Big brains are pricey, guppy study shows

Jan 03, 2013
Male and female guppies (Poecilia reticulata). Image: Wikipedia.

Bigger brains can make animals, well, brainier, but that boost in brain size and ability comes at a price. That's according to new evidence reported on January 3rd in Current Biology, in which researchers artificially selected guppies for large and small brain sizes.

The findings lend support to the notion that bigger brains and increased cognitive ability do go together, a topic that has been a matter of considerable debate in recent years, said Niclas Kolm of Uppsala University in Sweden. They also represent some of the first convincing evidence that large brains are expensive, evolutionarily speaking.

"We provide the first that evolving a larger brain really is costly in terms of both gut investment and, more importantly, reproductive output," Kolm said.

Together, the findings strongly support the idea that relative brain sizes among species are shaped through a balance between selection for increased cognitive ability and the costs of a big brain.

The results in have important implications for us humans. After all, one of the most distinctive features of the human brain is its large size relative to the rest of the body.

"The only makes up 2 percent of our total but stands for 20 percent of our total energy demand," Kolm said. "It is a remarkably costly organ energetically."

But support for the so-called "expensive-tissue hypothesis"—that there is a trade-off between the brain and the energy demands of other organs and reproduction—came only from comparative studies among species and were correlative in nature.

In the new study, Kolm's team took a different, within-species approach. They selected live-bearing guppies for large and small brains relative to the size of their bodies. Under that strong selection pressure, they found that could evolve "remarkably quickly."

After selection, large-brained guppies outscored their smaller-brained peers in a test of numerical learning. With more energy devoted to brain-building, brainy fish—males especially—did have smaller guts. They also left fewer offspring to the next generation.

Those effects were observed despite the fact that the fish were supplied with an abundance of food. The researchers say they are curious to see what will happen in future experiments with fish in a more competitive, semi-natural environment including limited resources and predators.

The findings lead Kolm and his colleagues to suggest that the relatively small family sizes of humans and other primates, not to mention dolphins and whales, might have helped to make our big brains possible.

Explore further: Lizard activity levels can help scientists predict environmental change

More information: Kotrschal et al.: "Artificial selection on relative brain size in the guppy reveals costs and benefits of evolving a larger brain." dx.doi.org/10.1016/j.cub.2012.11.058

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User comments : 4

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VendicarD
1 / 5 (2) Jan 03, 2013
Insufficient.

The genes may simply be homologous with genes for small guts, and so in the long run, as genes are shuffled during crossover, the connection would be lost.
Torbjorn_Larsson_OM
not rated yet Jan 03, 2013
Insufficient to test the hypothesis? They took a "random" species. If any such connections are temporary, they would show up as such.

Yes, this effect may very well be due to selective sweeps. So what? They still show up early, until genes are transferred.
VendicarD
not rated yet Jan 04, 2013
Then they need to run their experiment through another million generations in order to see how drift in the transcriptome alters the relationship in their results.

"If any such connections are temporary, they would show up as such." - Larsson

Have they done this?
VendicarD
not rated yet Jan 04, 2013

"They still show up early, until genes are transferred." - Larsson

The difference of course may very well be one of identifying correlated gene locality on a chromosome vs identifying a universal evolutionary trade off between brain size and the size of other organs and tissues.

If your goal is to confuse co-evolution of genes with simple homologous placement then your objection is valid.

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