Ethiopia's mysterious Bale monkey eats almost nothing but bamboo, according to the first study of the primate.
Discovered in 1902, little is known about the monkey, named after the region in Africa in which it lives.
But scientists have now discovered it spends most of its life in the trees of a bamboo forest, eating young leaves to avoid getting poisoned.
Very few primates depend on bamboo, and the Bale monkey's reliance on it makes the primate vulnerable to extinction.
Researchers from Ethiopia, US and Norway describe the behaviour of the Bale monkey for the first time in the International Journal of Primatology.
The Bale monkey (Chlorocebus djamdjamensis) is an arboreal and enigmatic primate restricted to the forests of the Bale Massif and Hagere Selam regions of southeastern Ethiopia.
Little information has been available on how this mysterious primate lives.
"They were always considered by scientists to be 'too difficult to study' due to the rough mountainous terrain and foggy conditions in the forests where they occur," says Dr Peter Fashing from California State University, California, US, one of the co-authors of the study.
Between 2007 and 2008 the team studied two neighbouring groups of Bale monkeys in the Obobullu forest in southeastern Ethiopia, which lies to the east of the Bale mountains.
The researchers spent many months deep in the forest, following the primates and recording their behaviour and ecology.
"At the beginning, I had to habituate the very shy monkeys to my presence, but over time they came to trust me enough to let me watch them from a distance," explains Mr Addisu Mekonnen from Addis Ababa University in Ethiopia, who led the study.
No taste for cyanide
The research team discovered several previously unknown Bale monkey populations.
The scientists also found that Bale monkeys are quite different to their closest relatives, other green or vervet monkeys belonging to the same genus.
"We found Bale monkeys to be highly specialised primates, relying entirely on the bamboo forest to meet their needs," Mr Mekonnen says.
The monkeys feed on just 11 plant species.
However, of those bamboo leaves account for a remarkable 77% of their diet.
Most other forest monkeys eat far richer diets, typically consuming between 50 and 100 different plant species or more, says Dr Fashing.
Bale monkeys also consume mainly young bamboo leaves, perhaps to avoid being poisoned by cyanide that accumulates in mature leaves.
Food for thought
Only one type of primate is known to rely more heavily on bamboo than Bale monkeys - the bamboo lemurs of Madagascar, of which there are three species, each consuming a diet that is 90% bamboo.
"Bamboo is a key resource for the existence of Bale monkeys," says Mr Mekonnen.
Yet bamboo in the Bale Massif is being commercially harvested.
"The loss of this resource would have [an equally profound] adverse effect on the long-term survival of this species."
The revelations about the Bale monkey also highlight how little we still know about some primate species, says Dr Fashing.
"If we are to ensure the survival of these mysterious primates, we must first study their basic ecology and behaviour to determine what their conservation needs are," he says.
"Prior to this study, we did not know just how dependent Bale monkeys are on bamboo for their survival."