Its worth noting that the idea that great apes have learnt sign language is largely a fabrication by a single person, and nobody has ever been able to replicate this. All the communication has to be interpreted through that individual, and anyone else (including people that speak sign language) have confirmed that they're just making random hand motions in exchange for food
They don't have the dexterity to really sign properly
They claimed fraud by a single person, with zero replication. That’s both testable so they should be able to support it.
At the very least, more than one researcher was involved and more than one ape was alleged to have learned ASL. There is a better discussion about what our threshold is for speech, along with our threshold for saying that research is fraud vs. mistaken, but we don’t fix sloppiness by engaging in more of it.
> In his lecture, Sapolsky alleges that Patterson spontaneously corrects Koko’s signs: “She would ask, ‘Koko, what do you call this thing?’ and [Koko] would come up with a completely wrong sign, and Patterson would say, ‘Oh, stop kidding around!’ And then Patterson would show her the next one, and Koko would get it wrong, and Patterson would say, ‘Oh, you funny gorilla.’ ”
More weirdly was this lawsuit against Patterson:
> The lawsuit alleged that in response to signing from Koko, Patterson pressured Keller and Alperin (two of the female staff) to flash the ape. "Oh, yes, Koko, Nancy has nipples. Nancy can show you her nipples," Patterson reportedly said on one occasion. And on another: "Koko, you see my nipples all the time. You are probably bored with my nipples. You need to see new nipples. I will turn my back so Kendra can show you her nipples."[47] Shortly thereafter, a third woman filed suit, alleging that upon being first introduced to Koko, Patterson told her that Koko was communicating that she wanted to see the woman's nipples
There was a bonobo named Kanzi who learned hundreds of lexigrams. The main criticism here seems to be that while Kanzi truly did know the symbol for “Strawberry” he “used the symbol for “strawberry” as the name for the object, as a request to go where the strawberries are, as a request to eat some strawberries”. So no object-verb sentences and so no grammar which means no true language according to linguists.
Can the dogs sign back? Even dogs that learn to press buttons are mostly just pressing them to get treats. They don't ask questions, and it's not really a conversation.
They don't have the dexterity to really sign properly