Rats are ticklish
Ticklishness, a trait that was long thought to be unique to humans and our closest primate relatives, evolved in great apes as a means of social bonding: It engenders light-hearted interactions between parents and children, as well as helping youngsters hone their self-defense skills during tickle battles with siblings.
Over the past decade, animal behaviorists have gathered considerable evidence that suggests rats, of all creatures, are ticklish too. When stroked in certain body regions, the rodents emit high-pitched chirps. These chirps seem to signify joy, because the rats will run mazes and press levers if they learn they'll be rewarded with a good tickle afterward. Rats' chirping, the researchers say, is akin to human laughter.
Ticklishness probably evolved in rats for similar reasons that it evolved in apes. Rats are extremely playful animals, engaging in rough-and-tumble play as juveniles just as young apes do, chirping all the while.