According to Gordon Burghardt, from the University of Tennessee, Knoxville, the concept of 'play' can also be applied to species not previously thought capable of playing, such as wasps, reptiles and invertebrates.
    
"Play is repeated behaviour that is incompletely functional in the context or at the age in which it is performed and is initiated voluntarily when the animal or person is in a relaxed or low-stress setting," said Burghardt.
    
He and his colleagues, including James Murphy of the Smithsonian National Zoological Park in Washington, DC, are the first to document play with objects in a cichlid fish species.
    
There are hundreds of species of cichlid, including tilapia, but the behaviour of the species they studied appears unique.
    
The team studied and filmed three male fish individually over the course of two years. They observed the fish repeatedly striking a bottom-weighted thermometer.
    
The presence or absence of food, or other fish within the aquarium or visible in an adjacent aquarium, had no effect on their behaviour. The thermometer-attacking behaviour satisfies Burghardt's criteria for play.
    
"The quick righting response seemed the primary stimulus factor that maintained the behaviour," said Burghardt.
    
"We have observed octopus doing this with balls by pulling them underwater and watching them pop back up again. This reactive feature is common in toys used for children and companion animals," said Burghardt.
    
By more accurately characterizing play and observing it throughout the entire animal kingdom, humans may better understand themselves, Burghardt believes.
    
His research illustrates how play is embedded in species' biology, including in the brain.
     
Play, like much of animals' psychology including emotions, motivations, perceptions and intellect, is part of their evolutionary history and not just random, meaningless behaviour, according to Burghardt.
    
The research is published in the journal Ethology.

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