The monkey species is definitely new to science, said Colin Groves, a biological anthropologist at the Australian National University in Canberra and an expert in the often contentious field of primate classification.
"The only question is whether it is correctly referred to the genus Lophocebus," Groves wrote in an e-mail to National Geographic News. "This, however, was the best option that the authors had until they could come up with some more complete material, especially a voucher [mounted and preserved] specimen."
There are two genera of mangabey, the genus Cerocebus and the genus Lophocebus. (Genus is the classification just above species.)
Cerocebus mangabeys are most closely related to the large baboons called mandrills, have pink or white eyelids that contrast with their black face color, and spend at least some of their time on the ground. Lophocebus are most closely related to other baboons, have black eyelids that are the same color as their faces, and spend most of their time in trees.
Jones, Davenport, and colleagues place the highland mangabey with the genus Lophocebus primarily because of the species' noncontrasting black eyelids and tree-dwelling nature.
Given the likely critically endangered status of the highland mangabey, the researchers have not captured an individual for detailed scientific analysis, nor do they plan to.
Researchers say they are stunned by their find. Tanzania is considered one of the most biologically well-known African countries. Davenport, the WCS biologist, said discovering a medium-size monkey new to science there demonstrates how little humans actually know.
"If two or three years ago someone discovered a new species of monkey in an African country, Tanzania would be near the bottom of the list for guessing which country it's most likely to be in," he said.
Jones, the Udzungwa Mountains National Park-based research biologist, added: "In a way, finding a monkey in Tanzania, to me, makes it more likely that other species might turn up, for example in the Congo Basin. And it really highlights the fact that there is still so much to learn in the 21st century."
Groves, the Australian biological anthropologist, said he too was "extremely surprised" to learn about the discovery.
Together with WCS colleagues Noah Mpunga, Sophy Machaga, and Daniela De Luca, Davenport first observed the highland mangabey in the southern highlands of southwest Tanzania in May 2003.
He said scientists and conservationists have largely ignored the region, believing that it contained little of interest in terms of large animals. The WCS researchers were there, in part, to test that notion.
While interviewing members of a local tribe, the Wanyakyusa, in January 2003, the researchers learned of a shy monkey known as the kipunji.
Real and mythical forest animals populate the tribe's oral traditions. That's why it took the researchers several months to validate the rumors of a new monkey species previously unknown to Western science.
The team confirmed the species's existence only after the May 2003 sighting and a subsequent sighting in December of that year, according to their account in Science.
Davenport said he is uncertain how long the Wanyakyusa have known about the highland mangabey, "but they certainly had a name for itkipunji." In recognition of the Wanyakyusa's likely generations-long knowledge of the species, kipunji is the second half of the species's scientific name.
Meanwhile, Jones first observed the highland mangabey last July, while serving as the field director on a project to study the critically endangered Sanje mangabey in the Udzungwa Mountains.
That study was led by Carolyn Ehardt, an anthropologist at the University of Georgia in Athens. It was Ehardt, together with Thomas Butynski of Conservation International in Nairobi, Kenya, who subsequently confirmed that the species was new to science.
At the time of discovery, Jones was following up on reports from ornithologists of a population of Sanje mangabeys in the region. What he found turned out to be something entirely different.
"As soon as I laid eyes on it, I was gobsmacked," Jones said. "I was a little off balance and grabbed onto my assistant to stop me from falling over. Then I put my binoculars back on the animal and stared in disbelief."
Researchers say the highland mangabey eluded recognition by the outside world until now for a host of factors. These include the monkey's low number, restricted range, shyness, and remote location.
"They're [forest] canopy animals," Jones said. "And the canopy is very tall, up to 50 meters [164 feet]. They hear you coming and can disappear before you get a good look at them."
Call for Conservation
The researchers estimate that the highland mangabey population in both the Udzungwa Mountains and southern highlands of Tanzania total no more than a thousand individuals.
While small, the Udzungwa Mountains population is protected by its isolation. It is "about a day and a half walk from the village at the end of the road, and it takes a long time to get to the end of the road," Jones said.
He will return to the Ndundulu Forest Reserve later this year to continue research on the Udzungwa population, including a detailed assessment of potential long-term threats to the species.
The outlook for the southern highland populations is much more serious, according to Davenport, who says highland mangabeys there are at immediate risk. The area's forest is severely fragmented due to logging and other forms of resource extraction, he said, and the species is hunted for the meat.
"I know every conservationist says that, but the point is this is true," he said. "We are optimistic, or at least hope, that this sort of discovery will help people realize this is an important area."
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