By Jody Bourton

Earth News reporter



A freshwater stingray using the tools it has around it

Freshwater stingrays use water as a "tool" in problem-solving tests, scientists reveal for the first time.

Researchers gave South American freshwater stingrays tests to evaluate their problem-solving ability.

The stingrays learned to use jets of water as a tool to extract a meal of hidden food from a plastic pipe.

It reveals that the fish, once thought a "simple reflex animal", has cognitive abilities to rival birds, reptiles and mammals, scientists say.

Scientists from Israel, Austria and the US publish their study in the journal Animal Cognition.

Tool use in fish is far from anything seen in birds or mammals

Dr Michael Kuba

Hebrew University of Jerusalem, Israel

Freshwater stingrays, found in many tropical waters such as the Amazon river, are related to ocean stingrays. Like sharks, they have skeletons made of cartilage, rather the bony skeletons of less closely related teleost fish.

In the past, scientists have assumed that such cartilaginous fish have limited cognitive abilities, in part because they have been difficult to study, says Dr Michael Kuba from the Hebrew University of Jerusalem, Israel who undertook the latest study.

His team tested the ability of captive South American stingrays (Potamotrygon castexi) to solve problems, by setting them a series of underwater tasks.

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Using a plastic pipe with one end sealed and containing hidden food, researchers observed how the fish overcame the challenge of getting the meal from the container.

They also tested the fish to see if it could discriminate between black and white ends of the tube.

The stingrays not only performed the tasks well but also demonstrated a range of problem-solving strategies, including using water as a "tool" to obtain the hidden reward.

"Tool use in fish is far from anything seen in birds or mammals, " explains Dr Kuba.

Dr Kuba says that the definition of tool use, using an agent to achieve a goal, was set by cognitive scientist Dr Benjamin Beck in 1980.

The stingrays meet this definition by using water as a tool, manipulating their bodies to create a flow of water that moves food towards them.

At least one other fish species is known to use water in a similar way.

The archer fish, a teleost, shoots spurts of water from its mouth to dislodge prey from leaves above the water's surface.

"Archer fish use water as a projectile to hunt insects," says Dr Kuba.

Like the archer fish, the stingrays also use jets of water to dislodge food stuck among plants on the surface of the fish tank, a behaviour caught on video by the researchers.

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Previously, stingrays have largely been on the sidelines of cognitive research for a number of reasons, says Dr Kuba.

"Firstly, they are bigger and more difficult to study than other model animals such as zebra fish, guppies or mice," he says.

"Second, they, like sharks, have often been considered to be reflex machines having very acute senses but limited cognitive capacities."

"What our study shows is that stingrays are capable of problem solving," he says.

Dr Kuba also suggests that research on stingrays may reveal important aspects of the vertebrate thought process.

"They are members of one of the oldest lines of vertebrates and to know more about their abilities will help us to learn more about the evolution of cognition in vertebrates."