Chimpanzees laugh differently if they're being tickled than if they're playing with each other, and we might be seeing something like that here, involuntary laughter, tickling laughter, being different from social laughter.
Los chimpancés ríen diferente si les hacen cosquillas que si se ríen jugando unos con otros, y puede que veamos algo así aquí, la risa involuntaria, la de cosquillas, es diferente de la risa social.