Western gorillas roam widely in the forest in search of their food, which consists mainly of fruit. However, each member of the group may have different information and needs. A team of scientists from the University of Neuchâtel and the Muséum national d’Histoire naturelle studied the vocalizations and behaviors of three groups of western gorillas in the tropical forests of the Central African Republic. The aim - to understand how these great apes choose when to stop resting and start moving. A surprisingly democratic process, despite the dominant role of the silverback gorilla.
In gorilla society
Western gorillas live in family groups, with a single adult male - the ’silverback’ - surrounded by females and their offspring. Both males and females leave the group when they reach sexual maturity, but males stay longer and leave when they start to become ’silverbacks’ themselves. After a period of solitude, some of these males recruit females and form their own group", explains Lara Nellissen, first author of the study. This species of gorilla is constantly on the move, either to feed or to avoid dangerous fights with other gorillas. The group relies on the silverback male, twice the size of the females, to protect it from other, unrelated silverbacks.
This is probably why western gorillas, more than other great apes such as chimpanzees or orangutans, prefer to travel together and stay permanently close to each other. For gorillas, maintaining group cohesion is crucial,’ explains Lara Nellissen. ’ Yet we’ve noticed that gorillas vocalize before departure to make sure everyone agrees. And despite its importance in the group, the ’silverback’ is not the only one to express himself!
At the start of a journey
The research team from the University of Neuchâtel and the Muséum national d’Histoire naturelle analyzed the vocalizations of three groups of western gorillas to determine how they choose the time and direction of their departure. The scientists observed that in the five minutes preceding departure, the apes’ vocal activity increased considerably. Around half of these grunt-like calls were part of exchanges with other members of the group. And the greater the number of individuals involved in these exchanges, the more likely they were to start moving. We
found that gorillas were more likely to move if a high number of group members had vocalized, suggesting that gorillas may be responding to a quorum: once a threshold number of individuals have vocalized in favor of a behavior, the whole group adopts it,’ explains Lara Nellissen. Such quorum responses have already been described in meerkats and wild dogs. They can enable animals to make rapid decisions, without the need for complex communication.
For the scientists, this flies in the face of the prediction that the ’silverback’ is the only individual to decide when to depart: his consent is in fact not even required! Although high-ranking individuals are more likely to initiate the departure and make it a success - and therefore determine the direction of departure - all group members can influence the group’s next steps. Now it’s a question of determining the content of these exchanges. We believe that there are variations in these cries, and we are continuing our research in this direction," adds Nellissen.
In the footsteps of the gorillas
To collect the data required for her work, Lara Nellissen spent 11 months studying three groups of gorillas in the Dzanga-Sangha protected area forest in the Central African Republic, in collaboration with WWF CAR. ’’We carried out focal animal monitoring, meaning we focused on a different individual each day and throughout the day’’, explains the researcher. When the gorillas were resting together, the scientists recorded the behavior of group members and their vocalizations, particularly those related to departure.
It was also essential for the researcher to be able to recognize the individuals by their voices. For this, she was able to rely not only on her supervisor, lecturer Shelly Masi, who has been studying these gorillas for over twenty years, and her student Silvia Miglietta, but also on the Aka trackers, hunter-gatherers of the forest. Determining which gorilla was vocalizing was difficult, especially at first, but the extraordinary Aka trackers were very helpful, and after following the gorillas for a few weeks, I too began to notice differences in their voices", recalls Lara Nellissen.
Recording gorilla vocalizations can also help decipher other types of cooperation. According to previous research by Silvia Miglietta, gorillas also collaborate in the area of food. We think they produce food-related calls to inform others of the presence of abundant, good-quality food, and to maintain group cohesion," says Lara Nellissen.
Text: NCCR Evolving Language The Swiss National Centre of Competence in Research (NCCR) Evolving Language is a national interdisciplinary research consortium that brings together research groups from the humanities, language and computer sciences, social sciences and natural sciences at an unprecedented level. Together, we seek to solve one of humanity’s great mysteries: What is language - How did our species develop the capacity for linguistic expression, language processing in the brain and the systematic transmission of new variations to the next generation - How will our linguistic capacity evolve in the face of digital communication and neuro-engineering - The University of Neuchâtel hosts the consortium as a co-parent institution.
