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.