Scientists have long known that brains are extremely power-hungry organs — in the case of human beings accounting for up to 20% of the energy we use. It’s been thought that large brains might be so costly that as they evolved, other organs in the body grew correspondingly smaller. It might also be the reason that large-brained animals such as humans, chimps and whales have relatively few offspring: there’s just not enough energy in the tank to crank out repeated litters over the course of all the breeding years.
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But plausible as these trade-off theories seem, they have rarely been tested experimentally, in part because the work is so painstaking. The experiments, done right, involve raising, breeding, and dissecting thousands of animals, and until recently, the only attempts to do so in vertebrates were a set of small mouse experiments in the 60s and 70s. But one day several years ago, Niclas Kolm, a professor of evolutionary biology at University of Uppsala in Sweden, decided to take the plunge, using guppies as his sample species, in part because they breed so quickly — needing as little as three months to reach sexual maturity.
Kolm, along with post-doc Alexander Kotrschal and other colleagues, started out with 225 breeding pairs of guppies, whose offspring they carefully separated into sex-segregated tanks. Then, they dissected the parents, weighed their brains and measured their bodies to find out which had the largest and the smallest brains, relative to body size. The offspring of the top 25% in terms of brain weight were then allowed to mate among themselves, as were the offspring of the bottom 25%. When the next round of guppies was born, Kotrschal repeated the process. Overall, more than 1,100 tanks of fish were involved.
The team next tested whether the last generation, whose parents’ brains were 9% larger than those of the smaller-brained group, were better at a cognitive task. Male and female fish from the large- and small-brained populations were each placed for three days in a tank outfitted with a card on either end. One card had two symbols on it, the other had four, and twice a day, each guppy was fed in front of one of these cards, never the other. Then, to see whether the number of symbols on the card had made an impression, the researchers let the guppies go a day without feeding, and watched to see if they swam over to the card where they had been fed before.
The female large-brained guppies were champions at returning to the right number of symbols, outperforming their smaller-brained counterparts by about 40%. Curiously, there was no difference between the large- and small-brained males. The researchers have a possible explanation for this disparity:
“We think that the reward of food is not so important for males as for females,” Kolm says. Kotrschal adds: “In wild guppies, the females forage a lot…but the males, all they think about is copulation.” A more suitable temptation for male guppies, perhaps, might be a female guppy, an idea the researchers intend to follow up on.
As Kolm and his colleagues also found, there were indeed penalties associated with a big brain. Overall, guppies with larger brains had smaller guts — relative to their brain size — and produced 19% fewer babies. “This is the most interesting finding in the study by far,” Kolm says, referring to the drop in offspring, “because this suggests that there are true fitness costs to having a large brain, at least in [this] situation.”
The smaller guts intrigues other scientists in the field as well, because that is exactly what is predicted by a theory in one of the first papers: as the second most energy-intensive organ, the gut might have to shrink to allow the brain to grow. Perhaps, says Jeremy Niven, a research fellow in neuroscience at the University of Sussex, there is a fixed energy budget for developing all organs, meaning that gut and brain are by no means the only two in play. ”There are many expensive tissues in an animal body,” he says.” There may be other effects on muscle-mass or the size of the kidneys for instance that haven’t necessarily been investigated.”
Certainly, what happens in a fish tank — with no predators and no competition for always-available food — hardly parallels what happens in the wild. But the investigators are planning to probe the guppies’ brain development further, trying in particular to factor in some of those natural variables. And they might as well. After all that hard work evolving large-brained guppies, they now have more than 5,000 of the little critters. “It takes over an hour just to feed all the fish,” Kolm says. But here too, the tradeoff may be worth it.
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