It's a laughing matter - apes make "weird" sounds when they are tickled, and these "tickle-induced vocalizations" are akin to human laughter, according to a study discussed on NPR.

"The results suggest that the evolutionary origins of human laughter can be traced back at least 10 to 16 million years to the last common ancestor of humans and modern great apes," NPR says.

But humans and apes aren't the only ones with tickle-induced vocalizations.
Robert Provine, a neuroscientist at the University of Maryland, Baltimore County who studies laughter, notes that dogs pant in a certain way when they're playing, and rats are known to chirp.

So he thinks the roots of laughter might go even further back and would like to see more studies like this one that compare different animals' playful sounds.

"I think that it's about time we get out there, start tickling the dogs and the cats, and the pigs, the rats, as well as the chimpanzees," Provine says. "I think we'll learn a lot about what we have in common, as well as our differences."

Play-associated sounds have hardly been studied in animals, says Provine. "What about whales?" he asks. "You know, do whales have some form of laughter? People really haven't looked into this."
Check out a Live Science video on zookeepers tickling an orangutan and a gorilla here, or a slightly longer one from NPR here.