You’ve asked a really complex question on which researchers are still divided. There’s far too much info to be able to sum up easily.
I’ve been fascinated by this whole subject of animal comminication for a long time. There are several animals who have been taught “language” to one degree or another. Several primates as well as a thirty year study with an African Grey parrot.
Many of the people working with these animals have indeed estimated their understanding level as equivalent to human toddlers 2½ to 3 yrs old.
But others feel that they are reading too much into the communication efforts and are biased since they are the ones working with these animals and want to validate their work.
You can judge for yourself. I’m going to include websites for both Koko and Alex. You can also do an info search on Kanzi, a Bonobo who communicates slightly differently.
There are numerous articles on all of these animals with varying viewpoints on how much they can understand and communicate to us. There are also numerous short video clips of each of these animals so everyone can see for themselves.
Years ago there was a documentary about Koko which aired on PBS but I can’t remember the name of it. I found it really fascinating, especially the part when he ended up with a little kitten for his pet. Imagine that !
This Q definitely has few pat, one- size- fits- all answers.
The one thing for certain is that what these animals know is a whole lot more than what was ever considered possible in the past.
www.koko.org
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www.alexfoundation.org