It's not entirely clear what the evolutionary benefit of this sort of ticklishness might be. Some argue that laughter — including the ticklish kind — is linked to social intelligence. If you're an animal that has to live and work in groups, it pays to be able to pal around. Robert Provine, a psychologist at the University of Maryland, has that tickling provides a means of communication between parents and infants before the babies are able to talk. Laughing in response to a playful touch could provide positive feedback for parents, making the adults more attentive and their babies more likely to survive.
Interestingly, a found that lab rats respond to being tickled by scientists with the same chirping noises they use while playing with each other.
Lots of mammals exhibit this response to a light touch, but only primates are known to get "gargalesis." Primatologist Marina Davila Ross has conducted experiments in which she tickled young orangutans, gorillas, chimps and bonobos and. Chimps even , smiling with their teeth bared, while playing. Ross believes that humans inherited our ability to laugh from our last common ancestor with great apes.
Another theory suggests that ticklishness promotes protection of vulnerable areas — on the stomach, under the armpits, the soles of the feet — resulting in an adaptive advantage during hand-to-hand combat. (Harris pointed out that this theory doesn't explain why you should laugh and smile while someone tickles your belly.) Alternatively, perhaps tickling makes us both giggly and squeamish in order to encourage other people to tickle us while motivating us to defend ourselves. "This would promote rough-and-tumble play that may help the development and acquisition of combat or other skills that have survival benefits," Harris .