Paul Manger of Johannesburg's University of the Witwatersrand says the super-sized brains of dolphins, whales and porpoises are a function of being warm-blooded in a cold water environment and not a sign of intelligence.
"We equate our big brain with intelligence. Over the years we have looked at these kinds of things and said the dolphins must be intelligent," he said.
"The real flaw in this logic is that it suggests all brains are built the same ... When you look at the structure of the dolphin brain you see it is not built for complex information processing," he said.
A neuroethologist who looks at brain evolution, Manger's views are sure to cause a stir among a public which has long associated dolphins with intelligence, emotion and other human-like qualities.
They are widely regarded as one of the smartest mammals.
But Manger, whose peer-reviewed research on the subject has been published in Biological Reviews of the Cambridge Philosophical Society, says the reality is different.
Brains, he says, are made of neurons and glia. The latter create the environment for the neurons to work properly and producing heat is one of glia's functions.
"Dolphins have a super abundance of glia and very few neurons ... The dolphin's brain is not made for information processing it is designed to counter the thermal challenges of being a mammal in water," Manger said.
Fish out of water
Manger said observed behaviour supports his iconoclastic take on dolphins as dim-wits.
"You put an animal in a box, even a lab rat or gerbil, and the first thing it wants to do is climb out of it. If you don't put a lid on top of the bowl of a goldfish it will eventually jump out to enlarge the environment it is living in," he said.
"But a dolphin will never do that. In the marine parks the dividers to keep the dolphins apart are only a foot or two above the water between the different pools," he said.
Why not? Because, Manger says, the thought would simply not cross their unsophisticated minds.
They jump through hoops at marine parks only because they have been conditioned to for a food reward - which may suggest the brain of a single-minded predator rather than a reasoned thinker.
"Dolphins can actually chain up to 16 stimulus response events, but this is indicative of good trainers and not intelligent animals. Stimulus-response conditioning is thought to be a low level of intelligent behaviour," Manger said.
Manger also points to the tuna industry, which under consumer pressure has gone to great lengths to prevent dolphins from being caught and killed by accident in nets.
"If they were really intelligent they would just jump over the net because it doesn't come out of the water," he said.