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.
Tawa mereka ketika digelitik berbeda dibandingkan saat mereka bermain bersama, dan mungkin kita bisa melihat kemiripannya, tawa sungguhan, karena digelitik, berbeda dengan tawa dalam sosialisasi. Suaranya pun sangat berbeda.