Ape sign language studies mostly, but they've also tried to teach language to birds, dolphins, etc and it never works.
The old idea of language development is that when you babble as an infant, you occasionally make words, and your parents reward you for it - thus teaching you language. Chomsky argued that language is too complex to be explained by this mechanism, and it was a major debate in language development for a long time. So the way to test this theory is to try to teach simple language via this mechanism to the most intelligent non-humans we have access to.
The animal studies that looked into this were mostly done with apes and monkeys. You've probably heard of Coco the Gorilla - well she was just one of many that we tried to teach sign language too. Unfortunately, despite the hype and marketing around Coco the results reported by her trainers were never replicated, and a systematic review of all ape language studies (which were done for decades) revealed that basically no ape had ever actually learned sign language - at best they learned a couple of signs and learned to associate those signs with food and attention from their trainers (the same way a dog can learn individual words), at worst they learned basically nothing at all but the trainers (who usually were not fluent in ASL themselves) interpreted random arm movements as valid signs.
Ape sign language studies mostly, but they've also tried to teach language to birds, dolphins, etc and it never works.
The old idea of language development is that when you babble as an infant, you occasionally make words, and your parents reward you for it - thus teaching you language. Chomsky argued that language is too complex to be explained by this mechanism, and it was a major debate in language development for a long time. So the way to test this theory is to try to teach simple language via this mechanism to the most intelligent non-humans we have access to.
The animal studies that looked into this were mostly done with apes and monkeys. You've probably heard of Coco the Gorilla - well she was just one of many that we tried to teach sign language too. Unfortunately, despite the hype and marketing around Coco the results reported by her trainers were never replicated, and a systematic review of all ape language studies (which were done for decades) revealed that basically no ape had ever actually learned sign language - at best they learned a couple of signs and learned to associate those signs with food and attention from their trainers (the same way a dog can learn individual words), at worst they learned basically nothing at all but the trainers (who usually were not fluent in ASL themselves) interpreted random arm movements as valid signs.