Sun, Apr. 3rd, 2005, 10:51 am Make a joyful noise!
Research by Jaak Panksepp has found that rats laugh--when you tickle their belly, when they play, when they make love. Their laugh is beyond the range of human hearing, but this is what they do when they feel joy.
Well when I used to tickle my rat, she chittered her teeth, and kept moving her eyes. So I don't think that she was laughing, but the chittering of her teeth was a way of her showing that she was happy.