If bigger brains were always better, every animal would have them. Thus, scientists reasoned, there must be a downside.

Human brains, for example, are only 2 percent of our bodies, but they take up a whopping 20 percent of our energy requirements. Which body parts have paid the price? The expensive-tissue hypothesis, developed in the early 1990s, suggests that our guts took the hit but that intelligence made up for the loss through more efficient foraging and hunting.

To examine the cost of big brains, researchers at Uppsala University in Sweden exploited the natural variation of brain size in guppies (Poecilia reticulata).

First, the team successfully created guppies that had brains about 9 percent larger than their counterparts through artificial selection. Then the researchers gave the fish a numerical learning task (guppies have a rudimentary ability to count). Whereas the males seemed to gain no benefits from possessing larger noggins, the females with bigger brains were much better at the task. But what was really remarkable was the cost of these larger brains. Gut size was 20 percent smaller in large-brained males and 8 percent smaller in large-brained females. The shrunken digestive system seemed to have serious reproductive consequences: the smarter fish produced 19 percent fewer offspring. The researchers reported their results online in January in Current Biology.

By providing empirical evidence for the physiological costs of brains, this study provides the first direct support for the expensive-tissue hypothesis and can provide us with insights into how our own big brains evolved. One of the prevailing hypotheses for our own brain growth is that more animal products in our diets allowed us to obtain more energy from less food, thus offsetting the cost of a reduced gut. The debate, however, is far from over. Comparative analyses in primates do not support a gut-brain trade-off, and there are certainly plenty of other hypotheses as to how and why we developed our massive lobes, as well as what price our bodies paid for them.

Adapted from Science Sushi at blogs.ScientificAmerican.com/science-sushi