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.
One se drukčije smiju kada ih se škaklji i kad se igraju jedna s drugom, a to je možda nalik situaciji koju imamo kod nenamjernog i škakljajućeg smijeha  koji se razlikuje od onog društvenog.