well as other dangerous animals like elephants and buffalo?" said Mr Hicks.

"I don't like to paint them as being more aggressive, but maybe they prey on some of these predators and the predators kind of leave them alone." He is keen to point out though that they don't howl at the moon.

"The ground nests were very big and there was obviously something very unusual going on there. They are not unknown elsewhere but very unusual," said Colin Groves, an expert on primate morphology at the Australian National University in Canberra who has observed the nests in the field.

Prof Groves believes that the Bili apes should prompt a radical rethink of the family tree of chimp sub-species. He has proposed that primatologists should now recognise five different sub-divisions instead of the current four.

Mr Hicks said the animals also have what he calls a "smashing culture" - a blunt but effective way of solving problems. He has found hundreds of snails and hard-shelled fruits smashed for food, seen chimps carrying termite mounds to rocks to break them open and also found a turtle that was almost certainly smashed apart by chimps.

Like chimp populations in other parts of Africa, the Bili chimps use sticks to fish for ants, but here the tools are up to 2.5 metres long.

The most exciting thing about this population of chimps though is that it is much bigger than anyone realised and may be one of the largest remaining continuous populations of the species left in Africa. Mr Hicks and his colleague Jeroen Swinkels surveyed an area of 7,000 square kilometres and found chimps everywhere. Their unique culture was uniform throughout.

However, the future for the Bili apes is far from secure. "Things are not promising," said Karl Ammann, an independent wildlife photographer who began investigating the apes 1996. "The absence of a strong central government has resulted in most of the region becoming more independent and lawless. In conservation terms this is a disaster."