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A dolphin, Saga, walks on its tail at the Mirage hotel and casino in Las Vegas on April 23, 2006. ((Jae C. Hong/Associated Press))

The large size of a dolphin's brain doesn't indicate superior intelligence, a South African scientist has suggested.

Paul Manger of Johannesburg's University of the Witwatersrand told Reuters that dolphins, whales and porpoises have large brains because they are warm-blooded creatures that live in a cold-water world.

Manger says large human brains are built to process complex information, something dolphin brains are not built for, says the report.

The dolphin brain is largely made up of glia, which work to produce heat so the brain's other main component, neurons, can work properly.

Dolphins have a "superabundance" of glia so they can survive in cold waters, he told Reuters.

The mammals perform tricks, such as jumping through hoops, because they are conditioned to do so for rewards, such as food, he said.

Manger's peer-reviewed research was published in the Biological Reviews of the Cambridge Philosophical Society.

Findings at odds with popular belief

His findings are at odds with the popular belief that dolphins are one of the smartest mammals, with human-like intelligence and emotions.

The intelligence of the dolphins has been a fixture of popular culture from film to television.

In George C. Scott's 1973 film The Day of the Dolphin, a man trains dolphins to speak and understand English, while the long-running television series The Simpsons showed English-speaking dolphins plotting to take over the land and drive humans into the sea.

But perhaps the most well-known was the 1964-1967 television series Flipper, which followed the adventures of a park ranger's family and their pet dolphin.

The dolphin could push wayward boats back on course and knock weapons from the hands of would-be criminals.