Can they, or can't they? The question of whether non-human primates are able to fundamentally change their vocal output in relation to social or acoustic experiences from their environment has been on scientists' minds for decades. This process, known as "vocal learning", is a key approach to better understanding the evolution of human language. It has long been known that the vocal structures of different primate species are very similar within a genus and therefore probably innate. Modifications are only possible within very strict, species-dependent constraints. However, some recent studies also discuss vocal learning in different primate species such as chimpanzees and common marmosets. The DPZ researchers Julia Fischer and Kurt Hammerschmidt, who have been conducting studies on the communication of monkeys for many years, have now reopened the subject in an review article. They have compiled the current studies on the subject, present their own research results and discuss them against the background of the debate on vocal learning in primates. The authors propose to focus future research on the different mechanisms that influence primate vocalization with respect to auditory experiences. The article was published in the issue "What can animal communication teach us about human language?" of the journal Philosophical Transactions of the Royal Society B.
“The aim of the article was to provide an overview of current studies on vocal learning in primates and to draw lessons for future research approaches,” says Julia Fischer, head of the Cognitive Ethology Laboratory at the DPZ. “In recent years, some studies with chimpanzees, common marmosets and macaques have appeared in which the authors want to show that vocal learning in primates basically exists.”
In the case of chimpanzees, it was mainly the vocalizations of animals from different groups that were studied and compared, both in the field and in captivity. Among other things, the influence of parental vocalization on the development of young animals' vocalization was investigated in common marmosets and macaques. “Although variations between chimpanzee groups and populations could be detected, the general vocal structure is innate and can only be modified to a small extent,” Julia Fischer evaluates the work. “The studies with marmosets and macaques showed that the vocalization of the adult animals is important for the development of vocal communication, but that the young animals also learn the specific calls of the adult animals when they grow up separately from their parents, i.e. they do not receive any auditory input from them.
Julia Fischer and her team were also able to make similar observations in their own studies. They examined the vocalizations of various baboon species and compared special call types such as the typical grunts and loud calls. Fischer and her colleagues found out that the two call types sound very similar in the different species. Small variations could be found in the call duration, the mean fundamental frequency and the mean peak frequencies, which might be due to the different body sizes of the different species. The subsequent comparison of the entire vocal repertoire of chacma and Guinea baboons did not reveal any fundamental differences in the vocal structure of the two species. This was all the more astonishing as both species live in very different social systems, but these seem to have no effect on their vocal diversity.
In further studies Fischer and their team investigated the alarm calls of West African green monkeys. The scientists confronted the animals with a new, potential danger from the air: a drone that flew over them at a height of 60 meters. The sounds of the drone were recorded and later played to the animals. The researchers wanted to find out how quickly the animals learned the meaning of the sounds. In the playback experiment, the animals reacted to the drone sounds with alarm calls, some searched the sky and hid. These alarm calls were very different from the sounds the animals made in the presence of snakes and leopards. The calls, however, resembled the alarm calls that the related East African vervet monkeys utter when an eagle approaches from the air.
"The animals quickly learned what the previously unknown sounds meant and remembered this information," says Julia Fischer. "This shows their ability for rapid auditory learning. The structure of alarm calls, however, seems to have been established early in the evolutionary history of vervet monkeys.”
Fischer and Hammerschmidt emphasize two fundamental findings on vocal learning based on the current state of research: on the one hand, the patterns of vocalization in primates are relatively strongly genetically fixed at the species or genus level; on the other hand, vocal output can be modified in relation to auditory experience, but only within certain constraints.
"It is important that we overcome the dichotomous view of the subject and in future investigate more closely the mechanisms that enable the adjustment of primate vocalization," summarizes Julia Fischer. "For example, certain auditory experiences can increase the probability that the corresponding call type is also activated and produced in the listener – just as yawning is contagious. Alternatively, the use of a call that is more similar to the calls of the other group members may produce the desired response. Both mechanisms eventually result in the vocal output in a group resembling each other, but the ways to get there differ. Future studies should therefore focus on the mechanisms of vocal learning rather than arguing about whether or not there is vocal learning in non-human primates."
Fischer J and Hammerschmidt K (2019): Towards a new taxonomy of primate vocal production learning. Philosophical Transactions of the Royal Society B 375(1789), https://doi.org/10.1098/rstb.2019.0045