The olive colobus, a discreet primate found in the forests of Côte d’Ivoire, is surprisingly proficient in vocal communication. According to a study by the University of Neuchâtel, despite its limited vocal repertoire, this little monkey is able to convey a variety of information by combining its calls according to precise rules. A discovery published in the journal iScience that sheds light on the evolution of complexity in primate vocal communication - including human language!
There’s no need for complex communication when you’re an unsocial animal - according to research by the University of Neuchâtel, the olive colobus may well prove the opposite. Although this small monkey living in the tropical forests of Côte d’Ivoire has a surprisingly limited vocal repertoire, it compensates by assembling these calls into sequences.
Through their study, recently published in the journal iScience, the scientists were even able to decode the meaning of some of these combinations, revealing a rudimentary "grammar". Far from being arbitrary, each sequence of calls can convey information about the nature of a hazard in the environment. This discovery offers a unique model for exploring the origins of linguistic complexity in our ancestors.
Olive colobus monkeys are discreet, living hidden in the tropical forests of Côte d’Ivoire. To avoid being noticed, they blend into their environment thanks to their coloring and their calm, (almost!) silent behavior, which makes them difficult to observe!
This makes them particularly difficult to locate in the dense forests of Côte d’Ivoire," says Quentin Gallot, first author of the study on the vocalizations of these small monkeys. To be able to study their calls, we had to travel long distances every day and be patient’.
Previous studies have determined that the olive colobus is a very unsocial species, for reasons as yet unknown. Individuals live in small groups of 2 to 15 individuals at most, with little 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!
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 apart is their ability to combine them. These calls are almost never produced alone, but in long sequences, assembled according to a set of syntactic rules", reveals Quentin Gallot. An unsuspected complexity that far exceeds that of other non-human primate species living in the same environment!
By systematically analyzing over 10 years of Olive Colobus call recordings, the scientists were able to identify three rules for call combination.
- 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 enabled us to extract the structure of the data in the form of mathematical formulae", explains the researcher.
By combining calls in this way, olive colobus communication seems to incorporate a form of compositionality, characteristic of human linguistic communication and a precursor of grammar. Compositionality is the principle whereby the meaning of a complex expression derives from that of its simpler parts and their arrangement," explains Quentin Gallot. Grammar then formalizes this composition, establishing rules for organizing the elements to produce coherent, comprehensible statements. ’
While scientists disagree 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 characterizing its communication system, we saw an opportunity to better understand how vocal complexity can emerge in our closest cousins, and thus learn a little more about our own evolutionary history", reports Quentin Gallot.
Through playback experiments, some of the sequences were associated with environmental contexts, allowing their meaning to be surmised. Quentin Gallot and colleagues then exposed groups of olive colobus monkeys to recordings of eagle and panther calls, as well as falling tree noises. 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 calls ’ BA ’ 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 instead emit a ’ BA ’ sequence followed by several ’ A ’ calls.
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 (eagle and panther) from a less dangerous event (falling tree), by differences 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, but we have been able to associate certain syntactic rules with a precise environmental context, such as the presence of a type of 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 serves to convey meaning - they want to go further. We’d like to go deeper and see what level of detail is encoded in the call sequences and what information is actually used by the conspecifics", concludes the researcher.
