Brain size comparison

Brain-to-body mass ratio, also known as the brain-to-body weight ratio, is the ratio of brain mass to body mass, which is hypothesized to be a rough estimate of the intelligence of an animal, although fairly inaccurate in many cases. A more complex measurement, encephalization quotient, takes into account allometric effects of widely divergent body sizes across several taxa. The raw brain-to-body mass ratio is however simpler to come by, and is still a useful tool for comparing encephalization within species or between fairly closely related species.

The relationship between brain-to-body mass ratio and complexity of behaviour is not perfect as other factors also influence intelligence, like the evolution of the recent cerebral cortex and different degrees of brain folding, which increase the surface of the cortex, which is positively correlated in humans to intelligence. The noted exception to this, of course, is swelling of the brain which, while resulting in greater surface area, does not alter the intelligence of those suffering from it.

In the essay "Bligh's Bounty", Stephen Jay Gould noted that if one looks at vertebrates with very low encephalization quotient, their brains are slightly less massive than their spinal cords. Theoretically, intelligence might correlate with the absolute amount of brain an animal has after subtracting the weight of the spinal cord from the brain. This formula is useless for invertebrates because they do not have spinal cords, or in some cases, central nervous systems.

Recent research indicates that, in non-human primates, whole brain size is a better measure of cognitive abilities than brain-to-body mass ratio. The total weight of the species is greater than the predicted sample only if the frontal lobe is adjusted for spatial relation. The brain-to-body mass ratio was however found to be an excellent predictor of variation in problem solving abilities among carnivoran mammals.

In humans, the brain to body weight ratio can vary greatly from person to person; it would be much higher in an underweight person than an overweight person, and higher in infants than adults. The same problem is encountered when dealing with marine mammals, which may have considerable body fat masses. Some researchers therefore prefer lean body weight to brain mass as a better predictor.

Encephalization quotient (EQ), encephalization level (EL), or just encephalization is a relative brain size measure that is defined as the ratio between observed to predicted brain mass for an animal of a given size, based on nonlinear regression on a range of reference species.[

• Perspective on intelligence measures

Currently the best predictor for intelligence across all animals is forebrain neuron count.[17] This was not seen earlier because neuron counts were previously inaccurate for most animals. For example, human brain neuron count was given as 100 billion for decades before Herculano-Houzel[18][19] found a more reliable method of counting brain cells.

• Variance in brain sizes

Rules for brain size relates to the number brain neurons have varied in evolution, then not all mammalian brains are necessarily built as larger or smaller versions of a same plan, with proportionately larger or smaller numbers of neurons. Similarly sized brains, such as a cow or chimpanzee, might in that scenario contain very different numbers of neurons, just as a very large cetacean brain might contain fewer neurons than a gorilla brain.

• Limitations and possible improvements over EQ

There is a distinction between brain parts that are necessary for the maintenance of the body and those that are associated with improved cognitive functions. These brain parts, although functionally different, all contribute to the overall weight of the brain. Jerison (1973) for this reason, has considered 'extra neurons', neurons that contribute strictly to cognitive capacities, as more important indicators of intelligence than pure EQ. Gibson et al. (2001) reasoned that bigger brains generally contain more 'extra neurons' and thus are better predictors of cognitive abilities than pure EQ, among primates.[

Factors, such as the recent evolution of the cerebral cortex and different degrees of brain folding (gyrification), which increases the surface area (and volume) of the cortex, are positively correlated to intelligence in humans.[

In a meta-analysis, Deaner et al. (2007) tested ABS, cortex size, cortex-to-brain ratio, EQ, and corrected relative brain size (cRBS) against global cognitive capacities. They have found that, after normalization, only ABS and neocortex size showed significant correlation to cognitive abilities. In primates, ABS, neocortex size, and Nc (the number of cortical neurons) correlated fairly well with cognitive abilities. However, there were inconsistencies found for Nc. According to the authors, these inconsistencies were the result of the faulty assumption that Nc increases linearly with the size of the cortical surface. This notion is incorrect because the assumption does not take into account the variability in cortical thickness and cortical neuron density, which should influence Nc

The notion that encephalization quotient corresponds to intelligence has been disputed by Roth and Dicke (2012). They consider the absolute number of cortical neurons and neural connections as better correlates of cognitive ability.[28] According to Roth and Dicke (2012), mammals with relatively high cortex volume and neuron packing density (NPD) are more intelligent than mammals with the same brain size. The human brain stands out from the rest of the mammalian and vertebrate taxa because of its large cortical volume and high NPD, conduction velocity, and cortical parcellation. All aspects of human intelligence are found, at least in its primitive form, in other nonhuman primates, mammals, or vertebrates, with the exception of syntactical language.

• Brain-body size relationship

Brain Size

More glia (supporting brain cells)

Neocortex (complex processing)

Human victory

But what, exactly, makes our brains so special? Some leading arguments have been that our brains have more neurons and expend more energy than would be expected for our size, and that our cerebral cortex, which is responsible for higher cognition, is disproportionately large—accounting for over 80 percent of our total brain mass.

Suzana Herculano-Houzel, a neuroscientist at the Institute of Biomedical Science in Rio de Janeiro, debunked these well-established beliefs in recent years when she discovered a novel way of counting neurons—dissolving brains into a homogenous mixture, or “brain soup.” Using this technique she found the number of neurons relative to brain size to be consistent with other primates, and that the cerebral cortex, the region responsible for higher cognition, only holds around 20 percent of all our brain’s neurons, a similar proportion found in other mammals. In light of these findings, she argues that the human brain is actually just a linearly scaled-up primate brain that grew in size as we started to consume more calories, thanks to the advent of cooked food.

Other researchers have found that traits once believed to belong solely to humans also exist in other members of the animal kingdom. Monkeys have a sense of fairness. Chimps engage in war. Rats show altruism and exhibit empathy. In a study published last week in Nature Communications, neuroscientist Christopher Petkov and his group at Newcastle University found that macaques and humans share brain areas responsible for processing the basic structures of language.

Although some of the previously proposed reasons our brains are special may have been debunked, there are still many ways in which we are different. They lie in our genes and our ability to adapt to our surroundings.

Plasticity may be what underlies the specific differences in our brain that lead to our unique cognitive abilities. A study published last week in Proceedings of the National Academy of Sciences revealed that human brains may be less genetically inheritable, and therefore more plastic, than those of chimpanzees, our closest ancestors.

Neuroscientists have become used to a number of “facts” about the human brain: It has 100 billion neurons and 10- to 50-fold more glial cells; it is the largest-than-expected for its body among primates and mammals in general, and therefore the most cognitively able; it consumes an outstanding 20% of the total body energy budget despite representing only 2% of body mass because of an increased metabolic need of its neurons

Here, I review this recent evidence and argue that, with 86 billion neurons and just as many nonneuronal cells, the human brain is a scaled-up primate brain in its cellular composition and metabolic cost, with a relatively enlarged cerebral cortex that does not have a relatively larger number of brain neurons yet is remarkable in its cognitive abilities and metabolism simply because of its extremely large number of neurons.

If the basis for cognition lies in the brain, how can it be that the self-designated most cognitively able of animals—us, of course—is not the one endowed with the largest brain? The logic behind the paradox is simple: because brains are made of neurons, it seems reasonable to expect larger brains to be made of larger numbers of neurons; if neurons are the computational units of the brain, then larger brains, made of larger numbers of neurons, should have larger computational abilities than smaller brains. By this logic, humans should not rank even an honorable second in cognitive abilities among animals: at about 1.5 kg, the human brain is two- to threefold smaller than the elephant brain and four- to sixfold smaller than the brains of several cetaceans

Humans also do not rank first, or even close to first, in relative brain size (expressed as a percentage of body mass), in absolute size of the cerebral cortex, or in gyrification (Hofman, 1985). At best, we rank first in the relative size of the cerebral cortex expressed as a percentage of brain mass, but not by far. Although the human cerebral cortex is the largest among mammals in its relative size, at 75.5% [Rilling and Insel, 1999], 75.7% [Frahm et al., 1982], or even 84.0% [Hofman, 1988] of the entire brain mass or volume, other animals, primate and nonprimate, are not far behind: The cerebral cortex represents 73.0% of the entire brain mass in the chimpanzee ([Stephan et al., 1981], 74.5% in the horse, and 73.4% in the short-finned whale ([Hofman, 1985].

The incongruity between our extraordinary cognitive abilities and our not-that-extraordinary brain size has been the major driving factor behind the idea that the human brain is an outlier, an exception to the rules that have applied to the evolution of all other animals and brains. A largely accepted alternative explanation for our cognitive superiority over other mammals has been our extraordinary brain size compared with our body size, that is, our large encephalization quotient [Jerison, 1973].

However, the notion that higher encephalization correlates with improved cognitive abilities has recently been disputed in favor of absolute numbers of cortical neurons and connections ([Roth and Dicke, 2005], or simply absolute brain size (Deaner et al., 2007. However, the former animals with a smaller brain are outranked by the latter in cognitive performance [Deaner et al., 2007].

NOT ALL BRAINS ARE MADE THE SAME: NEURONAL SCALING RULES

The cerebral cortex grows across rodent species as a power function of its number of neurons with a large exponent of 1.7 ([Herculano-Houzel et al., 2011], which means that a 10-fold increase in the number of cortical neurons in a rodent leads to a 50-fold increase in the size of the cerebral cortex. In insectivores, the exponent is 1.6, such that a 10-fold increase in the number of cortical neurons leads to a 40-fold larger cortex. In primates, in contrast, the cerebral cortex and cerebellum vary in size as almost linear functions of their numbers of neurons ([Herculano-Houzel et al., 2007]; [Gabi et al., 2010], which means that a 10-fold increase in the number of neurons in a primate cerebral cortex or cerebellum leads to a practically similar 10-fold increase in structure size, a scaling mechanism that is much more economical than in rodents and allows for a much larger number of neurons to be concentrated in a primate brain than in a rodent brain of similar size.

Comparing the human brain with other mammalian brains thus required first estimating the total numbers of neuronal and nonneuronal cells that compose these brains, which we did a few years ago (Azevedo et al., 2009). Remarkably, at an average of 86 billion neurons and 85 billion nonneuronal cells (Azevedo et al., 2009), the human brain has just as many neurons as would be expected of a generic primate brain of its size and the same overall 1:1 nonneuronal/neuronal ratio as other primates

HUMAN ADVANTAGE

COST OF BEING HUMAN

Growing a large body comes at a cost. Although large animals require less energy per unit of body weight, they have considerably larger total metabolic requirements that, on average, scale with body mass raised to an exponent of ~3/4 ([Kleiber, 1932]; [Schmidt-Nielsen, 1984]; [Martin, 1990]; [Bonner, 2006]. Thus, large mammals need to eat more, and they cannot concentrate on rare, hard-to-find, or catch foods

It has been proposed that the advent of the ability to control fire to cook foods, which increases enormously the energy yield of foods and the speed with which they are consumed ([Carmody and Wrangham, 2009]; [Carmody et al., 2011], may have been a crucial step in allowing the near doubling of numbers of brain neurons that is estimated to have occurred between H. erectus and H. sapiens [Wrangham, 2009]. The evolution of the human brain, with its high metabolic cost imposed by its large number of neurons, may thus only have been possible because of the use of fire to cook foods, enabling individuals to ingest in very little time the entire caloric requirement for the day, and thereby freeing time to use the added neurons to their competitive advantage

CONCLUSION: REMARKABLE, YET NOT EXTRAORDINARY

Researchers have been exploring the question for 3 decades, and the answer, it turns out, is pretty darn smart. In fact, according to panelist Lori Marino, an expert on cetacean neuroanatomy at Emory University in Atlanta, they may be Earth's second smartest creature (next to humans, of course).

Marino bases her argument on studies of the dolphin brain. Bottlenose dolphins have bigger brains than humans (1600 grams versus 1300 grams), and they have a brain-to-body-weight ratio greater than great apes do (but lower than humans). "They are the second most encephalized beings on the planet," says Marino

But it's not just size that matters. Dolphins also have a very complex neocortex, the part of the brain responsible for problem-solving, self-awareness, and variety of other traits we associate with human intelligence. And researchers have found gangly neurons called Von Economo neurons, which in humans and apes have been linked to emotions, social cognition, and even theory of mind—the ability to sense what others are thinking. Overall, said Marino, "dolphin brains stack up quite well to human brains."

Cognitive psychologist Diana Reiss of Hunter College of the City University of New York brought the audience up to speed on the latest on dolphin behavior. Reiss has been working with dolphins in aquariums for most of her life, and she says their social intelligence rivals that of the great apes. They can recognize themselves in a mirror (a feat most animals fail at—and a sign of self-awareness). They can understand complex gesture "sentences" from humans. And they can learn to poke an underwater keyboard to request toys to play with. "Much of their learning is similar to what we see with young children," says Reiss

Fun fact: There is an international Cephalopod Sequencing Consortium, which includes scientists from the University of Chicago; University of California, Berkeley; and Okinawa Institute of Science and Technology. By sequencing the genome of the California two-spot octopus (a.k.a. Octopus bimaculoides), they discovered that octopi possess brain-building genes called protocadherins, which were thought to exist only in vertebrates (things with spines, like humans or sentient carnivorous books). While octopi have the equivalent to eight spinal cords—one running down each arm—the cephalopods are clearly invertebrates and aren’t supposed to have this brain-building protein.

While humans have about 60 protocadherins, the octopus genome was found to have 168, nearly three times the neural wiring capacity than humans (who tend to be several times larger than octopi, except in our nightmares).

Octopi are demonstrably smart, and they stole all our best brain-genes, so why aren’t we visiting octopus cities on the ocean floor these days? It’s not because they don’t have a key evolutionary ability of humanity–the emergent ability to conceptualize and imagine scenarios–but because they don’t get enough time to use that ability. An octopus only lives three to five years; long enough to get their Bachelor’s degree in Literature, but not long enough to get hired to write articles for sassy websites.

Octopuses, cuttlefish and squid belong to a class of marine mollusks called cephalopods

As the cephalopod body evolved toward these modern forms—internalizing the shell or losing it altogether—another transformation occurred: some of the cephalopods became smart.

First of all, these animals evolved large nervous systems, including large brains. Large in what sense? A common octopus (Octopus vulgaris) has about 500 million neurons in its body. That is a lot by almost any standard. Human beings have many more—something nearing 100 billion—but the octopus is in the same range as various mammals, close to the range of dogs, and cephalopods have much larger nervous systems than all other invertebrates.

Vertebrate brains all have a common architecture. But when vertebrate brains are compared with octopus brains, all bets—or rather all mappings—are off. Octopuses have not even collected the majority of their neurons inside their brains; most of the neurons are in their arms.

The most famous octopus tales involve escape and thievery, in which roving aquarium octopuses raid neighboring tanks at night for food. Those stories—the basis for octopod hijinks in the 2016 Disney-Pixar film Finding Dory—are not especially indicative of high intelligence. Neighboring tanks are not so different from tide pools, even if the entrance and exit take more effort. But here is a behavior I find more intriguing: in at least two aquariums, octopuses have learned to turn off the lights by squirting jets of water at the bulbs and short-circuiting the power supply. At the University of Otago in New Zealand, this game became so expensive that the octopus had to be released back to the wild. BrainBook/Task/Material

octopuses have an ability to adapt to the special circumstances of captivity and to their interactions with human keepers. Anecdotally at least, it has long appeared that captive octopuses can recognize and behave differently toward individual human keepers. In the same lab in New Zealand that had the “lights-out” problem, an octopus took a dislike to one member of the staff, for no obvious reason. Whenever that person passed by on the walkway behind the tank, she received a half-gallon jet of water down the back of her neck.

in an octopus, the majority of neurons are in the arms themselves—nearly twice as many in total as in the central brain. The arms have their own sensors and controllers. They have not only the sense of touch but also the capacity to sense chemicals—to smell or taste. Each sucker on an octopus's arm may have 10,000 neurons to handle taste and touch. Even an arm that has been surgically removed can perform various basic motions, such as reaching and grasping.

Despite their many differences, cephalopods bear some striking similarities to vertebrates. For instance, vertebrates and cephalopods separately evolved “camera” eyes, with a lens that focuses an image on a retina. The capacity for learning of several kinds is also seen on both sides. Learning by attending to reward and punishment, by tracking what works and what does not work, seems to have been invented independently several times in evolution. If, on the other hand, it was present in the human/octopus common ancestor, it was greatly elaborated down each of the two lines.

There are also more subtle psychological similarities. Research indicates that octopuses, like us, seem to have a distinct short- and long-term memory. They seem to have something like sleep.

in an octopus, it is not clear where the brain itself begins and ends. The octopus is suffused with nervousness; the body is not a separate thing that is controlled by the brain or nervous system. The usual debate is between those who see the brain as an all-powerful CEO and those who emphasize the intelligence stored in the body itself. But the octopus lives outside both the usual pictures.

It has a body—but one that is protean, all possibility; it has none of the costs and gains of a constraining and action-guiding body. The octopus lives outside the usual body/brain divide. —

For decades, researchers have studied how certain animals evolved to be intelligent, among them apes, elephants, dolphins and even some birds, such as crows and parrots.

Cephalopods behave in ways that certainly suggest they’re highly intelligent. An octopus named Inky, for example, made a notorious escape recently from the National Aquarium of New Zealand, exiting his enclosure and slithering into a floor drain and, apparently, out to sea. BrainBook/Task/Material

Another feature that cephalopods share with other smart animals is a relatively big brain. But that’s where the similarities appear to end. Most of the neurons that do the computing, for example, are in the octopus’s arms.

losing their shells left cephalopods quite vulnerable to hungry predators. This threat may have driven cephalopods to become masters of disguise and escape. They did so by evolving big brains, the ability to solve new problems, and perhaps look into the future — knowing that coconut or clam shells may come in handy, for example.

Koalas have a ‘smooth’ brain.  This means that they lack higher-level recognition and understanding that many other animals have.

If you gather a bunch of Eucalyptus leaves, which the koalas eat, and put them on a plate in front of the koala, the koala won’t know what to do with them; they just sit there and gawk at it.

They lack the ability to discern that it’s still food given that the leaves have moved off the tree and onto a new source that they’re unfamiliar with.

Another fun biproduct of their smooth brain is that koalas don’t really seem to understand what rain is. They will just sit in that rain wondering why they get wet until the rain passes.

Elephant brain weighs 4-5kg, human only 1.2-1.5kg, whale up to 9kg, but we perform better cognitive function than them. What caused this?