Alt Text
Office of Neuroscience Research > WUSTL Neuroscience News > Why big brains are rare

Why big brains are rare



From the WUSTL Newsroom...

As a species, we’re so brain-proud it doesn’t occur to most of us to ask whether a big brain has disadvantages as well as cognitive benefits.

“We can think of tons of benefits to a larger brain, but the other side of that is brain tissue is incredibly ‘expensive’ and increasing brain size comes at a heavy cost,” said Kimberley V. Sukhum, a graduate student in biology in Arts & Sciences at Washington University in St. Louis.

So evolving a large brain requires either a decrease in other demands for energy or an increase in overall energy consumption, said Bruce Carlson, Sukhum’s advisor and professor of biology in Arts & Sciences.

Previous studies in primates, frogs and toads, birds and fish found support for both hypotheses, leaving the evolutionary path to a larger brain unclear.

Carlson’s lab studies mormyrid electric fishes from Africa, which use weak electric discharges to locate prey and to communicate with one another.

The mormyrids have a reputation as large-brained fish and indeed one species (the fish in the top photo) has a brain that constitutes 3 percent of its body size, comparable to human brains, which range from 2 to 2.5 percent. But it was unclear whether other mormyrids were equally brainy.

Examining 30 out of the more than 200 species in the mormyrid family, the scientists discovered that they have a wide variety of brain sizes.

“We realized this meant the fish presented a great opportunity to study the metabolic costs of braininess,” Sukhum said.

Using oxygen consumption and the ability to tolerate hypoxia as proxies for energy use and energy demand, the scientists put the fish to the test. Sure enough, they found that the largest brained species had the highest demand for oxygen and the smallest brained species the lowest.

The results, published in the Dec. 21 issue of Proceedings of the Royal Society B, make a provocative pairing with an article published in the May 19, 2016 issue of Nature finding that large-brained humans have a much higher metabolic rate than the great apes.

For the complete article, click here.