Oxford Scientific / Getty Images Stock Researchers found that it was easy to breed guppies with bigger brains, but the fish had smaller guts and also produced fewer progeny. The results serve as experimental evidence of evolutionary trade-offs.





Scientists have long suspected that big brains come with an evolutionary price — but now they've published the first experimental evidence to support that suspicion, based on their efforts to breed big-brained fish.

A Swedish team found it relatively easy to select and interbreed common guppies to produce bigger (or smaller) brains — as much as 9.3 percent bigger, to be precise. But the bigger-brained fish also tended to have smaller guts and produce fewer babies.

This finding is consistent with what's known as the "expensive-tissue hypothesis" — the idea that there's a trade-off between the demands of the brain and the demands of other organs. For example, we humans have bigger brains than other primates, relative to body size. About 20 percent of the energy we take in is used up by the brain, which represents just 2 percent fo our body mass. But the amount of energy devoted to digestion is smaller, relatively speaking.

Some evolutionary biologists have speculated that when our distant ancestors shifted to an easier-to-digest diet, that freed up the energy for bigger brains. But that speculation has been based primarily on comparative studies of brain size and gut size as they are in present-day species. And some of the recent studies on the subject have been interpreted as refuting the expensive-tissue hypothesis.

Niclas Kolm and his colleagues at Uppsala University used artificial evolution — that is, selective breeding — to show the tissue trade-off in action. Their results were published online today by the journal Current Biology.

The experiment put 48 of the guppies through an underwater arithmetic test to see whether better cognitive abilities came with the bigger brains. It turned out that the brainier fish were better at learning to recognize how many geometric symbols were on a door in order to get to the food on the other side (at least if there were up to four symbols).

But in the gut-size department, the bigger-brained fish, especially the males, came up short (20 percent smaller for males, 8 percent for females). What's more, the big-brained fish had 19 percent fewer offspring than the small-brained fish. That result suggests that bigger brains are somehow associated with smaller broods — a phenomenon that researchers have noticed with regard to primates as well as cetaceans.

Although the raw numbers seem to support the expensive-tissue hypothesis, Kolm and his colleagues weren't able to tease out the genetic mechanism for the trade-off. Thus, it's not fully clear which comes first: smaller guts or bigger brains.

"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 in an 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."

The same caveat goes for the findings on reproduction. Kolm said he and his colleagues interpret their results as supporting the view that reduced reproductive capacity is one of the evolutionary costs associated with bigger brains.

"This would seemingly suggest a lower fitness in large-brained individuals, which would not be intuitively 'possible' from an evolutionary point of view," Kolm acknowledged. "Here it is important to remember that in our selective set-up, we have no additional selection pressures from avoiding predators, finding food, competing for mates, etc., that would occur in the natural environment. Hence, there might still be an overall fitness benefit from increased brain size in the wild, at least in certain environments/situations, that would allow selection for increased brain size despite the lowered fecundity."

The Swedish researchers are planning a new round of experiments to see how big-brained (and small-brained) guppies handle a more realistic evolutionary environment.

"Watch this space," Kolm said.

More about brains and evolution:

In addition to Kolm, the authors of "Artificial Selection on Relative Brain Size in the Guppy Reveals Costs and Benefits of Evolving a Larger Brain" include Alexander Kotrschal, Björn Rogell, Andreas Bundsen, Beatrice Svensson, Susanne Zajitschek, Ioana Brännström, Simone Immler and Alexei Maklakov.

Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with Cosmic Log as well as NBCNews.com's other stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.