Scientists looking to see whether animals play were not just on some fishing expedition. The University of Tennessee researchers applied criteria for identifying animal “play” in their study of a cichlid fish species and found that the fish actually play to have fun.

“Play is repeated behavior 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 lead author Gordon Burghardt, whose report was published at the end of September in the journal Ethology.

Play has been identified in several animal groups, including wasps, reptiles, and invertebrates, all groups previously long-thought to lack the ability for such recreation. The jury has been out regarding fish.

“Whether play occurs in fishes has long been a contentious issue, but recent observations document that social, object, and locomotor play can all be found in some species of teleosts,” wrote the researchers.

Burghardt and colleagues are the first to document play with objects in the fish they studied. They filmed three male fish individually over a period of two years and found that the fish repeatedly attacked a bottom-weighted thermometer. The behavior was independent of the presence or absence of food and other fish, both in the same aquarium and visible in an adjacent aquarium. The behavior, they concluded, satisfied the criteria for play.

“The quick righting response seemed the primary stimulus factor that maintained the behavior,” 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.”

“Play is an integral part of life and may make a life worth living,” added Burghardt.

That is certainly no fish story!