animals, including all ape and monkey species, use vocal signs. For example, green monkeys use calls of various kinds to warn one another 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.
