In the Rise of the Planet of the Apes, when Caesar was moved to the ape sanctuary, it was shown that Caesar was with another big ape that was able to communicate through sign language.
This was before Caesar shares the intelligence virus (ALZ-113) on his fellow apes.
Did that specific ape gain intelligence earlier his life? Simple vet training? Or was he also exposed to the treatment?
Is there any canon answer to this? Even deduced answers are accepted.