—“What do believe is the qualitative difference between human and animal language?”—Bob Robertson
As far as I know animals do not possess language, it only exists within humans. All other creatures merely manage to communicate.
Charles Hockett (1967) introduced a generally accepted check list for language, a set of features that all human languages possess. His seven key properties are:
1 – productivity (the ability to create and understand new utterances): system which makes it possible to construct an unlimited number of sentences from a limited set of rules.
2 – arbitrariness (when signs/words do not resemble the things they represent),
3 – displacement (the ability to refer to the past and to things not present), and
4 – duality of pattern (the combination of a phonological system and a grammatical system),
5 – interchangeability (the ability to transmit and to receive messages by exchanging roles),
6 – specialization (when the only function of speech is communication and the speaker does not act out his message),
7 – cultural transmission (the ability to teach/learn from other individuals, e.g. by imitation).
As far as I know the reason humans can speak is simply brain size and complexity (long chains). But I won’t go into all of it here. But the ability to imagine futures, imagine stories, ‘self-observe’ and describe those stories in a series of symbols, using a series of rules’ takes a great deal of processing power.
While we can see elements of these patterns in parrots, corvids, dolphins, and the apes, we must teach them, and they say very simple things – because they only think very simple things.
Assuming we selected and trained enough chimpanzees to use sign language to build a self sustaining community of them, and assuming we could leave them on an island for a few centuries, it’s possible that sign language would persist. I suspect the problem is that it’s hard to produce a community of chimps with the intelligence necessary for perpetuation.