Comparisons of different animals have shown that larger brains provide greater intelligence, but they also guzzle more energy and have other consequences. Now scientists have observed the effects of varying brain size within a single species: guppies. The work is in Current Biology. Swedish researchers bred two different lines of guppies, selecting one for larger brains and one for smaller. The fish quickly modified until brains were nine percent larger in the big-brained line than in the other. Not surprisingly, when 48 guppies were given learning tests, large-brained female fish outperformed small-brained females. However, males from both lines scored about the same, possibly because the female guppies' visual system was more suited to the type of intelligence test used. But big brains also had a downside—the brainier fish had smaller guts, by 20 percent for males and 8 percent for females.