Makumba, the silverback gorilla of one of the three study groups, at rest. Photo credit: Shelly Masi.
“Let’s go there!”— Cooperation is key to making group decisions, for example on where and when to travel. For western gorillas, traveling is needed to find food, scattered in the forest. However, every individual in the group may have different needs and knowledge. So how do they coordinate to travel all together? A team of researchers from the University of Neuchâtel and the Museum National d’Histoire Naturelle studied the vocalizations and behaviors of three groups of western gorillas in the tropical forest of Central African Republic, to figure out how the apes choose when to stop resting and get moving. A surprisingly democratic process, despite the dominant role of the silverback.
Within the gorilla society
Western gorillas live in family groups, with only one fully adult male – the silverback – surrounded by females and their offspring. “Both male and female offspring leave the group when they reach maturity, but the males stay longer and leave once they start to become silverbacks. After a solitary period, some of these males will then recruit females and form their own group”, explains Lara Nellissen, first author of the study. In order to survive, this species of gorilla is constantly moving to find food, or to avoid dangerous fights with other gorillas. The silverback is twice the size of the females, and the group relies on him for protection from non-related silverbacks.
This is probably why western gorillas, more than other great apes such as chimpanzees or orangutans, prefer to travel together and stay close. “For gorillas, it is crucial to maintain group cohesion,” says Nellissen. “As a result, to make sure everyone agrees, we’ve noticed gorillas vocalize prior to departure.” And despite his importance in the group, the silverback isn’t the only one talking!
At the beginning of a journey
Though it is possible to coordinate group movements without active communication, in social animals with complex social interactions such as gorillas, individuals usually communicate to coordinate group movements. The research group from the University of Neuchatel and the Museum National d’Histoire Naturelle analyzed the vocalizations of three groups of western gorillas to determine how they choose when and where to go.
The scientists observed that in the five minutes before departure, the vocal activity of the apes increased drastically. Around half of these grunt-like calls were part of exchanges with other members of the group. And the more individuals participated in the exchanges, the more likely they were to depart. “We found that gorillas were more likely to depart, the more group members had vocalized, which suggests that gorillas might react to a quorum,” explains Lara Nellissen. “A quorum response means that once a threshold of individuals has signalled in favour of a certain behaviour, the group then adopts this behaviour.” Such quorum responses have been described for meerkats and wild dogs. It can allow animals to make fast decisions, without needing complex communication.
For the researchers, this goes against the prediction that the silverback is the only individual deciding on the timing of departure: his consent isn’t even necessary for departure. Though higher-ranked individuals are more likely to initiate departure and be successful – and in consequence to determine the direction of departure – all the members of the group can influence the group’s next steps. The question now is to determine what they are saying in these exchanges. “We indeed think that there are some variations in these calls, and we are further investigating that now,” Nellissen adds.
Following the gorillas’ tracks
To collect the data necessary for her work, Lara Nellissen spent 11 months studying the three groups of gorillas in the forest of Dzanga-Sangha Protected Areas, Central African Republic, in collaboration with the WWF CAR. “We did focal follows, which means that we followed a different individual each day and for the whole day,” explains the researcher. When the gorillas were resting together, the scientists recorded the behaviour of group members and their vocalizations, particularly those related to departure.
It was also key for her to be able to recognize the individuals by their voice. For this, she could count on her supervisor, Dr Shelly Masi, who has over 20 years of experience studying these gorillas, and her student Silvia Miglietta, and the Aka trackers, forest people hunter-gatherers. “Learning who vocalized was tricky, especially in the beginning, but I got help from amazing Aka trackers. They taught me how to recognize the different individuals and their vocalizations, and after having followed the gorillas for a few weeks, I started to notice the differences in their voices too,” Nellissen remembers.
Recording gorilla vocalizations can help us understand other aspects of their life in which they need to cooperate. According to previous research by Silvia Miglietta, gorillas also collaborate regarding food. “We think they produce food-related calls to inform the others about the presence of good quality and abundant food, and to maintain group cohesion,” states the researcher.
Reference
Lara Nellissen, Terence Fuh, Klaus Zuberbühler, Shelly Masi. 2024. Vocal consensus building for collective departures in wild western gorillas. Proceedings of the Royal Society B. https://doi.org/10.1098/rspb.2024.0597
About the NCCR Evolving Language
The Swiss National Centre of Competence in Research (NCCR) Evolving Language is a nationwide interdisciplinary research consortium bringing together research groups from the humanities, from language and computer science, the social sciences, and the natural sciences at an unprecedented level. Together, we aim at solving one of humanity’s great mysteries: What is language? How did our species develop the capacity for linguistic expression, for processing language in the brain, and for consistently passing down new variations to the next generation? How will our capacity for language change in the face of digital communication and neuroengineering?
About the Muséum national d’Histoire Naturelle
The Muséum national d’Histoire Naturelle is a major cultural and scientific institution dedicated to exploring the wonders of the natural world. Founded in 1820, it houses vast collections ranging from zoology and mineralogy to paleontology and botany, making it an essential center for research and education. Inaugurated in June 1938, the Musée de l’Homme presents the evolution of man and society, combining biological, social and cultural approaches.