Answer:
I found this from a site. I hope this helps:
But according to new research, Koko’s ability to cough on command is not just a neat party trick or a manufactured heartwarming moment. It might help rewrite our narrative of what great apes — humans’ closest living relatives — are capable of. And in doing so, it may teach us more about our own history of communication.
I think its the ape's ability to control their voice and mouth activity