Laughter in animals
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Numerous species demonstrate vocalizations similar to human laughter. A significant proportion of these are mammals, including non-human primates, which suggests that the neurological functions involved in expressing cheer occurred early in the process of mammalian evolution. 
Chimpanzees, gorillas, bonobos and orangutans show laughter-like vocalizations in response to physical contact, such as wrestling, play chasing, or tickling. This is documented in wild and captive chimpanzees. Chimpanzee laughter is not readily recognizable to humans as such, because it is generated by alternating inhalations and exhalations that sound more like breathing and panting. It sounds similar to screeching. The differences between chimpanzee and human laughter may be the result of adaptations that have evolved to enable human speech. It is hard to tell, though, whether or not the chimpanzee is expressing joy. There are instances in which non-human primates have been reported to have expressed joy. One study analyzed and recorded sounds made by human babies and bonobos (also known as pygmy chimpanzees) when tickled. It found that although the bonobo’s laugh was a higher frequency, the laugh followed the same spectrographic pattern of human babies to include as similar facial expressions. Humans and chimpanzees share similar ticklish areas of the body such as the armpits and belly. The enjoyment of tickling in chimpanzees does not diminish with age.
Research has noted the similarity in forms of laughter among humans and apes (chimpanzees, bonobos, gorillas, orangutans) when tickled, suggesting that laughter derived from a common origin among primate species, and has subsequently evolved prior to the origin of humans.
It has been discovered that rats emit long, high frequency, ultrasonic, socially induced vocalization during rough and tumble play and when tickled. The vocalization is described as distinct "chirping". Humans cannot hear the "chirping" without special equipment. It was also discovered that like humans, rats have "tickle skin". These are certain areas of the body that generate more laughter response than others. The laughter is associated with positive emotional feelings and social bonding occurs with the human tickler, resulting in the rats becoming conditioned to seek the tickling. Additional responses to the tickling were that those that laughed the most also played the most, and those that laughed the most preferred to spend more time with other laughing rats. This suggests a social preference to other rats exhibiting similar responses. However, as the rats age, there does appear to be a decline in the tendency to laugh and respond to tickle skin. The initial goal of Jaak Panksepp and Jeff Burgdorf’s research was to track the biological origins of joyful and social processes of the brain by comparing rats and their relationship to the joy and laughter commonly experienced by children in social play. Although the research was unable to prove rats have a sense of humor, it did indicate that they can laugh and express joy. Chirping by rats is also reported in additional studies by Brian Knutson of the National Institutes of Health. Rats chirp when wrestling one another, before receiving morphine, or when mating. The sound has been interpreted as an expectation of something rewarding. High frequency ultrasonic vocalizations serve an important communicative function, namely to elicit social approach behavior in the recipient.
Dogs sometimes pant in a manner that sounds like a laugh. By analyzing the pant using a sonograph, this pant varies with bursts of frequencies. When this vocalization is played to dogs in a shelter setting, it can initiate play, promote pro-social behavior, and decrease stress levels. In one study, 120 dogs in a mid-size county animal shelter were observed. Dogs ranging from 4 months to 10 years-of-age were compared with and without exposure to a recorded "dog-laugh" . This recording significantly reduced stress behaviors, increased tail wagging, the display of a "play-face" when playing was initiated, and pro-social behavior such as approaching and lip licking.[dead link]
- Panksepp, Jaak (December 2000). "The Riddle of Laughter Neural and Psychoevolutionary Underpinnings of Joy". Current Directions in Psychological Science. 6 9. doi:10.1111/1467-8721.00090.
- "Tickled apes yield laughter clue", BBC, June 4, 2009
- Reconstructing the evolution of laughter in great apes and humans. doi:10.1016/j.cub.2009.05.028
- Science News 2001
- Ultrasonic Communication in Rats: Can Playback of 50-kHz Calls Induce Approach Behavior?
- Simonet, Versteeg, & Storie 2005