Suppose a scientist working in Africa has discovered a troop of monkeys that
have a complex communication system. These monkeys make two kinds of
sound – howls and grunts – which they combine to make a "call". For example,
"howl-grunt" is the call for "snake" and "grunt-grunt" is the call for "leopard". The
monkeys never make two howls in a row. Based only on the information here,
which aspect of human language is not present in the monkeys' communication?
