In a Swedish lab, Alexander Kotrschal has deliberately moulded the intelligence of small fish called guppies. From a starting population, he picked individuals with either unusually large or small brains for their bodies, and bred them together. It’s what farmers and pet-owners have done for centuries, selectively breeding animals with specific traits, from shorter legs or more muscle.
Or bigger and smaller brains. After just two generations, Kotrschal had one lineage of guppies with brains that were 9 percent bigger than the other. And these individuals proved to be smarter—they outclassed their peers at a simple learning task, where they learned to discriminate between two and four symbols. This may seem like child’s play for us, but it’s a “relatively advanced cognitive task” for a fish, says Kotrschal.
Their boosted smarts came at a price—the big-brained fish developed smaller guts and produced fewer offspring. Brains are expensive energy-guzzling organs. Ours, for example, make up just 2 percent of our body weight but consume 20 percent of our energy. Many scientists think that to pay for our larger brains, we had to scale back other parts of our bodies like our guts or fat stores, and that’s exactly what Kotrschal found in his guppies.
This deceptively simple experiment is an important one. It provides direct evidence for two important ideas about brain evolution: that bigger brains make for more intelligent animals; and that animals must pay some cost for bigger brains.
Most of the support for these ideas, and there’s rather a lot, comes from comparing different animals or populations. For example, more than 50 studies have shown links between brain size and social complexity, flexible behaviours, innovation, and more. This comparative approach is useful but has many problems. The fact that two traits are correlated does not mean that one caused the other, and there could always be other factors at work. For example, big-brained humans do tend to have higher IQs, but maybe that’s because wealthier people raise healthier children and provide them with better education.
That’s why experimental evidence, of the kind that Kotrschal has found, is so valuable. If we actively manipulate brain size to produce creatures with bigger brains, do they become more intelligent? Yes. Do other body parts get smaller? Yes. It’s “the strongest evidence for a direct effect of brain size on cognitive abilities within a species,” says Karin Isler, who studies brain evolution at the University of Zurich.
There’s a long history of studies into brain size evolution, and it’s not hard to see why. For our bodies, human brains are relatively huge compared to those of other animals, and it’s reasonable to think that this boost in size was important for our vaunted intellect. On the one hand, comparisons in primates and other animals support this link. On the other, social insects like ants and bees can show sophisticated behaviour, form complex societies and perform advanced mental tasks with brains no bigger than a pinhead.
But Kotrschal writes, “Our results now show that larger brains really can be better.” Brain size may be a crude and unsophisticated indicator of mental abilities, but the link is good enough that selecting for the former can boost the latter.
Kotrschal’s study also reminds us that evolution can’t just pull adaptations out of thin air. There’s always a cost. In the case of the guppies, that took the form of smaller guts and fewer young.
The guts result is important—it’s the first direct support for an 18-year-old hypothesis that during human evolution, we sacrificed guts for smarts. This “expensive-tissue hypothesis”, first proposed in 1995 by Leslie Aiello and Peter Wheeler, noted that our guts and brains are our most energetically expensive organs. As our ancestors started eating a richer diet of meat and tubers, and eventually started cooking food, they lifted some of the digestive load from their bowels. This allowed their guts to shrink, and freed up some surplus energy to fuel our expanding brains.
It’s still a controversial idea. Just last year, Isler led a study that seemed to disprove it, with an intense series of dissections that showed no connection between the size of a mammal’s brain and its other organs (although big-brained species did have smaller fat stores). Again: more comparisons and correlations. “Careful experimental work is what has been lacking,” says Aiello.
That’s what Kotrshcal has provided, even though he was originally sceptical of the idea. “There were multiple comparative studies suggesting such a trade-off but I always found it hard to believe,” he says. “In fact this is one of our strongest results now. [That was] the biggest surprise for me.”
It’s perhaps even more important that Kotrschal’s big-brained guppies produced 19 percent fewer offspring than the small-brained ones. Big brains are such an obvious part of our lives and anatomy that it’s tempting to see them as naturally desirable adaptations. But studies like these show that there are huge costs to braininess. For example, you might sideline reproduction, which is also an energetically expensive activity
This fits with a pattern seen across other animals. Among mammals, the most intelligent groups—the primates and cetaceans (whales and dolphins)—also have unusually low fertility. And humans, in particular, have the largest brains of any primate and the lowest number of offspring. “Our results suggest that the reduction in offspring number may have been a major cost associated with the evolution of a larger brain among the primates and especially hominids,” says Kotrschal.
This tug-of-war between brains and fertility sets a “grey ceiling” for animals—a point where their brains become so big and they reproduce so slowly, that they flirt with extinction. For bigger brains to take hold, the advantages they offer, such as greater intelligence leading to higher odds of survival, have to outweigh the fact that their owners cannot raise as many young.
There’s one final twist to Kotrschal’s study: Only the female big-brained guppies became smarter. This may just be because females are more active and take more risks while foraging than males, so are naturally suited to the test that Kotrschal used. Females also choose their males based on black spots on their bodies, so their eyes may be better tuned to the black symbols in the test. Or, it’s possible that both sexes use their extra brains in different ways.
Next, the team wants to see what would happen if they repeated their experiment in more realistic settings, where the fish get to compete with one another over limited supplies of food, or face the threat of predators.
It would also be interesting to see more experiments of this kind in other animals. “I am a bit concerned with extending the conclusion beyond guppies,” says Aiello.” The lesson of recent work is a variety of trade-offs that are possible across species to support large brain sizes. I would like to see similar research done in other species and particularly in mammals like mice, to begin to get a handle on what other trade-offs might be involved, or if the brain gut trade-off really is more universal than many would accept now.”
Reference: Kotrschal, Rogell, Bundsen, Svensson, Zajitschek, Brannstrom, Immler, Maklakov & Kolm. 2013. Artificial Selection on Relative Brain Size in the Guppy Reveals Costs and Benefits. Current Biology http://dx.doi.org/10.1016/j.cub.2012.11.058
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