In this (1978) video from our KokoArc research database, 7-year-old Koko tickles Penny’s left foot, then signs to Penny to remove her other shoe so Koko can tickle Penny’s right foot.
Gorillas, like many other primates, engage in tickling as a form of play and social bonding. Tickling can be a way for gorillas to communicate and establish social relationships with others, including other gorillas as well as humans.
Koko had a very close relationship with Penny and would often engage in playful behaviors with her. Koko tickled Penny’s foot as a playful gesture; she laughs and smiles afterward.
Gorillas are known to have a sense of humor and enjoy making others laugh, and tickling can be a way for them to do so.
You can see another good example of this in the “Koko and Robin Williams” video on our YouTube channel: