A tour operator first noticed that there might be substantial numbers of De Brazza's in Mathews Range more than three years ago. At the time, Mwenja was completing a survey of De Brazza's in the wet forests of western Kenya.
In that study, Mwenja found evidence of nearly twice the number of De Brazza's that he'd expected. But the prognosis for their survival was not good in the face of burgeoning development around their habitat, Mwenja said.
"The De Brazza's grey-green coloring offers excellent camouflage from predators such as leopards, eagles, pythons, and other primates, but this does not save them from the tactically superior Homo sapiens," Mwenja wrote in his 2004 report.
"Hundreds are killed for crop raiding and bush meat through poisoning, traps, and the use of dogs."
If the Kenyan population of the species were to survive, western groups would have to be relocated to new forests, he believed.
That's why Mwenja was eager to confirm reports of De Brazza's living to the east of Kenya's Great Rift Valley.
While the environment surrounding Mathews Range wasn't particularly hospitable to researchers—there were armed cattle rustlers and road bandits to avoid—the ecology inside the forest, coupled with local taboos against killing wildlife, proved ideal for De Brazza's.
Marina Cords, a biologist at Columbia University in New York who has studied De Brazza's, said the discovery of the new population is an important find.
"People in Kenya are very excited about the De Brazza's, and [the monkeys] seem to be quite rare, because they seem to be just hanging on in western Kenya," she said. "So it's great to have this discovery in Mathews Range."
Climate Change
Mwenja found that the newly discovered monkeys are surviving off of many of the same plants and insects as the De Brazza's in western Kenya.
He also noticed that their physical appearance and social behavior were the same as that seen in other groups in Kenya.
Richard Leakey, a leading conservationist and former head of the Kenya Wildlife Service, pointed out that these similarities suggest the De Brazza's in Mathews Range are quite closely related to their cousins west of the Great Rift Valley.
The groups must have been part of the same population long after the valley opened up some two million years ago, dividing Kenya's eastern savannas from the forested hills of the west, he said.
"If the group in Mathews had been isolated several hundred thousand years ago, they would be quite different," Leakey said.
Today the Rift Valley is arid. Trees are sparse. But not so long ago, De Brazza's must have lived there, he said.
"For the De Brazza's to appear as far east—150 kilometers across the Rift Valley—it implies that there was some fairly continuous natural habitat or swampy ground across the Rift Valley until relatively recently," Leakey said.
"This complete disappearance of that forest belt must be in the last 8,000 years."
According to Leakey, that relatively rapid disappearance of forest suggests that climate change is affecting the world's habitats faster than we think.
Unique Population
Researchers say the thousand or so De Brazza's monkeys dwelling throughout East Africa are unique.
"They have a totally different social system than the ones in western Africa," said Joseph Muriuki Wahome, a zoologist at Mississippi Valley State University in Itta Bena, Mississippi, who has studied De Brazza's in western Kenya.
"In Gabon they found fairly small monogamous troops. But the ones in Kenya live in pretty large groups, and they were all made up of single males who were polygamous."
That uniqueness means the plight of East Africa's De Brazza's may be more critical than many people think.
"If it's only in East Africa that the population has become extremely small and very isolated, [the newfound population] is still very important," Wahome said. "Because you know what that kind of isolation means for inbreeding."
Anthony Rylands, a primatologist with Conservation International in Washington, D.C., sees Mwenja's study as an important step toward preserving the monkeys' future.
"We need to research, document, understand the nature of the wildlife around us in order to save it," Rylands said.
"Iregi Mwenja's work may well, we hope, have saved this population just by documenting its existence."
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