Uncovering the origins of syntax with sooty mangabeys

The whole repertoire of sooty mangabeys 

For over a year, researcher Auriane Le Floch and colleagues observed sooty mangabey monkeys in the tropical forest of Côte d’Ivoire. Following two groups of sooty mangabeys habituated to human presence, each composed of more than 70 individuals, the researchers recorded vocalizations produced by the monkeys and their specific contexts. “We then had to track the group from the moment they woke up and started moving, before 7 am, until they went to sleep around 6 pm, following them in the forest” explains Le Floch. Recognizing each individual was definitely a challenge, as it’s hard to tell them apart at first. “Fortunately, both for learning individuals and monitoring and following the group, we were working in collaboration with expert local field assistants,” she says. “After spending a month with them, we could definitely tell the difference between individuals.” 

With this set of over 2’000 recordings, the researchers could analyze the syntax, or the rules underlying the combination of multiple calls together. According to their results, in both sexes, out of the 10 call types produced by the species, 8 were combined at least once in their recordings. However, sooty mangabeys mainly utilized a few sequence types. Females usually combined grunts with twitters, and males mostly combined shrills and hoos. 

“Within this subset, certain sequences follow precise rules of ordering and repetition,” says Auriane Le Floch. From the whole-repertoire analysis, a few syntactic patterns emerged: shrills were consistently in first position, and hoos in second. Grunts were usually first, and twitters in second. In repeating sequences starting with a grunt (so, containing several grunts and several twitters), the ending call was a grunt as well, whereas no such rules existed when it began with a twitter.

“We need more comprehensive studies examining entire vocal repertoires, alongside production contexts, to better understand the full range of syntactic abilities a given species is capable of,” states Auriane Le Floch. Indeed, knowing more about the syntactic rules can have potential for meaning expansion. It is a critical lens to explore whether sequence structures may generate new meanings or not.  

Conveying meaning with sequences 

Now using the context of the recordings, the researchers tested whether sooty mangabeys’ call sequences were meaningful combinations or mere strings of sounds. In animal research, the context of production of a call can be used as a proxy for its meaning.  

In their analysis, the researchers saw that some sequences were order-sensitive: in females, while grunt, twitter and grunt-twitter were typically produced in feeding contexts, twitter-grunt was produced during infant-directed affiliation. Repetitive combinations (twitter-grunt-twitter…) were also more likely to occur during infant-directed affiliations, hinting that they may have a role in meaning generation. 

“These rules appear essential for generating meaning, notably during social interactions between females,” says Auriane Le Floch. “For example, when one is carrying an infant, the signaler wants to show a peaceful intention using these specific sequences and rules.”

“Some sequences could not be analyzed thoroughly due to limited sample size, because of their contexts of production that were challenging to capture, like for aggressive interactions and dangerous events such as the presence of a predator”, the first author explains. Also, how the receivers understand the sequences remains unclear. To address these gaps, playback experiments, exposing monkeys to specific call combinations and measuring the response, or predator model experiments, simulating their presence and observing the monkeys’ response, could be useful. Though this approach is time-consuming because it only allows one sequence to be tested at a time, and is challenging to do in the wild, it can be a good complement to a full-repertoire analysis.

From animal to human language 

One of the central linguistic features explored in the study is compositionality. “It is defined as when the meaning of a sequence derives from the meanings of its components and the way they are combined,” explains Auriane Le Floch. This property is widely used in human language to generate meanings. This amazing capacity is, however, rarely seen in non-human animals. But in sooty mangabey monkeys, the researchers found that call sequences exhibit compositionality.

“Our results reinforce the claim that the use of combinatorial rules to modify the meaning of calls may have been present in a common ancestor of human and Afro-Eurasian monkeys around 30 million years ago,” says the researcher. In previous studies, this capacity was mostly observed in the alarm call sequences of monkeys. However, it is only in chimpanzees and bonobos that this capacity seems to be used more broadly across their repertoire. This suggests that a wider use of compositionality may be a more recent development within hominids, i.e. within great apes and human species around 8 millions years ago.

The team highlights that an avenue worth developing in sooty mangabeys would be to better understand the social factors linked with sequence production, in particular during affiliative interactions between females when one is carrying an infant. The management of social interactions has been suggested as a strong driver in the evolution of complex communication and understanding the role of social rank on such sequences, for instance, would offer valuable insight into the emergence of language.