Posted on 5 February 2015
Chimpanzees give distinct grunts when they find different types of food, and other chimpanzees understand the meaning of those grunts. Previously, scientists have assumed that the structure of these calls is a read-out of arousal (i.e. excitement about or preference for the food), with the chimps having little control over the structure of the calls.
Researchers had a unique opportunity to investigate whether chimpanzees can change the structure of their food calls when a group of adult chimpanzees from Beekse Bergen Safari Park in the Netherlands were integrated with the resident chimpanzees at Edinburgh Zoo in 2010.
Researchers found before integration, the animals had different grunts for apples as well as different preferences for apples. They discovered that the incoming Dutch chimpanzees modified the structure of their grunts referring to apples so that, three years after integration of the two groups, their grunts were very similar to those produced by the resident Edinburgh chimpanzees.
Though non-human primates can produce vocalisations, such as alarm calls and food calls, that refer to external objects in the environment, it is generally accepted that their acoustic structure is fixed and a product of arousal state. Some scientists have argued that an apparent lack of flexible control over the structure of referential vocalisations is a crucial way in which non-human primate vocalisations differ from language.
But the York researchers, led by Dr Katie Slocombe, of the University’s Department of Psychology, have challenged this long standing assumption, finding that captive chimpanzees ‘learned’ grunts that referred to specific foods.
In the research published in Current Biology, they also discovered that after integration of the separate groups of chimpanzees, the acoustic structure of the grunts of the immigrant group of chimpanzees converged to be similar to those of the resident group.
They made acoustic recordings of grunts produced in response to apples before integration in 2010, then after integration in 2011, and again in 2013. The research team found that simply living together for a year and being exposed to the other group’s different calls for apples was not enough to motivate any changes in call structure: it was not until 2013, when social network analyses indicated strong friendships were established between members of the original subgroups, that the call structures converged.
Importantly, the chimpanzees’ preferences for apples remained stable over this three year period, showing for the first time that structure of these referential calls is not simply a product of arousal, and that the structure can be altered independently of preference for the food.
Dr Slocombe said: “An extraordinary feature of human language is our ability to reference external objects and events with socially learned symbols, or words. These data represent the first evidence of non-human animals actively modifying and socially learning the structure of a meaningful referential vocalisation.”
“Our findings indicate that primate referential call structure is not simply determined by arousal, and that the socially learnt nature of referential words in humans is likely to have ancient evolutionary origins.”
Dr Simon Townsend, of the University of Zurich, added: “These findings might shed some light on the evolutionary origins of these abilities. The fact that both humans and now chimpanzees possess this basic ability suggests that our shared common ancestor living over 6 million years ago may also have been socially learning referential vocalisations”.