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Study attempts to explain the scarcity of big brains

The latest findings support previous research showing the same correlation between brain size and metabolic rates among humans and their great ape relatives.

By Brooks Hays
Study attempts to explain the scarcity of big brains
The brain of the elephant fish species Gnathonemus petersii accounts for a higher percentage of bodyweight than the human brain. Photo by David Kilper/Washington University

ST. LOUIS, Dec. 21 (UPI) -- New research details the tremendous costs of a big brain. But researchers have struggled to explain how and why species pay these costs.

The latest study -- detailed in Proceedings of the Royal Society B -- suggests species pay for bigger brains with an increased metabolic rate.

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The development of a big brain requires energy. Scientists have hypothesized species can acquire the necessary energy by consuming more or by skimping on the energy devoted to other organs. Studies involving primates, frogs, toads, birds and fish have offered support for both explanations.

In the latest study, scientists at Washington University, St. Louis analyzed the brains and metabolism of 30 fish species from the mormyrid family. Sometimes called elephantfish, mormyrids use weak electric discharges to locate prey and communicate with one another.

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Mormyrids are known as big-brained fish. One species boasts a brain constituting 3 percent of its body weight. Human brains account for between 2 and 2.5 percent of body weight.

Scientists found mormyrid species boast a wide range of brain sizes.

"We realized this meant the fish presented a great opportunity to study the metabolic costs of braininess," Kimberley V. Sukhum, a graduate student in biology, said in a news release.

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Analysis of the 30 mormyrid species revealed species with the largest brains required the highest levels of oxygen intake, while species with the smallest brains required the least.

The findings support previous research showing the same correlation between brain size and metabolic rates among humans and their great ape relatives.

Researchers suggest some species may be able to pay for medium brains by skimping on other organs, but to develop really big brains, more energy intake is a must.

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Taking in more energy isn't easy, which is why big brains are rare. For elephantfish, species with big brains tend to specialize and adapt to high-oxygen niches. Specializing can provide more energy, but it also makes a species more vulnerable to ecological changes and less able to adapt. Small-brained elephantfish are more likely to be generalists and less vulnerable to change.

Humans are also at greater risk of food shortages. New risks require new adaptations. Bipedalism allowed humans to move across greater distances in search of food, while body fat allowed for more efficient food storage and ameliorate lean times.

The latest research not only explains how brains develop, but highlights the reasons why they are relatively rare.

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