Elephants and gorillas that live in the Republic of Congo rainforests spend a lot of time hiding in the shadows—or so we thought. Using drone surveys and artificial intelligence processing, scientists have discovered an extensive network of mysteriously open clearings of grass and sedges. Elephants, gorillas and other iconic animals visit these muddy concourses, called bais in languages of the Indigenous Ba’aka people, to soak up vital nutrition and maintain their intricate social networks. The numerous playgrounds are visible on satellite images, and processors that use AI are helping researchers find them more effectively.
Recognizing the surprising extent of the bai network began on the ground. In May 2021 Sylvain Ngouma, a local botanist at Odzala-Kokoua National Park in the north of the Republic of Congo, led a small team of researchers through the verdant arrowroots of rainforest. Evan Hockridge, then a second-year Harvard University graduate student, was with Ngouma searching for a thesis. Inside the forest, 150-foot canopies of kapok and red ironwood trees came to an abrupt end at an unobstructed meadow the size of Times Square. Ngouma pointed to a trail of wet, pot-sized footprints leading through the woods to the open rotary and murmured, “Les éléphants.”
“You can’t understand animal interactions without first understanding bais.” —Evan Hockridge Harvard University
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Hockridge, who was planning to study forest animal behavior and thought bais were anomalies in his data collection, says, “I had it backward. It kind of hit me when I was there, these freaking enormous bais, with buffalo at the front and elephants right in the middle ... you can’t understand animal interactions without first understanding bais.”
For the next four summers researchers investigated the clearings. They first sifted through more than two million camera trap images placed around 13 known bais. They confirmed what many local Congolese people had told them—that these natural clearings are critical gathering grounds for some of the world’s most endangered mammals. Among the regulars: Forest elephants congregate to consume nutrient-rich soil. Western lowland gorillas feed on salt-rich roots of bai plants. Forest buffalo, blue duikers, sitatungas and even bongos—forest-dwelling antelopes worshipped as spirits by area inhabitants—graze around bais. The ungulates in turn attract predators such as spotted hyenas and lions. The bais, the researchers realized, are big melting pots, big playgrounds, for a menagerie of forest dwellers.
Elephants and others congregate at a bai playground in the Republic of Congo.
Irene Galera/African Parks
“There is something quite magical in watching a family of elephants, gorillas or giant forest hogs emerge from the forest edge and bask in sunlight and social opportunities before slipping back into the cool shade of the forest interior,” says Vicki Fishlock, deputy director of research at the Amboseli Trust for Elephants, who is not involved in the current study. Bais are like Viennese cafés: social arenas where animals hang out. Elephant families meet, and their children get introduced. In open space, they can see one another clearly. The calves play in the mud or, according to Odzala park managers, spend a lot of time chasing birds. Female gorillas get a better look at solitary males and decide whether to join them. Herbivores graze with their calves, perhaps because clearings allow them to more easily spot predators.
During the initial surveys, the team often followed elephant trails—so-called elephant boulevards—in the rainforest to move from bai to bai. This led Hockridge to wonder about a network. Although previous studies documented animal behavior in selected bais, no one had counted them nor had the means to map their distribution.
Doing so requires taking to the sky. Partnering with the Odzala park management African Parks, the team flew drones equipped with high-resolution lidar over some of the 220 bais that park rangers already knew about, gathering structural and spectral signatures. The researchers used this information to train a machine-learning algorithm that picked out bais from satellite images. The results were published on October 1 in Ecology. Hockridge and Ngouma mapped all the bais in a national park the size of Connecticut and found 2,176 of them—10 times more than the park management had previously known. (Disclosure: The writer of this Scientific American article was formerly affiliated with the senior study author’s lab at Harvard but was not involved in the work.)
A bai in Odzala-Kokoua National Park in the Republic of the Congo.
Gwili Gibbon/African Parks
The forest agoras are always located close to rivers and streams. Most of them are smaller than a city block, but some, at more than 100 acres, are larger than some college campuses. “Especially in the West, we often view rainforest as a continuous sea [of trees], but we need to consider where the forests cease to be,” says Hockridge, lead author of the paper. “Bais are ultimately islands of resources, and animals produce these networks of trails to essentially navigate to and from a nodal network of bais.”
The researchers’ map of bais, the first of its kind, is also a map of conservation priorities. “We find that a huge portion of the animal community are dependent on this unique ecosystem. These species don’t have alternative habitats other than bais,” Hockridge says. “We name a lot of the animals as the forest elephants, the forest buffalos, but if you look at their movement patterns and the amount of time they spend in the bais, they are almost like clearing specialists.”
Observers have only really been able to clearly see the network recently as satellite resolution and computation capacity has improved, says Andrew Davies, an assistant professor at Harvard’s department of organismic and evolutionary biology and senior author of the paper. Davies hopes to apply the same algorithm to chart bais across the entire Congo basin—the second-largest rainforest in the world.
The study is also a step toward solving the ultimate mystery: How did bais form? Many ecologists have argued that a combination of hydrology and animal landscaping, especially by forest elephants, is at play. But definitive proof has been elusive; no one has ever documented the formation or recession of bais. Now with a map in hand, and camera traps still in place, the research team is tracking the changes of bais in the long term. This has implications for the maintenance of these critical animal habitats. For example, would rampant ivory poaching affect elephant population and thus bais? Would changing climate cause the bais to shrink or expand?
“We have hypotheses,” Davies confides, “but the short answer is: we don’t know.”
A version of this article entitled “Rainforest Playgrounds” was adapted for inclusion in the January 2025 issue of Scientific American.