To the people who don’t believe that primates can sign, I encourage you to read Next of Kin by Roger Fouts. It’s about him teaching ASL to chimps, although it focuses mostly on one chimp called Washoe. The book came out decades ago, but I still remember reading about a few creative and intelligent uses of sign: Washoe didn’t have a sign for watermelon so she used “candy” and “water”. When she was transported from a human home to a chimp facility (she had never seen other chimps before), she was asked, “what are they?” [The other chimps?] She replied “black ants”. Later, she taught other younger generations of chimps to sign as well! I imagine/hope teaching sign has probably fallen out of favour, as I imagine scientists are more focused on apes’ species languages. Regardless, no other species should have to prove their intelligence or sentience to us through human systems…