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Secretive and colorful dryas monkey isn’t as rare as once thought

Drawing of a dryas monkey courtesy of Daniel Alempijevic.

  • In 2014, biologists discovered a population of critically endangered dryas monkeys (Cercopithecus dryas) living 400 kilometers (250 miles) south of their only known range in the Democratic Republic of Congo.
  • Multi-level camera traps revealed that these stealthy monkeys are more common — and a lot weirder — than previously thought. They digest young leaves, snuggle up in impenetrable vine thickets, and sometimes boast an outrageous blue behind.
  • In 2019, the IUCN downgraded their conservation status to endangered, and scientists are predicting a potentially positive future for the dryas.

On a July day in 2014, biologists Henri Silegowa and Jean Pierre Kapale were buying rice in Bafundo, a village in the Democratic Republic of the Congo, when they noticed a dead monkey about to be cooked as bushmeat. In seven years of surveying the region’s primates, neither they nor their colleagues at the Lukuru Foundation’s TL2 Project had laid eyes on a monkey like this.

“We were inside the fenced compound of a local woman, buying rice for our patrol teams, when I saw this tiny monkey hanging off the side of the kitchen. I’d never seen anything like it, and the woman said she did not know its name,” Silegowa recalls.

The specimen was a small female, the size of a housecat. Her black face was cushioned in lushrusset fur and haloed by a white diadem. She had a pale blue rump. The hunter, when Silegowa tracked him down, called the monkey inoko. He’d traded the carcass for a few measures of rice.

John Hart, scientific director of the TL2 Project, described her as “beautiful,” a “miniscule little animal.”

Henri Silegowa and Jean Pierre Kapale noticed this mysterious monkey hanging in Bafundo, a village in the Democratic Republic of Congo. In seven years of studying the region’s primates, they’d never seen anything like it. Image courtesy of Henri Silegowa.

At first, Silegowa and Hart thought the monkey might be new to science.

“I checked our field guides, and couldn’t find anything that matched the photo we had of the inoko,” Silegowa says.  “There was not a single monkey in the field guide with that black face rimmed by orange, and with blue buttocks.”

Hoping for more information, the TL2 Project posted a photo of the mystery monkey on its blog. Comments trickled in from researchers as far away as Japan and the U.S., identifying it as the dryas monkey (Cercopithecus dryas), the smallest member of the genus of monkeys known as guenons. The species was first described in 1932 by a German zoologist, Ernst Schwarz, but it was only known from a tiny patch of forest in the Wamba-Kokolopori region, nearly 400 kilometers (250 miles) to the northwest. Not surprisingly, at the time, the IUCN classified the species as critically endangered.

The revelation struck Silegowa as a “big surprise.”

“We knew we had found something special,” he says.

So how did this little dryas monkey end up hanging as bushmeat in Bafundo?

Bafundo sits in the Congo Basin, the world’s second-largest tropical rainforest, on the border of Lomami National Park. The region is a sparsely populated wilderness known as TL2 for the three rivers — Tshuapa, Lomami and Lualaba — that drain it toward Africa’s center.

The TL2 Project, founded in 2007, has discovered biological riches here including bonobos (Pan paniscus), the striped giraffe-like animal known as the okapi (Okapia johnstoni), and around 700 African forest elephants (Loxodonta cyclotis). In 2012, the team even stumbled upon a species of monkey unknown to science, which they named lesula (Cercopithecus lomamiensis).

The mysterious monkey’s black face was encircled by a white diadem and wrapped in lush russet fur. Image courtesy of Henri Silegowa.

When the bushmeat dryas appeared, the researchers had trouble believing this forest held yet another surprise for them.

“At seven years in, they get this animal showing up dead in a village that they had no idea was there!” says Daniel Alempijevic, a graduate student at Florida Atlantic University who works with the TL2 Project. “The big surprise was, how has this [monkey species] been going under the radar for so long now?”

Camera trap mission

Alempijevic came in to tackle that question by setting up dozens of “multi-strata” camera traps in Lomami National Park and its buffer zone. Nobody knew where to look for the dryas, so he and his mentor, Kate Detwiler, an anthropologist at Florida Atlantic University, decided to place cameras on the ground, in the understory, and up in the canopy, sometimes at a dizzying 30 meters (100 feet) high.

In the first month, not one of his cameras caught a dryas, but in the second round his persistence was rewarded with a few tantalizing seconds of video.

“That first time we got one on a camera, I was just thrilled,” Alempijevic says. “After that, they just started trickling in. Now, it’s actually not an irregular sighting in the understory. It’s one of the more common monkeys we’re getting in that stratum.”

A camera trap video shows a group of dryas monkeys moving through the rainforest’s dense understory in their careful, quiet manner. Video courtesy of Daniel Alempijevic/Primatology Lab/Florida Atlantic University and Ephrem Mpaka/TL2 Inoko Project/Frankfurt Zoological Society.

This young dryas monkey posed for a minute-long video selfie. Video courtesy of Daniel Alempijevic/Primatology Lab/Florida Atlantic University and Ephrem Mpaka/TL2 Inoko Project/Frankfurt Zoological Society.

An innovative strategy of placing camera traps at multiple levels in the forest has earned researchers an hour-and-a-half of footage of the elusive dryas monkey. This video collage includes the highlights. Video courtesy of Daniel Alempijevic/Primatology Lab/Florida Atlantic University, Ephrem Mpaka and Koko Bisimwa/TL2 Inoko Project/Frankfurt Zoological Society.

Camera trap results show that dryas monkeys rarely venture up to the canopy, and the cameras have yet to catch them on the ground. Their preferred habitats are dense, swampy vine thickets in the understory, especially gaps where an old tree has fallen and dragged its tangle of woody lianas and skin-ripping rattans down with it.

“It is a really difficult environment to penetrate,” Alempijevic says. “Total bushwhacking to get in. Lots of spines. I’m still pulling rattan spines out of my skin from a year ago!”

This might explain how dryas monkeys have evaded human notice for so long. But now, thanks to the cameras, scientists have accumulated a total of an hour-and-a-half of footage.

“We’re seeing so much more on the cameras than we could ever see with our eyes,” Alempijevic says.

Portrait of a monkey

Researchers are piecing together a portrait of the dryas monkey as a master of stealth with a taste for leaves and an outrageous blue butt. Unlike other guenons, which make exuberant leaps and booming calls, dryas move silently. They whisper, murmur, chuckle and chirp.

“In all our camera trap videos of this thing, we don’t have them making any calls we could use to locate them. That adds to their cryptic nature,” Hart says.

Think of the dryas as that annoying little sister who wins every game of hide-and-seek by contorting herself into a nook where nobody would think to look, then stays hidden for hours after the game has ended to make sure she’s won.

Hart describes the most recent encounter with this “hunker-down monkey,” in March 2019. The TL2 Project patrol came upon a single monkey on the forest floor. It vanished in a blur, then “snuggled right up into the vine thicket,” becoming an invisible furball. No running away. No alarm calls.

“We spent over half an hour there, but the monkey would not move,” recalls Aimedo Onale, a member of the patrol. “We knew where it was, but we could not see it. Finally, I asked the machete man to climb nearby and shake the lianas to get the monkey to move, so we could get a photo.”

After 30 minutes of searching a vine thicket, Onale and his patrol team finally earned this snapshot of a dryas monkey in their March 2019 encounter. Image courtesy of Terese Hart via Flickr (CC BY-NC-SA 2.0).

The patrollers had never seen this kind of monkey before. They were deep in Lomami National Park, far from any previous sightings, but photos confirmed it was an adult male dryas, once again popping up in an unexpected place.

That night, Onale sent a satellite message out to the TL2 Project team. It said simply, “Inoko ino” — dryas is here.

The dryas monkey is a master of avoiding human notice, but don’t let that fool you into thinking it has a drab fashion sense.

“When you get it in front of you, the coloration is just unbelievable. There’s no monkey that’s so vividly colored,” Hart says.

The backside of a mature male dazzles like Fourth of July fireworks. A large patch of bare skin gleams aquamarine under the tail, extending to the testicles. Above that, a smaller patch of cherry-red skin highlights the anus. This colorful display is encircled by a ring of bright white fur, contrasting nicely with the monkey’s black limbs.

Hart recalls the first male dryas he saw.

“It was breathtaking. There was nothing like it in the field guides. The field guides remind me of those renaissance statues that someone subsequently had to come and put the fig leaf over. This thing is just — you can’t miss it!”

When Daniel Alempijevic started researching dryas monkeys in 2016, he needed to ask local communities to report sightings of the species, but he had no photos of it alive, and he didn’t want people getting the idea that he wanted dead specimens. So he sketched this image of a dryas based on bushmeat. Image courtesy of Daniel Alempijevic.

Not all males have the brilliant blue backside and testes. It might be age-related, hormonally induced, or a signal of health or status within the group.

The monkey’s diet was another surprise for researchers. Alempijevic calls it “really strange.” Dryas eat insects, terrestrial mushrooms, and the green inner bark of lianas, but their favorite food appears to be young leaves, an infamously difficult food to digest.

“They’re eating a lot of herbaceous material, which is unusual for a small monkey,” Alempijevic says.

Some animals have evolved adaptations for digesting leaves, such as the goat’s four stomachs or the leafcutter ant’s fungus gardens. Some large primates, like colobus and proboscis monkeys, eat leaves too, but small monkeys usually rely on energy-dense foods like fruits and insects. Just how the dryas extracts energy from its leafy diet remains a mystery.

A positive future?

In January 2019, the IUCN downgraded the dryas monkey’s conservation status from critically endangered to endangered. Theworldwide population is still extremely small, totaling 100 to 250 adults by IUCN estimates, but Hart predicts the number will rise. Neither habitat loss nor hunting poses an existential threat at this time.

“[The dryas monkey] can’t make it in an oil palm plantation as far as we know, but it’s able to sneak around and do pretty well in vine thickets on the edges of little towns,” Hart says.

The first male dryas monkey found by the TL2 Project was killed by a hunter and laid out on a mongongo leaf, showing off its bright blue behind. Image courtesy of Daniel Alempijevic.

Dryas prefer forest disturbed by wind storms, floods and elephants. That evolutionary preference is now working to their advantage, as humans dot the landscape with abandoned gardens and fallow fields.

“We’re actually finding more dryas monkeys around the edges of these abandoned gardens than we are within the national park in protected sites,” Alempijevic says.

As for hunting, dryas avoid snares by staying off the ground, and they evade hunters’ notice with their small size and cryptic behavior.

“They don’t seem to be a target for hunters. If a hunter opportunistically has a chance, they will kill one, but they’re not seeking them out like the larger-bodied primates,” Alempijevic says.

Hart says he hopes more scientists will search for dryas, now that they know how to look for it with camera traps, and work to conservethe pockets of forest where it lives. The goal, he says, is to protect the habitat of these quiet, colorful monkeys, “so they can continue to hunker down and do their thing.”

A rare photo of two dryas monkeys “hunkering down” outside Bafundo village in 2016. The IUCN downgraded the species’ conservation status from critically endangered to endangered after a population was discovered in Lomami National Park. Image courtesy of Pablo Ayali.