Monkeys lose the ability to consciously control their calls as they age, which may have limited the evolution of language in non-human primates.
Steffen Hage and his colleagues at the University of Tu¨bingen in Germany studied the vocalizations of two male captive rhesus macaques (Macaca mulatta; pictured) over a roughly five-year period. The monkeys were trained to produce a specific call in response to a coloured cue to receive a reward. At five years old, the macaques scored highly, but by eight years of age neither monkey could perform the task. Adult macaques still produced spontaneous, instinctive calls in their enclosure, indicating that they maintained vocal ability.
Language may have evolved in humans by first extending the vocal flexibility of juveniles into adulthood.