Bigger brains make us smarter. However, a new study proves that you really can't have the brains and the brawn after researchers found that a boost in brain size and cognitive ability comes at an expensive price for guppy fish.

After artificially selecting guppies for large and small brain sizes, researchers found evidence to support the so-called "expensive-tissue hypothesis" after they found that while large-brain guppies outscored their pea-brained peers in a test of numerical learning, brainy fish had smaller guts and left fewer offspring.

"We provide the first experimental evidence that evolving a larger brain really is costly in terms of both gut investment and, more importantly, reproductive output," Researcher Niclas Kolm of Uppsala University in Sweden said in a statement.

The "expensive-tissue hypothesis" explains why we can't have it all by stating that there is a trade-off between the brain and the energy demands of other organs and reproduction.

In the past the support for the "expensive-tissue hypothesis" came only from comparative studies among species that were correlative in nature.

However, the latest study published January 3 in the journal Current Biology, takes on a different, within-species approach.

Kolm and his team selected live-bearing guppies for large and small brains relative to the size of their bodies. Kolm and his team found that under strong selection pressure the guppies' brain size could evolve "remarkably quickly".

After selection, researchers found that the large-brained guppies scored better than small-brained guppies on numerical learning tests, because more energy was devoted to brain building, the smarter fish, particularly the males, developed smaller guts and also left fewer offspring to the next generation.

Kolm said that the latest findings support the theory that bigger brains and enhanced cognitive ability actually go together. More importantly, he said that the latest findings also represent some of the first convincing evidence that, in terms of evolution, large brains are expensive.

Researchers said that together, the findings strongly support the theory that relative brain sizes among species are formed through a balance between selection for increased cognitive ability and the costs of a big brain.

Kolm said that the results also have important implications for humans because the most distinctive feature of the human brain is its large size compared to the rest of the human body.

"The human brain only makes up 2 percent of our total body mass but stands for 20 percent of our total energy demand," Kolm said in a statement. "It is a remarkably costly organ energetically."

In the latest experiment, all the fish in the experiment were supplied with an abundance of food. In the future, researchers hope to test the effects of fish placed in a more competitive, semi-natural environment with limited resources and predators.

Kolm and his colleagues say that the latest findings also suggest that the relatively small family size of humans and other big-brained mammals such as primates, dolphins and whales, might have helped make big brains possible.