This is a lovely tutorial, but I have to disagree about humans being the only ones that use eyes to gauge social cues. I can't speak for animal social structure in general, but cats definitely use eyes in their body language. The old "cat staring contest" isn't just a humorous thing, it's a threatening dominance thing (it's essentially perceived as agressive to stare at a cat for too long, this is part of why they tend to like people who aren't cat people... they don't stare at them). Cats vary how much of the white of their eyes shows when they're really upset to show extreme distress or agression (much like horses, although I don't know if horses read that cue from each other).