Nat sez, "Rats laugh in the ultrasound when they're tickled. They also seek the hand that tickled them (not to bite). Listen to the Radio Lab segment on it, where the researcher who discovered the rodent guffaw talks about how he came to do so. I first found out about it in the excellent QI: The Book of Animal Ignorance, where you can learn that a kangaroo can shuffle foetuses between its two uteruses to wait for droughts to end before giving birth, monotremes are literally "one hole" (for pee, poo, and procreation), and the New Zealand kea are parrots strong enough to rip the kidney fat out of sheep. It's certainly changed my after-dinner conversation!"