The camera crew find the troop of red-capped mangabeys
The habits of one of the world's least studied monkeys have been revealed by a BBC film crew.
Filming for the programme Living with Monkeys, the team managed to film a troop of elusive red-capped mangabeys living in the forests of Gabon.
The crew captured footage of the mangabeys sleeping in the treetops for the first time, as well as learning new insights about the monkeys' behaviour.
Dwindling numbers of the monkey are confined to small areas of West Africa.
Primatologist Dr Julie Anderson led the BBC crew, accompanied by explorer Guy Grieve. They were aided by Catherine Cook, a researcher from Ohio Sate University who has just set up the first scientific study of the monkeys' behaviour.
To get close to the monkeys, Anderson, Grieve and a cameraman had to live for five weeks in a specially constructed tree house with a top deck built 16m off the ground.
It is a highly social and intelligent primate with a wide range of vocabulary
Primatologist Dr Julie Anderson
During their expedition, the team's tree house was invaded by highly poisonous centipedes, while the camera crew had to avoid stumbling into forest elephants as they filmed among the trees at night. But it was worth it to get close to such a rare monkey, says Anderson.
"I find that amazing in this day and age, that we are still finding out things about species that are unknown to science in terms of their behaviour and ecology," she says. "And the red capped mangabey is one of those primates."
For example, it was thought that red-capped mangabeys live in groups of 10 to 20 individuals.
But the troop filmed by the camera crew contained 70 to 80 individuals.
"The group is not all together all the time all the day," says Anderson. "They all come together to sleep on occasions or eat if there is a big mass fruiting going on in a certain season when they will all be found in a certain area. But they bud off during the day into little sub troops."
While the team also recorded a number of solitary males, the super-troop was governed by a one dominant male.
"He was really aggressively putting other males in place within that group. He must have his work cut out for him. It must be quite stressful in the group being so big, as an alpha male protecting all these females."
A glimpse of some of the forest's other species
During the expedition, the team also discovered the red-capped mangabey was more vocal than they expected.
"I found their vocalisations fascinating," says Anderson.
"We are just getting a feel for the vocalisations, such as different predator calls, cataloguing what they mean. It is a highly social and intelligent primate with a wide range of vocabulary."
But the most striking secret the team discovered was the range of the monkeys' diet.
Anderson herself saw the monkeys feeding on mushroom, bark and fruits and most striking of all, the BBC crew filmed the mangabeys feeding on crabs, an incredibly rare food for primates.
No-one knows how the monkeys learnt to eat crustaceans and whether the practise is passed down the generations as a form of culture, says Anderson.
But she thinks the monkeys are able to eat crabs due to their immensely strong jaws.
"They can crack open nuts that have been on the forest floor for months," says Anderson, nuts which only elephants can break open, or chimps yielding stone hammers.
She hopes that more will be learnt about this remarkable primate before it's too late.
Little data exists about the distribution and population status of the red-capped mangabey.
But being such a large, mainly terrestrial monkey, it remains vulnerable to being hunted for the bushmeat trade. Isolated local populations have already been hunted to extinction in parts of Nigeria where the monkey also lives.
Living with Monkeys is broadcast on BBC One each Tuesday at 2100.