From 2018 to 2022, the researchers recorded 114 observations of Likweli over a range of about 1,700 square kilometers in Lomami National Park and its buffer zone on highlands located between three tributaries of the Congo River: the Tshuapa, Lomami, and Lualaba. The monkeys, which were most frequently observed in small groups of about six individuals, are difficult to spot and photograph because they live high in dense forest canopy, the researchers noted. 

“Only after 10 years of exploring the Lomami Forest did we have enough clear views and convincing photos to allow us to say that there was yet another new monkey,” said Terese Hart. “This primate was not only new – it was extremely rare and with a limited range.”

In addition, the researchers spoke with residents of 52 villages bordering or located within Likweli’s range. Residents in only eight villages reported knowledge of the species and could accurately describe it. Since people in the region typically have detailed knowledge of local flora and fauna, this supports the notion that Likweli is a cryptic species, the researchers said. 

In April 2021, ICCN officers confiscated three deceased Likweli specimens from hunters within the Lomami National Park buffer zone. Those specimens — two adult females and one adult male — were acquired by the researchers with authorization from ICCN. The skins and skeletons are now housed in the Yale Peabody Museum’s mammalogy collection.

Likweli are small, long tailed, and have black coats. Long, black hairs frame their foreheads and faces. Their small size, striking orange-cream patches on their faces, and white patches on their backsides underneath their tails distinguish them from Colobus satanas, its sister species and the only other Colobus monkey with all-black coats, the researchers said. Characteristics of its teeth and skull also differ from those of Colobus satanas, according to the study.

The researchers also compared Likweli vocalizations with those of other Colobus species. They found that its roars are similar to vocalizations of its sister species (Colobus satanas) in some ways but differ in sequence and frequency, among other aspects. 

Based on a genetic analysis, the researchers estimated that the most recent common ancestor of Colobus congoensis and Colobus satanas existed between about 4.7 and 5.8 million years ago, which represents the deepest split in lineages among Colobus monkeys, further suggesting that Likweli is a distinct species. 

The researchers note that the new species and its sister species, though very similar, inhabit ranges that are at least 1,200 kilometers apart, with the latter living to the west of the former. 

Based on Likweli’s limited range and small population, coupled with the effects of increased hunting and human encroachment on the monkeys’ habitat, the researchers recommend that the species be designated as endangered. 

“This is a rare discovery with significant conservation implications,” said Sargis, a curator of mammalogy and vertebrate paleontology at the Yale Peabody Museum and director of the Yale Institute for Biospheric Studies. “Our analyses provide strong evidence that this is a distinct new species that is endangered. 

“Documenting new primate species like Likweli and Lesula illustrates the incredible biodiversity in this region and points to the importance of Lomami National Park and other conservation efforts in protecting that diversity.”