If your teacher catches you goofing around in class, she might tell you to quit acting like a monkey. Or tell you to stop horsing around. But it would probably surprise you to hear her say: “Stop acting like a fish.” Yet a new study suggests the comparison might be apt.

Most people recognize play when they see monkeys, horses, dogs or people doing it. But fish?

“It’s an animal we don’t expect to play,” says Gordon Burghardt. He is a scientist who studies animal behavior at the University of Tennessee in Knoxville.

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His team now has evidence suggesting that a type of small fish called cichlids (SIK-lids) clearly play. These scientists are not the first to witness fish at play. But the behavior they saw — and recorded for hours on video — seems new. The researchers published their findings online September 30 in Ethology.

Aquatic movies

The scientists started videotaping the fish after herpetologist James Murphy saw a male cichlid behaving strangely in one of his home aquariums. These small fish had been born in captivity. Their parents or grandparents, however, had been brought over from Lake Tanganyika in Zambia, a country in south-central Africa.

Murphy works at the Smithsonian’s National Zoological Park in Washington, D.C. There, he studies amphibians and reptiles. But at home, he keeps pet fish.

Murphy had removed a male cichlid from an aquarium in which it had been attacking other cichlids. The bully shared his new home with other small species of fish. But he didn’t attack them.

This second aquarium contained a thermometer. It would have floated except that a heavy weight in its base anchored it in place. The weight also allowed the thermometer to bounce upright if it got knocked over.

Soon after moving into his new tank, the cichlid began striking the thermometer over and over. Each time, the thermometer bounced back. Again and again, the fish swam by to strike it.

When Burghardt heard about this behavior from Murphy, he urged the fish hobbyist to set up a video camera outside the aquarium. Over the next two years, the research team videotaped three male cichlids repeatedly bat at the thermometer.

Each fish had been removed from another aquarium because of aggressive behavior. Their new tank sometimes had other fish in it, but never any cichlids. And like the first male cichlid to be moved, the other transferred bullies showed no aggression towards their new tank-mates.

All three cichlids repeatedly struck the thermometer. Each, however, did it somewhat differently. This suggested they had individual personalities.

One fish sometimes struck the bottom of the thermometer, gradually moving the device around the bottom of the tank. The other two attacked only the top of the thermometer. One of these two fish also swirled around the thermometer while striking it.

How to test if this was playing

The researchers wanted to make sure the fish were truly playing. After all, they reasoned, the fish might just be aggressors letting off some steam, or hungry guys angry at not having enough food — or even lonely guys frustrated by not having a mate.

Male cichlids vibrate their whole bodies when looking for a mate, Burghardt says. But these fish were not doing that. Their thermometer-bashing also didn’t seem to have anything to do with being hungry. These guys did it before and after eating.

Finally, the videos showed no link between thermometer batting and aggression toward other fish. Former bullies who could sometimes see others of their species in a neighboring tank might race at the glass wall, as if they were trying to attack. But even when they could see these other cichlids, the sequestered males still played with their tank’s thermometer.

In the end, what the fish were doing looked a lot like Burghardt’s definition of play. He defines that as repeated behavior that has little to do with finding food, finding a mate or fighting off predators. Animals do it when they’re relaxed or bored, he says.

The role of this play is still unexplained

Burghardt suspects it was important that the thermometer bounced back after each strike. “A lot of kids and dogs like a toy that is responsive,” he explains.

It’s hard to know what role this type of play might serve. But social play, such as the wrestling like rats will do, seems to help them react appropriately to other members of their group. Through mock-fighting, “they learn how to inhibit their behavior when things don’t go right,” Burghardt says. In a sense, he says, it teaches them how to behave “so they are not killed by a bigger animal.”

Sergio Pellis is a neuroscientist, someone who studies the brain. At the University of Lethbridge in Alberta, Canada, he probes how play helps animals learn to modify their behavior with others.

He says Burghardt’s study is important. “When you look carefully, as Burghardt’s study did, examples of play can be found in other types of animals,” not just obvious ones, says Pellis. Indeed, he notes, Burghardt and others have been assembling some “pretty convincing” data on play in a range of critters — from turtles and wasps to octopuses.

Power words

amphibians A group of animals that includes frogs, salamanders and caecilians. Amphibians have backbones and can breathe through their skin. Unlike reptiles, birds and mammals, unborn or unhatched amphibians do not develop in a special protective sac called an amniotic sac.

biology The study of living things. The scientists who study them are known as biologists.

cichlids A freshwater fish that has become popular in the aquarium trade. This animal’s family is large and diverse. It includes at least 1,650 species, many of which are eaten. Although found all over the world, they are most diverse in Africa and South America.

ethology The science of behavior in animals, including humans, from a biological point of view.

herpetology The biology of reptiles and amphibians. Scientists who work in this field are known as herpetologists.

neuroscience Science that deals with the structure or function of the brain and other parts of the nervous system. Researchers in this field are known as neuroscientists.

reptile Cold-blooded vertebrate animals, whose skin is covered with scales or horny plates. Snakes, turtles, lizards and alligators are all reptiles.