"The newly identified primate is not only genetically and anatomically distinct from other African colobus monkeys but is further distinguished by its vocalizations. Its deep, resonant roaring calls resemble those of related Colobus species but possess a distinct acoustic structure.

Researchers also incorporated local ecological knowledge to better understand the species’ distribution and behavior, speaking with residents and hunters across villages in the Lomami National Park’s buffer zone. Only eight villages recognized the species or could accurately describe it. Local communities referred to the monkey as “Likweli” and “kasaba nkoni,” the latter meaning “the branch shaker,” and described it as elusive and rarely seen."

"Between 2018 and 2022, researchers recorded 114 sightings across an estimated range of just 1,700 square kilometers – an unusually small range for colobus monkeys. C. congoensis appears naturally isolated by rivers and forest barriers and dependent on scattered upland forest patches in the Congo Basin."

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My understanding of the local terrain and jungle there is that 'upland forest' is not well suited for manual labor type human agriculture, so there would be few if any villages directly in its habitat. And not frequent reasons for the people who live near it to go directly into its habitat, and even when they do, it's living high up in the tree canopy and may not be directly observed.

  The species, Colobus congoensis, is a rare and cryptic monkey largely unknown even among people living within its range. But those who are familiar with the small, black monkeys — an arboreal creature marked by distinctive orange-cream patches around their mouths and noses — call them “Likweli,” which the researchers recommend remain the species’ common name.

Edit: also see the point that there is a very similar Colobus monkey more widely known by locals, but this is a distinct species known only to a few, and until 2018 was previously unknown even to local naturalists/explorers.