Since the 19th century, researchers have noticed that animals with large brains had survival advantages in complex environments, but also appeared to have fewer offspring. To test whether the two observations were causally linked, researchers from Uppsala University in Sweden bred guppies and selected for those that either had small brains or large ones. They tested the abilities of the fish on learning tests and found that, not surprisingly, the larger-brained fish did better.

But there was also a downside of the big brains: the fish produced fewer offspring. Small-brained fish produced an average of seven offspring, while large-brained fish had an average of six, the researchers reported at the Evolution Ottawa meeting on Tuesday (July 10).

"This is a real experimental result," David Reznick, an evolutionary biologist at the University of California who was not involved in the study, told ScienceNOW. "The earlier results were just correlations."

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