For example, green monkeys use calls of various kinds to warn each other of danger. Zoologists have identified one call that means, ‘Careful! An eagle!’ A slightly different call warns, ‘Careful! A lion!’ When researchers played a recording of the first call to a group of monkeys, the monkeys stopped what they were doing and looked upwards in fear. When the same group heard a recording of the second call, the lion warning, they quickly scrambled up a tree.
