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Expensive Organs: Guppies Reveal The Cost Of Big Brains

The views expressed are those of the author and are not necessarily those of Scientific American.


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There’s a lot to be said for smarts—at least we humans, with some of the biggest brains in relation to our bodies in the animal kingdom, certainly seem to think so. The size of animal brains is extravagantly well-studied, as scientists have long sought to understand why our ancestors developed such complex and energetically costly neural circuitry.

One of the most interesting evolutionary hypotheses about brain size is The Expensive Tissue Hypothesis. Back in the early 1990s, scientists were looking to explain how brain size evolves. Brains are exceedingly useful organs; more brain cells allows for more behavioral flexibility, better control of larger bodies, and, of course, intelligence. But if bigger brains were always better, every animal would have them. Thus, scientists reasoned, there must be a downside. The hypothesis suggests that while brains are great and all, their extreme energetic cost limits their size and tempers their growth. When it comes to humans, for example, though our brains are only 2% of our bodies, they take up a whopping 20% of our energy requirements. And you have to wonder: with all that energy being used by our brains, what body parts have paid the price? The hypothesis suggested our guts took the hit, but that intelligence made for more efficient foraging and hunting, thus overcoming the obstacle. This makes sense, but despite over a century of research on the evolution of brain size, there is still controversy, largely stemming from the fact that evidence for the expensive tissue hypothesis is based entirely on between species comparisons and correlations, with no empirical tests.

With beauty and brains, Poecilia reticulata help give insights into cognitive evolution

A unique study published this month in Current Biology has taken a new approach to examining this age old question. Rather than comparing species with bigger brain-to-body ratios to smaller-brained relatives, they exploited the natural variation of brain size in guppies (Poecilia reticulata). Guppies, as it turns out, aren’t as dumb as they look. They’re able to learn, and show rudimentary ability to count. Researchers from Uppsala University in Sweden were able to use their numerical abilities to test whether brain size affects intelligence in these simple fish.

Fig. 2 from the paper; Large-brained females outperform small-brained females in a numerical learning task. Depicted are the mean and standard error values for the number of times, out of eight tests, that an individual chose the correct option of either two or four objects in females and males selected for large and small brain size.

First, the team selected for larger and smaller brains from the natural variation in guppies. They successfully created smarty-pants guppies that had brains about 9% larger than their counterparts through artificial selection. Then, they put them to the test. While the males seemed to gain no benefits from possessing larger noggins, the females with bigger brains were significantly better at the task.

But what was really remarkable was the cost of these larger brains. Gut size was 20% smaller in large-brained males and 8% smaller in large-brained females. The shrunken digestive system seemed to have serious consequences reproductively, as the smarter fish produced 19% fewer offspring in their first clutch, even though they started breeding at the same age as their dumber counterparts. And, the authors noted, this was in an idealized tank setting with an plenty of food—what about in the wild, where resources are harder to come by? How much of a cost does a reduced gut have when meals aren’t guaranteed?

“Because cognitive abilities are important to facilitate behaviors such as finding food, avoiding predation, and obtaining a mate, individuals with increased cognitive abilities are likely to have higher reproductive success in the wild,” explain the authors. These benefits, though, don’t come cheap. “Our demonstration of a reduction in gut size and offspring number in the experimental populations selected for larger relative brain size provides compelling experimental evidence for the cost of increased brain size.”

There are still many questions to be answered. For example, the authors aren’t entirely sure why females were the only ones to show cognitive improvement with larger brains. They suggest that, perhaps, the researcher’s measure of intelligence (the numerical task presented to the guppies) may be be geared toward female behaviors. “In the guppy, females are more active and innovative while foraging,” they explain. “Because females feed more, they may thus have had more time to associate the cue with food in our experimental design.”

The clear trade-off between brains and guts, though, is an important finding. 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 hypothesis for our own brain growth is that the incorporation of more animal products into our diets, through hunting or cooking or however, allowed us to obtain more energy from less food, thus offsetting the cost of a reduced gut. The less food we needed to eat for the same amount of energy, the more our brains could grow even if our guts suffered for it. The debate, however, is far from over. Comparative analyses in primates don’t support a gut-brain tradeoff, and there are certainly plenty of other hypothesis as to how and why we developed our massive lobes, and what prices our bodies paid for them.

Kotrschal A., Rogell B., Bundsen A., Svensson B., Zajitschek S., Brännström I., Immler S., Maklakov A. & Kolm N. (2013). Artificial Selection on Relative Brain Size in the Guppy Reveals Costs and Benefits of Evolving a Larger Brain, Current Biology, DOI:

Photo credit: Guppies from gucio_55 / 123RF Stock Photo

Christie Wilcox About the Author: Christie Wilcox is a science writer and blogger who moonlights as a PhD student in Cell and Molecular Biology at the University of Hawaii. Follow on Google+. Follow on Twitter @NerdyChristie.

The views expressed are those of the author and are not necessarily those of Scientific American.





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  1. 1. DiscomBob 4:47 pm 01/3/2013

    I would think to show if big brains are an advantage you would have to have an environment that could be exploited by them, say by making food harder to obtain. If you have an environment where everyone has plenty to eat and it takes no extra brains to find food, then the bigger gut would be expected to win.

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  2. 2. popseal 1:00 am 01/4/2013

    I think…therefore I am. I’ve wondered about the ‘self awareness’ of animals. It seems obvious that some species simply exist, not very aware of anything at all, totally instinctive. I’ve heard it said that every time an aquarium denizen swims through his castle, he admires it as though its the first time he’s been there. My rat terrier on the other hand is very aware, easily trainable, happy, excitable, fearful (fireworks), and a people ‘person’. Max, just what are you ‘thinking’?

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  3. 3. Dinoking 4:09 am 01/4/2013

    I thought its was jaw muscle size that effects primate brain size. Every creature probably has different give and take percentages for different abilities. A little of everything effects everything else of course.

    There’s an idea a map of all the give and take genetic triggers for every species. Would be a worthwhile endeavor.

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  4. 4. cpurrin1 8:26 am 01/4/2013

    I know there are plenty of genetic, cultural, familial issues that can cause correlation between mass and IQ in humans, but variation in O2 demands among people’s brains might be interesting factor as well. Probably nothing at all new to that possibility, but it’s really remarkable that authors don’t feel OK to discuss it. If more generally known, people might be a tad slower to blame their “slow metabolism” for weight gain relative to peers who seem to eat the same amount of food.

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  5. 5. jtdwyer 8:43 am 01/4/2013

    popseal – On the other hand, perhaps your rat terrier is totally self-unaware, having been selectively bred to be aware of its environment and, especially, of people… Can an old dog remember what it was like to be young?

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  6. 6. jtdwyer 3:48 pm 01/4/2013

    BTW, isn’t it the development of extensive specialized brain structures that distinguishes human brain physiology from other primates, more than brain weight to body weight and other arbitrary measures? What happens to people whose frontal lobes have been disconnected – do their brains consume significantly less energy? I understand that their intellectual capabilities are significantly impaired…

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  7. 7. cping500 7:02 pm 01/4/2013

    People are easier than fish of dogs for looking at this issue. So why not start with people who start off with much reduced brains, or have been unfortunate enough to loose parts of their brains. (“Tiny Brains”)

    Maybe looking at some relevant clinical papers might offer some clues. I seem to recall that you can get along with much less grey matter than the current provision and hence less energy. (lean and hence no so hungry)

    Another line which I am sure has been up followed is the dissipation of the energy used both while awake and asleep as a limit on size (Hot Heads)

    And here is a question when interviewing prospective neuroscientists…. Is there any limitation to brain size in organisms which are grown for example from eggs if there is environmental pressure favouring big brains. (Bird Brains)

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  8. 8. David Evans 6:38 am 01/5/2013

    The assumption that for a given functionality, brain size must be related to body size seems slightly odd – and I suspect rooted in the human desire to confirm human superiority. Is it logical though? I can see that processing of sensory inputs might require more neurons if there are more nerve endings to be serviced, but what about thinking – surely that should not be a function of body size. Is it possible that the human brain is not large (relatively) because of its thinking ability, but for some other reason – language processing perhaps, or visual processing (recognising people from subtle clues for example is a skill that computers can’t emulate). I would love to have comments on this.

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  9. 9. hungry doggy 9:53 am 01/5/2013

    Is it possible that what took the “hit” in human evolution to pay for increased brain size was muscular strength? Chimpanzees are a lot stronger than humans. Maybe reduced muscle size and maybe fewer mitochondria in our muscle cells was how are ancestors paid for increased brain size? Just a speculation.

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  10. 10. hungry doggy 9:56 am 01/5/2013

    Sorry for the bad grammar in the second to last sentence. The word “are” should of course be “our.”

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  11. 11. bucketofsquid 1:16 pm 01/7/2013

    @hungry doggy – You beat me to the observation about human/chimp muscle and brain density. Clearly when the blog or the study say size they don’t actually mean an indication of volume so much as they mean mass. A large number of creatures have big brains that have lower density levels than humans. If we were to combine dense muscles with a complex, densely packed brain, I wonder if we would be capable of having a digestive system that could support it.

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  12. 12. RobertJ 8:13 am 01/8/2013

    What is the greatest threat to humans after, say, the zombie apocalypse?

    Right, it’s other humans. The zombies, or any other difficulty for that matter (drought, disease…) are engineering issues, subject to analytic solution (irrigation, sanitation…).

    But other humans always pose the iterated Prisoners’ Dilemma (IPD).

    The real reason we have big brains in the first place is the arms race set off by the costs and benefits of cooperation.

    IPD has no analytic answer, only transiently stable approximations. The great driver of human culture is codification of those local solutions.

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  13. 13. diducthat 5:28 pm 01/8/2013

    Robert, I take your point (even if vaguely off-topic), but many forms of life have evolved to find local (heuristic) solutions to the IPD. You have to look no further than your gut to find bacteria, which whilst warring, with each other, still find optima to allow cooperation. Indeed, it is likely that most examples of symbiosis probably arose as a result (eg. eukaryotic mitochondria). The TSP (travelling salesman problem) has also been widely explored by evolution (ants, slime-moulds, etc.).

    Back to the point, however, although the article is interesting, I can’t see that this guppie model in a non-stressed environment does anything but demonstrate that there is a protein trade-off with little evolutionary advantage.

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  14. 14. flatigre1 9:56 pm 01/8/2013

    Hungry Doggy , I fully agree with you . Of course , in the long run , our ability to use our brains to invent and use weapons made a difference , but in the previous millions of years of our evolution ( until we were able to chip flint into utensils and weapons , allowing for other materials ) I would rather prefer the chimpanzees muscle strenght ( a chipanzee is some 3.5 times our power weight by weight ). And the difference looks like some minor protein-producing DNA variation . In the begining , it seems was a potentially extintion-threating trade off .

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