Grammar lessons with olive colobus monkeys
The olive colobus, a discreet primate from the forests of the Ivory Coast, surprises by the richness of its vocal communication. According to a study by the University of Neuchâtel and the NCCR Evolving Language, despite its limited vocal repertoire, this little monkey is able to transmit a variety of information by combining its calls according to precise rules. This discovery, published in the journal iScience, sheds light on the evolution of complexity in primate vocal communication – including human language!
An olive colobus monkey. © Clémentine Bodin.
No need for complex communication when you are a not-so-social animal? According to research by the University of Neuchâtel and the NCCR Evolving Language, the olive colobus may just disprove the hypothesis. Although this little monkey, which lives in the tropical forests of the Ivory Coast, has a surprisingly limited vocal repertoire, it compensates by assembling these calls into sequences. Through their study, published recently in the journal iScience, scientists were even able to decode the meaning of some of these combinations, revealing a rudimentary ‘grammar’. Each sequence of calls, far from being arbitrary, can convey information about the nature of a danger in the environment. This discovery offers a unique model for exploring the origins of linguistic complexity in our ancestors.
The secret life of the olive colobus monkey
Olive colobus monkeys are discreet primates, living hidden in the tropical forests of the Ivory Coast. To avoid being noticed, they blend into their environment thanks to their colouring and their calm and (almost!) silent behaviour, which makes them hard to spot! “This makes them particularly difficult to spot in the dense forests of Ivory Coast,” says Quentin Gallot, first author of the study on the vocalizations of these small monkeys. “To study their vocal production, we had to travel long distances every day and be very patient.”
Previous studies have determined that the olive colobus is a very unsocial species, for reasons that are still unknown. “Individuals live in small groups of 2 to 15 individuals at most, with low cohesion,” explains Quentin Gallot. “They also show very little social interaction, such as grooming or playing with each other.” And low social complexity means low communication complexity… unless that’s not always the case?
Rules for talking in binary
Olive colobus monkeys have one of the lowest call diversities among terrestrial species. The study published in iScience shows that these small primates have only a very basic repertoire, consisting of just two types of call: the ‘A’ call and the ‘B’ call. But what sets colobus monkeys apart is their ability to combine them. “These calls are almost never produced in isolation, but rather in long sequences that are assembled according to a set of syntactic rules,” reveals Gallot. An unsuspected complexity that far exceeds that of other non-human primate species living in the same environment!
By systematically analysing more than 10 years of recordings of Olive Colobus calls, the scientists were able to identify three rules for combining calls.
- The number of ‘B’ calls is always less than the number of ‘A’ calls in a sequence.
- Sequences always end with an ‘A’ call.
- A ‘B’ call is never followed by a second ‘B’ call.
“We were able to compile all the sequences ever recorded into a decision tree, which allowed us to extract the structure of the data in the form of mathematical formulas,” explains the researcher.
A precursor for human grammar
By combining calls in this way, olive colobus communication seems to incorporate a form of compositionality, a characteristic of human linguistic communication and a precursor of grammar. “Compositionality is the principle that the meaning of a complex expression is derived from the meaning of its simple parts and their arrangement,” explains Quentin Gallot. “Grammar then formalises this composition, establishing rules for organising the elements to produce coherent, comprehensible sentences.”
While scientists do not agree on whether or not this phenomenon of compositionality is present in non-human animals, the olive colobus remains an excellent model of language evolution. “By characterising its communication system, we saw an opportunity to gain a better understanding of how vocal complexity can emerge in our closest relatives, and thereby learn a little more about our own evolutionary history” reports Quentin Gallot.
Call combination for a new meaning
By means of playback experiments, some of the sequences were associated with environmental contexts, allowing their meaning to be guessed. Quentin Gallot and his colleagues then exposed groups of olive colobus monkeys to recordings of eagle and panther calls, as well as the sound of falling trees. They then compared the calls produced by the monkeys in response.
Their results show that, depending on the noises they hear, colobus monkeys produce sequences with a different structure, the succession of ‘BA’ calls being the core of these sequences: after hearing a leopard, colobus monkeys produce ‘BA’ alone; after hearing an eagle, they produce a ‘BA’ sequence preceded by several ‘A’ calls; and, after hearing a falling tree, they emit a ‘BA’ sequence followed by several ‘A’ calls.
Combination of calls in response to a leopard growl from an olive colobus monkey. © Quentin Gallot.
With these rules, even if a monkey does not hear the beginning of the sequence, it is still possible to differentiate the presence of predators (eagles and panthers) from a less dangerous event (a falling tree) by variations in the end of the sequence (‘BA’ for panthers and eagles and ‘AA’ for falling trees). “At this stage of the research project, we are not in a position to know the exact meaning of the calls. However, we have been able to associate certain syntactic rules with specific environmental events, such as the presence of a specific predator or other dangers,” comments Quentin Gallot.
Now that the research group has described the basics of this combinatorial and syntactic communication system – i.e. one that conveys meaning – it wants to go further. “We would like to dig deeper and see what level of detail is encoded in the call sequences and what information is actually used by potential receivers,” concludes the researcher.
Reference
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?
