I wonder if the primates had been previously exposed to mirrors, and had a chance to figure out what they were seeing.
There aren't many regularly eaten animals that I've been around smarter than pigs. I have to admit, knowing the depth and breadth of emotions that animals like cattle, pigs, sheep, goats experience, it is a bit disconcerting to raise them as food. It is for me anyway, and I grew up on a farm, and have worked in a meat processing plant. To extend to those animals the same compassion and love as the beloved family dog is accurate and logical, yet most of us do not.