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.
Ils rient différemment si on les chatouille et s'ils jouent avec d'autres ; et on pourrait observer que le rire involontaire, des chatouilles, est différent du rire social.