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Researchers: Breeding big-brained guppies comes at a cost, evolutionarily speaking

Researchers from Uppsala University in Sweden say that bigger brains make animals smarter, but that increase in brain size and cognitive ability comes at a price, evolutionarily speaking. In the study, researchers artificially selected guppies, one of the most popular freshwater aquarium species in the world, for big and small brain sizes.

Their evidence suggests that bigger brains and increased cognitive ability are linked, a topic that has been at the center of a heated scientific debate in recent years. According Niclas Kolm of Uppsala University, the findings also represent some of the first satisfying evidence that big brains are expensive, evolutionarily speaking.

“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,” Kolm said in a statement.

Researchers believe that their findings support the idea that relative brain sizes among species are created by a parity between selection for increased cognitive ability and the evolutionary costs of a big brain.

Researchers note that their findings have significant ramifications for humans.

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

However, support for the “expensive-tissue hypothesis,” the idea that there is a trade-off between the brain and the energy demands of other organs and reproduction, was limited to comparative studies among species and was correlative in nature.

“Despite over a century of research on the evolution of brain size, empirical support for the trade-off between cognitive ability and energetic costs is based exclusively on correlative evidence, and the theory remains controversial,” the authors wrote in a summary of their study.

Kolm and his team selected live-bearing guppies for large and small brains relative to the size of their bodies. They discovered that brain size had the ability to evolve “remarkably quickly.”

Researchers found that large-brained guppies outperformed smaller-brained peers in a test of numerical learning. However, they also discovered that big-brained guppies had smaller guts because most of their energy went to brain-building.

“We used artificial selection for large and small brain size relative to body size in a live-bearing fish, the guppy, and found that relative brain size evolved rapidly in response to divergent selection in both sexes,” the authors wrote. “Large-brained females outperformed small-brained females in a numerical learning assay designed to test cognitive ability. Moreover, large-brained lines, especially males, developed smaller guts, as predicted by the expensive-tissue hypothesis, and produced fewer offspring.”

Researchers note that further experiments will be necessary to make a more definitive conclusion about the validity of the “expensive-tissue” hypothesis.

“Our results on the guppies demonstrate that the order of evolutionary transitions is starting with a change in brain size, followed by a decrease in gut size,” Kolm told NBC News via email. “At the same time, this does not automatically mean that the opposite response is not also possible. To test this would of course require further experiments. Currently, our cautious conclusion is that we have identified a new possible direction of events in that selection for increased brain size may indeed have ‘forced’ a reduction in gut size.”

Researchers observed these effects even after supplying the guppies with plenty of food. In future experiments, researchers want to see what will happen with fish in a more competitive, semi-natural environment with limited resources and predators.

Researchers contend that the relatively small family sizes of humans and other primates might have helped to make our big brains possible.

The study’s findings were recently reported in the journal Current Biology. You can read the full story here.