Social learning in wild sooty mangabey vocal communication
Authors:   , León Ortega Julián 
Published: August 2023

Elucidating the origin of language is considered one of the hardest problems in science. There is considerable support for the idea that the basic components of human cognition, including the language faculty, did not emerge de novo, but have deep roots in the primate lineage. Although much is known about how humans acquire language, and even how birds acquire song, a largely open question is how non-human primates acquire their communicative competence. Comparative research can help us this gap of knowledge undermines our understanding of how deep these evolutionary roots are. Here, I conducted a set of field experiments with free-ranging sooty mangabeys, a primate species occurring in Taï National Park, Côte d'Ivoire, to shed light on how non-human primates acquire their communicative capacities. First, I combined direct behavioural observations with playback experiments to study the development of con- and heterospecific alarm calls recognition in sooty mangabeys. I found that communicative competence was acquired during the juvenile stage, with alarm call comprehension learning preceding appropriate vocal usage and with no clear difference in learning of con- and heterospecific signals. I also found that, during early stages of life, social referencing, a proactive form of social learning, was key in the acquisition of competent alarm call behaviour. In a second experiment, I provided one of the few systematic, direct, empirical tests under natural conditions of Seyfarth & Cheney’s influential developmental model –the idea that primates learn to communicate by undergoing a cognitive pruning process. Using models of dangerous vipers and not-venomous colubrid snakes, I showed that young juveniles perceived all snakes as dangerous, frequently engaged in social referencing and indiscriminately alarm called, whereas adults responded faster to vipers than colubrids but never alarm called to the latter, unlike juveniles. These results empirically demonstrate that snake predator learning in sooty mangabeys starts with over-generalisation followed by subsequent refinement via social learning. In a third experiment, I assessed the capacity of sooty mangabeys to socially learn predator knowledge from alarm calls. During individual presentations, I exposed subjects to a novel, chimeric predator model with both snake- and leopard-like visual features in conjunction with playbacks of conspecifics’ leopard and snake alarms. I found that the subjects’ response patterns to the predator model corresponded to the meaning of the alarm calls they heard during the first encounter, suggesting that they classed the model as either belonging to the snake or leopard category. Nearly 2 years later, I exposed a third of the same individuals to the same predator model for a second assessment and found that subjects continued to show the same anti-predator response, demonstrating long-term semantic memory from one single experience. In conclusion, non-human primates learn to communicate by a cognitive pruning process that fine-tunes anti-predator and alarm call behaviour. Primate vocal learning is governed by an underlying cognitive development, which departs from innate, over-generalising responses to basic animal classes, followed by subsequent refinement driven by social learning. This social learning process starts during their early stages of life and is refined as the animals mature, and equally shapes the interpretation of alarm calls from their own and other species. Overall, this work made progress with an important scientific problem, that is, how primates acquire the ability to socially learn to communicate, by observing and interacting with others, a core component of the human language faculty.