I heard a story on NPR (probably the Radiolab program) about a guy doing research on rats.
Apparently, rats vocalize at too high a pitch for humans to hear, but special microphone equipment in a rat cage dials down the pitch enough to allow us to hear the occasional little chirps rats make at this frequency. After ten years of study that went nowhere, the idea came to the researcher to tickle a rat. Suddenly, the chirps came hard and fast and sounded oddly like . . . laughter.
Another rat, same thing. It turns out rats like to be tickled, and they laugh in a way that is eerily like a human baby being tickled. Check out this video:
Anyway, the thought immediately occurred to me that if rats can feel this very simple human joy, it's probably true that most mammals can. Even the ones we regularly slaughter and eat.
Which leads me inexorably to my next thought: I don't think I can morally justify personally eating higher order animals.
So I'm back to being a vegetarian. I'm afraid birds are out too--but I'll still eat eggs, milk (and cheese), and fish. This'll be the third time in my life I've tried to be a vegetarian. Maybe third time'll be the charm? My omnivore genes may power through my moral decision in the end, but I'm ready to give this another go.