Humans aren't the only animals who laugh, according to a new study. Orangutans engage in a primitive form of laughing, the BBC reports—when one exhibits a facial expression such as an open, gaping mouth, and a companion displays the same expression less than half a second later. This sense of empathy and mimicry is a key component of laughter.
The speed with which orangutans imitated certain facial expressions of those around them suggests an involuntary reaction similar to "contagious" laughing. The earliest form of laughter may date back to an ancestor of both apes and humans, say European researchers who studied the play behavior of 25 orangutans aged 2 to 12 years.