you can teach apes sign language and many animals can understand language even if they can't speak it themselves
Wasn't the ape sign language thing not really all that convincing? I remember hearing that it was not as incredible as it was originally reported to be.
And please don't say you're thinking of dogs when you say they can understand language. They can associate a stimulus with some action they're supposed to perform, but that is not understanding language.
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Connecticut Huskies