A dog, a chimpanzee and a rat walk into a bar. The dog huffs, the chimpanzee lets out some excited squeaks, and the rat makes chirping sounds that are inaudible to the human ear. Clearly someone told a joke, because scientists now believe that some animals laugh. It's not clear why animals laugh or what they find so funny, but some animals do indeed indulge in laughter.

Animal studies on rats, monkeys and dogs, show that certain sounds they make are indicative of laughter. Rats, for example, make highly pitched squeaks when playing with each other, and monkeys also appear to make laughing noises during play and interaction. The chuff or huff of a dog when he is excited to see you all suggest these animals laugh.

Some other scientific studies suggest that other animals may laugh too. For example, intelligence studies on dolphins have shown that two dolphins can refer to a third dolphin by name. Who can say they’re not telling a joke, or capable of laughing during play. Given the range of vocal performance by dolphins and whales, it would not be surprising to find out these animals laugh, as well.

What puzzles scientists is what the animals are laughing about. Some studies suggest animals laugh when they are excited or happy. Others believe animals laugh to gain the attention of their owners. It’s possible animals laugh when they are enjoying play. Clearly, none of the animals appear to be telling knock knock jokes but a dog might laugh at the sound of his owner’s knock on the door.

It’s possible that animals laugh because it confers health benefits to them, just as laughing is very good for people. Laughter can lower blood pressure, ease stress, produce dopamine and growth hormone, and actually be good for the circulatory system. Preschool children may laugh as often as 400 times a day. Since some animals have about the same intelligence level as a two year old human, maybe these animals laugh for the evolutionary benefits achieved by laughing. A chimp that laughs for example may be a bigger chimp because he stimulates growth hormones.

Though we do like to anthropomorphize animals, we can’t really see the smiling face of a dog as actual smiling. But if you hear that long huff from your dogs, you many recognize your animals laugh at you or with you, or possibly at some very good joke they heard about the neighbor’s cat.