Juvenile macaques lose vocal communication as they age

Ceaselessly chattering, humans are a garrulous species that have developed thousands of languages for communication over the course of evolution. Yet, it can be hard to appreciate just how far human verbal communication has come as we converse continually. ‘Basically, speech is the ability to produce arbitrary acoustic signals to communicate’, says Steffen Hage from the University of Tübingen, Germany, adding that some of our more distant relatives may have had some of the essential aptitudes that eventually allowed our ancient ancestors to evolve the rudiments of speech. Explaining that humans diverged from our macaque monkey ancestors about 25 million years ago, Hage was curious to find out whether modern macaques might have the ability to respond vocally to an abstract event that they would not experience in everyday life. ‘If these precursors of speech also exist in rhesus macaques, it would imply that they already existed in the last common ancestor’, says Hage, who teamed up with Andreas Nieder and Natalja Gavrilov to test the monkeys’ abilities.

Working with two male rhesus macaques, Caruso and Torkel, Hage and Gavrilov trained the animals to initiate an experiment by gripping a bar to make a white square appear on a computer screen and then to utter a call when the square changed colour. ‘This took us quite a while’, says Hage, remembering months of training before the animals mastered the task. And once the animals had grasped the concept at 4.9 years of age, Hage and Gavrilov periodically retested the animals’ ability to make a call in response to changes on the computer screen over several years.

At first, all went well. The two monkeys enthusiastically initiated hundreds of experimental runs in return for a juice reward and produced a call showing that they were capable of communicating vocally in response to an event. However, as the years progressed and the monkeys aged, something seemed to go wrong. ‘The performance decreased’, says Hage, explaining that the animals continued initiating the test, but they began taking longer to respond when the square changed appearance and made fewer and fewer calls over time. Eventually, by the age of 8, neither monkey made a vocal sound as the square changed colour. ‘This is something that we wouldn't have expected’, says Hage, adding, ‘Most animals that are trained in other tasks get better and better over the years’.

Perplexed, the team trained the monkeys to perform another – more complex – operation, where they had to grasp a bar and release it after they had been shown two matching boxes. Both animals picked up the new activity and performed it with relish: they had not lost the ability to learn. Hage and Gavrilov also monitored the animals’ interactions with other members of the troop, and Caruso and Torkel continued calling and communicating with them.

Having ruled out the possibility that the animals had lost the ability to learn or to communicate vocally, Hage and Nieder realised that the monkeys began to lose the ability to respond vocally to the changing square as they matured into adults. ‘We said, “OK, this is probably when something is changing in the connections in the brain”’, says Hage. He suspects that juvenile macaques have some of the prerequisite aptitudes that are necessary for the development of speech, but that these are lost as they develop into adults – which also ties in with the theory that an extended period of juvenile development was, and still is, crucial for the evolution of human speech.