OTTAWA—With big brains comes big intelligence, or so the hypothesis goes. But there may be trade-offs as well. Humans and other creatures with large brains relative to their body size tend to have smaller guts and possibly fewer offspring. Scientists have debated for decades whether the two phenomena are related. Now a team of researchers says that they are—and that big brains do indeed make us smart. The finding comes thanks to an unusual experiment reported here yesterday at the Evolution Ottawa evolutionary biology meeting in which scientists shrank and grew the brains of guppies over several generations.
"This is a real experimental result," says David Reznick, an evolutionary biologist at the University of California, Riverside, who was not involved in the study. "The earlier results were just correlations."
Researchers first began to gather evidence that big brains were advantageous after 19th century U.S. biologist Hermon Bumpus examined the brains of sparrows, some of whom had succumbed in a blizzard and some of whom survived. The survivors had relatively larger brains. More recently, evolutionary biologist Alexei Maklakov from Uppsala University in Sweden found evidence that songbirds that colonize cities tend to have larger brains relative to their body size than species still confined to the countryside. The challenge of urban life might require bigger brains, he and his colleagues concluded last year in Biology Letters.
Yet in humans and in certain electric fish, larger brain size seems to have trade-offs: smaller guts and fewer offspring. That's led some scientists to suggest there are constraints on how big brains can become because they are expensive to build and maintain.
To try to sort out the pluses and minuses of having a larger brain, Alexander Kotrschal, a postdoctoral fellow at Uppsala University, and his colleagues bred two lines of guppies. For each generation, he dissected the parents and measured their relative brain size to decide which offspring to breed, selecting those with the biggest and the smallest-brained parents to create two lines. After two generations the team documented up to a 10% difference in brain size between the two lines in females and an 8% difference in males.
The researchers then tested how smart the fish were by trying to teach them to count. The team trained the animals to look for food where a card had either two or four symbols. "The large-brained fish seem to be able to get it," Kotrschal reported at the meeting, but not the small-brained fish. "When you experimentally manipulate brain size, you get cleverer fish."
The guppies' big brains came at a cost, however. The small-brained fish averaged seven offspring in their first brood whereas the large-brained fish had only six. And the guts of the small-brained fish weighed 5.5 milligrams compared with just 4 milligrams for the large-brained fish. "If you select for a trait as complex as brain size, you going to lose with other traits," says Wesley Warren, a biologist at Washington University in St. Louis who was not involved with the study.
That primates have fewer offspring compared to other mammals and humans have fewer still, may be a consequence of the bigger brains, Kotrschal suggested.