How do whales hear music? They listen to orca -stras!

I told that joke to a lizard and got crickets. It made me wonder the same thing as Eid Muhammad Afridi, who asked Saturday's Weird Animal Question of the Week , "Do animals laugh?"

Going Ape

So far, apes and rats are the only known animals to get the giggles.

Koko , the western lowland gorilla famous for her facility with sign language, "thinks that me being clumsy is funny," and will make laughing noises, says Penny Patterson , president of the California-based Gorilla Foundation.

The great ape also has a special "ho ho" laugh for visitors she especially likes, Patterson says. (Related: " Conversations with a Gorilla " in National Geographic magazine.)

In 2009 Marina Davila Ross , a psychologist at the U.K.'s University of Portsmouth, conducted experiments in which she tickled infant and juvenile primates —such as orangutans, gorillas, and chimpanzees. The apes responded by laughing—technically called "tickle-induced vocalizations."

Ross, who studies the evolution of laughter, suggests we inherited our own ability to laugh from humans and great apes' last common ancestor , which lived 10 to 16 million years ago.

Now her latest study, published this week in PLOS ONE , goes a step further, showing that chimpanzees display "laugh faces"—smiling, with teeth bared—with or without actual laughter.

This indicates "that chimpanzees can communicate in more explicit and thus versatile ways" than we thought, she says. It's similar to how people may smile silently, while talking, or while laughing—each of which conveys a separate emotion.

Tickled Pink

Rats have also gotten the tickle treatment.

Jaak Panskepp , a psychologist and neuroscientist at Washington State University in Pullman, has found that tickled rats make happy noises. (Also see " Rats Remember Who's Nice to Them—and Return the Favo r.")

When scientists tickled the rodents, the animals made the same chirping sounds that they use during play, according to a study published in 2000 . (The noises are above the range of human hearing.)

Some of the lab rats liked being tickled so much they followed the hand that tickled them.

View Images A captive brown rat makes an angry display. Ongoing research shows that rats communicate a variety of emotions, including a sound that may be similar to laughter. Photograph by Vincent J. Musi, Nat Geo Image Collection

Since then, Panskepp and colleagues have shown studying play is serious business—for instance, he's found that brain circuits responsible for laughter in rats can be used to study human emotion. He's also identified seven basic emotional systems housed in the same areas of mammal brains.

His research has even helped combat depression in people. One antidepressant in clinical trials, called GLYX-13 , has its roots in the study of rat laughter. (Also see " Is Laughter the Best Medicine ?")

It's an example "of what can be achieved by taking the emotional feelings of animals seriously as targets for psychiatric medicinal development," he says.

Play Leads to Laughter

We think of rats and apes as smart, but intelligence isn't a requirement for laughter, Panksepp adds.

"Maybe one should look at [it] the the other way around," he says, since it's possible "that play in any species can increase social intelligence."

So identifying other animals that laugh, he says, may be a matter of listening to the sounds they make while having fun.

Who knows? One day we may discover an actual silly goose.