From the Wikipedia entry on dogs
Laughter in dogs
Laughter might not be confined or unique to humans, despite
Aristotle's observation that "only the human animal laughs". The
differences between chimpanzee and human laughter may be the result of
adaptations that have evolved to enable human speech. However, some
behavioral psychologists argue that self-awareness of one's situation,
or the ability to identify with somebody else's predicament, are
prerequisites for laughter, so animals are not really laughing in the
same way that humans do.
The dog-laugh sounds similar to a normal pant. However by analyzing
the pant using a spectrograph, this pant varies with bursts of
frequencies, resulting in a laugh. When this recorded dog-laugh
vocalization is played to dogs in a shelter setting, it can initiate
play, promote pro-social behavior, and decrease stress levels. In a
study by Simonet, Versteeg, and Storie, one hundred and twenty subject
dogs residing 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 dog-laugh recording. The stress behaviors measured
included panting, growling, salivating, pacing, barking, cowering,
lunging, play-bows, sitting, orienting and lying down. The study
resulted in positive findings. Exposure to the dog laughing recording
resulted in the following: significantly reduced stress behaviors,
increased tail wagging and the display of a play-face when playing was
initiated, and more frequent pro-social behavior such as approaching
and lip licking. This research suggests exposure to dog-laugh
vocalizations can calm and possibly increase shelter adoptions. (Simonet, Versteeg, & Storie 2005) A dog laughter sample: Simonet 2005.