translation.en: 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.
translation.zh: 黑猩猩在被挠痒时的笑 和他们玩耍时的笑也是不同的， 这一点和我们很像， 不自主的笑、被挠痒的笑， 与社交性的笑是不同的。