Q: Why don’t apes have bigger brains? A: They can’t eat enough to afford them

By Ed Yong | October 23, 2012 9:00 am

As animals get bigger, so do their brains. But the human brain is seven times bigger than that of other similarly sized animals. Our close relative, the chimpanzee, has a brain that’s just twice as big as expected for its size. And the gorilla, which can grow to be three times bigger than us, has a smaller brain than we do.

Many scientists ask why our brains have become so big. But Karina Fonseca-Azevedo and Suzana Herculano-Houzel from the Federal University of Rio de Janeiro have turned that question on its head—they want to know why other apes haven’t evolved bigger brains. (Yes, humans are apes; for this piece, I am using “apes” to mean “apes other than us”).

Their argument is simple: brains demand exceptional amounts of energy. Each gram of brain uses up more energy than each gram of body. And bigger brains, which have more neurons, consume more fuel. On their typical diets of raw foods, great apes can’t afford to fuel more neurons than they already have. To do so, they would need to spend an implausible amount of time on foraging and feeding. An ape can’t evolve a brain as big as a human’s, while still eating like an ape. Their energy budget simply wouldn’t balance.

Our ancestors overcame this constraint when they learned how to cook. Cooked food offers more calories than raw food, and is easier to chew and digest. These early chefs could gain more energy from the same amount of eating time. That, in turn, fuelled more neurons and larger brains.

Fonseca-Azevedo and Herculano-Houzel developed their argument using data on 17 species of primates, from the tiny common marmoset to the huge gorilla. They collected figures on the average brain and body sizes of many primates from past papers. They worked out the number of neurons in various primate brains, by painstakingly inspecting their own samples. And they took data collected by Callum Ross and others on the number of hours that these species spend eating.

From these figures, they calculated the maximum number of neurons that each species can afford, given different amounts of feeding time. You can see the results in the graph below. There are clear boundaries. As Fonseca-Azevedo and Herculano-Houzel write:

“A primate that fed the putative maximum of 10 hours per day could afford a brain of, at most, 113 billion neurons, in which case it could weigh no more than 64 kg; if it fed 8 hours per day, it could afford a brain of no more than 53 billion neurons, but a body no larger than 24 kg; and if it fed 6 hours per day, it could afford up to 23 billion neurons in the brain, but, in that case, its body could weigh only 8 kg.” (For comparison, our brains contain around 86 billion neurons.)

Here’s another way of looking at it: To afford a brain that’s 2 percent of its body weight (as in your head and mine), a primate that fed for 8 hours a day (like  gorilla) couldn’t be heavier than 42 kilograms, or have more than 49 billion neurons. If it fed for 7 hours a day (like a chimp or orangutan), it couldn’t be heavier than 26 kilograms or have more than 32 billion neurons. To pull off a human-like brain-to-body ratio, primates would have to sacrifice both mass and neurons. “The largest great apes cannot afford both a large body and a larger number of neurons,” wrote Fonseca-Azevedo and Herculano-Houzel.

How did our ancestors manage? In earlier work, Fonseca-Azevedo and Herculano-Houzel had estimated the numbers of neurons in early hominins (human relatives who arose after our lineage split from chimps). The earliest hominins, such as Australopithecus afarensis (of which the famous Lucy was an example), had around 30 to 40 billion neurons, and could’ve maintained those by eating an ape-like diet for 7 hours a day. No problems there.

But the later Homo erectus had around 62 billion neurons. If it ate raw food, it would’ve needed to feed for more than 8 hours a day. Even later species like Homo heidelbergensis, Neanderthals, and our own relatives, would’ve needed to eat for 9 hours a day. Fonseca-Azevedo and Herculano-Houzel think that our ancestors, probably starting with Homo erectus, only managed by learning to cook.

Opinion is divided on the study. Richard Wrangham from Harvard University likes it. After all, he has long argued that cooking helped to fuel the evolution of our large brains. “I love their study because it is the first to have yet commented on my proposal that cooked food enabled brain expansion,” he says. “Their calculations look reasonable; and their broad conclusion is of course directly in line with mine.”

But he adds that Fonseca-Azevedo and Herculano-Houzel haven’t accounted for the types of food that primates eat. They assumed that raw animals foods, such as meat, brains, marrow and fish, wouldn’t provide enough extra calories to break the constraints that apes face, and Wrangham agrees with them. But other scientists think otherwise.

Karin Isler from the University of Zurich, who studies brain size evolution, isn’t at all convinced by the study, especially since Fonseca-Azevedo and Herculano-Houzel haven’t accounted for the quality of a species’ diet. Since other great apes females are actually smaller than human women, “the whole argument is mainly based on the gorilla,” Isler says. Gorillas eat a lot of leaves, and have to spent more time eating to extract the same number of calories as apes with other diets. That alone might explain why their brains aren’t bigger. Could a move towards animal protein and fat explain the bigger brains of humans without needing to invoke cooking? The study doesn’t answer that question either way.

Wrangham clearly thinks otherwise, but he says that we need more research to settle the matter, including some estimates of how quickly primates can digest raw animal foods.

Reference: Fonseca-Azevedo and Herculano-Houzel. 2012. Metabolic constraint imposes tradeoff between body size and number of brain neurons in human evolution. PNAS http://dx.doi.org/10.1073/pnas.1206390109

Image of gorilla by Jarapet; graphs from paper

More on brain evolution:


Comments (14)

  1. Tony Mach

    Or why Vegetarianism is self limiting.

    BTW, one should look at the neuronal tissues of the brain or the eye, and their content of both Omega-3 and Omega-6 (in the animal form of EPA/DHA and AA) – and from what food one can get efficiently these type of fatty acids.

  2. Uranium Willy

    What I would like to know is why is brain size proportional to body size. Why isn’t it that the bigger the brain the smarter, independent of body size. It doesn’t make sense that the bigger the body the more brain cells are needed to control it and for intelligence why should there be any connection to body size except a bigger body allows for a big brain and therefore smart.

  3. Scott

    Ed, in the paragraph starting “But the later Homo erectus had around 62 million neurons.”, do you mean *billion* instead of *million*? Other neuron measurements in the article are in billions.

  4. @Scott -Oops! Corrected. What’s three orders of magnitude between friends?

  5. Archwright

    @Tony Mach,
    Anyone in the developed world can now pick-and-choose from the best, most nutritionally dense foods in the world. Your assertion that “Vegetarianism is self limiting” is absurd.

  6. James Harmer

    @Uranium Willy – For most mammals there is a direct correlation between body size and brain size. At a simplistic level, the more muscles you have, the bigger your brain has to be to control and co-ordinate their use. As mentioned in the article, brains are very expensive to run, so there are strong evolutionary pressures trying to make the brain as small as possible for a given body size, so that the animal concerned won’t starve. Humans are unusual in many ways, and both the reduced gut size and enhanced brain size compared to other apes, are a direct result of our harnessing the chemical energy in fire to pre-digest and thereby obtain more energy from our food. Also, because we don’t have to spend every waking moment eating, we have time for more complex social development…

  7. Nihaya Khateb

    I think that our big brain evolved as an outcome of eating cooked meat and not just cooked food. Our brain is composed mainly of fatty material and cooked meat supply this mostly other than vegetables. So, I suppose that humans will loose big fraction of their neurones if they go completely vegetarian after long period.

  8. Karin Isler

    @Uranium Willy and James Harmer: Yes, there is a correlation between brain and body size. however, this correlation is not proportional, but “allometric”. In larger animals, the brain amounts to a smaller percentage of body mass than in smaller animals. It has been proposed that this slope of the brain vs. body mass relationship is related to either body surface (the amount of sensorimotor input and output, slope of 0.66), or with the metabolic rate (Kleiber’s law: metabolic rate scales to body mass also with a slope of 0.66 or 0.75). In the very small sample of Fonseca-Azevedo and Herculano-Houzel, the slope is almost isometric (slope 1), which is an artifact of the small number of species.
    But Uranium Willy, you have a point here: A study by Deaner et al. (2007) showed that in primates, “intelligence” is better explained by total brain size than by relative brain size (relative to body mass). Thus, the best way to account for differences in overall body size between species is still unclear.

  9. John S.

    So, if animals (apes) could only consume more calories their brains would get bigger? Isn’t there some large piece of the puzzle missing here, like it being advantageous to have a bigger brain? Obviously the apes consume a massive amount of calories to be so huge, yet they are huge in stature and not brains. No explanation.
    Bigger brains=better is presumed, but it wouldn’t help to have a slightly bigger brain, and then have your limbs ripped off by a more powerful ape, for example. For the “more calories then more brains” theory to work, something must have happened to humans at the same time as a caloric increase to favor bigger brain size rather than brawn.
    The numbers comparison now appears merely academic without an actual (interesting) explanation.

  10. Derpina

    @Nihaya Khateb

    By your argument, wouldn’t a woman who has been a vegetarian their entire life give birth to a baby without a brain (or at least a disproportionately small brain compared to other babies)?

  11. KN

    This whole argument is interesting only under the assumption that neurons evolve(d) for cognition, here is an interesting paper that suggests this may not be true.

    “An alternative possibility is that the neuron evolved first to fulfil a growth regulatory role in relation to regeneration and asexual reproduction. Its post-mitotic status, and hence the fixed number of nerve cells in an adult organism, may have been critical to define the limits of growth in some species, whereas its ability to extend and regenerate axons allowed it to control growth at a distance. This growth-regulating aspect of neurons may deserve at least some consideration in discussions of the emergence of this cell type.”

    Basically, the “cognitive” role of neurons came later. What that means for cognition is worth thinking about. There also seems to be a “brain-driven” model of body growth and development in there somewhere, but that was not very clear.


    I don’t disagree with the idea that cooking has given our species more energy to play around with. But whether this *directly* contributes to brain growth is an open question. For instance, it is possible that the extra energy allowed us to sit around and develop tools, and language, and the resulting wider scope of activity led to the larger brain size.

    For those of you using the paper to push a non-vegetarian agenda, it is worth pointing out that around 500 million Indians seem to have survived and are surviving fine with their vegetarian food habits. Some of them even do fancy math with their veggie brains. (India has around 900 million Hindus, most are vegetarian.)

  12. Excerpt: “Fonseca-Azevedo and Herculano-Houzel developed their argument using data on 17 species of primates, from the tiny common marmoset to the huge gorilla.”

    I developed my argument using the molecular biology of adaptive evolution via ecological, social, neurogenic, and socio-cognitive niche construction in species from microbes to man. It is the common molecular biology that enables human brain development as exemplified in the honeybee model organism of nutrient chemical-dependent and pheromone-dependent brain development. Perhaps my argument is too detailed to be considered.

    “The concept that is extended is the epigenetic tweaking of immense gene networks in ‘superorganisms’ (Lockett, Kucharski, & Maleszka, 2012) that ‘solve problems through the exchange and the selective cancellation and modification of signals (Bear, 2004, p. 330)’. It is now clearer how an environmental drive probably evolved from that of food ingestion in unicellular organisms to that of socialization in insects. It is also clear that, in mammals, food odors and pheromones cause changes in hormones such as LH, which has developmental affects on sexual behavior in nutrient-dependent, reproductively fit individuals across species of vertebrates.” — Kohl, J.V. (2012) Human pheromones and food odors: epigenetic influences on the socioaffective nature of evolved behaviors. Socioaffective Neuroscience & Psychology, 2: 17338. http://dx.doi.org/10.3402/snp.v2i0.17338

  13. Early hominids began to use environmental objects such as stones and branches to get more food, just as contemporary chimps have been found to do. The marginally increased calories allowed slightly bigger brains. Bigger brains allowed the hominids to develop more complex stone tools, spears, etc. which allowed still further caloric supply. This positive feedback between increased brain size (needed to conceptualize tools) and increasing sophistication of tools (to get more calories) continues until today. This is the ONLY way large brains could have arisen. Genetics alone result in the examples like the gorilla. Hominids developed non-genetic technology which went beyond what genetics alone provide. Memes are the genes of technology, and they developed in hominid brains. Technological memes evolving in brains are what define humans; they are what makes humans different from all other biological organisms.

  14. Joe Miller

    Did the hominids’ prowess at cooperative hunting and foraging have anything to do with it?


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