Comparative facial anatomy in macaques: insight into the evolution of complex communication

This project will test whether social tolerance, the possibility to interact with a large number of individuals in many different ways, leads to the evolution of complex communication systems in primates. We will determine the gross anatomy of crested macaques’ (Macaca nigra) faces stored at the National Museum of Scotland, to investigate the evolution of complex communication via facial expression. Through dissections of rarely available specimens, we will compare the number and arrangement of facial muscles in the socially tolerant crested macaque and the despotic rhesus macaque (M. mulatta, anatomical data already available). We predict that the facial muscles of crested macaques allow for a higher complexity of facial movements in order to cope with the increased social complexity associated with high social tolerance. This study will allow us to better understand the evolutionary drivers of complex communicative systems and will provide a platform for future comparative studies in order to better understand the evolution of human’s uniquely complex communication system.

We wanted to investigate whether social tolerance, the possibility to interact with a large number of individuals in many different ways, leads to the evolution of complex communication systems in primates. To test this hypothesis, we conducted dissections of the face of crested macaques (dead from natural causes). We then compared the number and arrangement of facial muscles in the tolerant crested macaques and the despotic rhesus macaque for which anatomical data was already available. Our results show that there is a general similarity in muscle presence/absence between the two species, except around the ear. These differences might relate to the different social styles of these two species, with rhesus macaques having more numerous and more developed ear muscles allowing for a greater diversity of ear movements and sustained activation contributing to the production of unambiguous signals (but not necessarily more signals overall). These clear, unambiguous signals might reduce the uncertainty in the outcome of social interactions and minimise the risk of injury. This study shows that social tolerance might be an important factor to explain different patterns of communication in primates but not necessarily by leading to the evolution of more facial muscles.