Humans use symbols. Humans have the capability of symbolic representation. The language is based on the cognitive capability of symbolic representation. Do the nonhuman animals have the symbolic representation? Since 1960s, there has been a lot of study so-called "Ape language." Some studies claimed that the great apes mastered a rudimental form of language skills such as American Sign Language. However, the symbols (or signs) learned by the apes are limited to several hundreds and never exceeding thousands. The acquisition rate is not exponential like humans. There is no strong evidence of the grammatical rules of combining the symbols. Moreover, it is very difficult to establish the symbols that have the symmetrical (or equivalent) relationship.
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