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63-436: Encephalization quotient was developed in an attempt to provide a way of correlating an animal's physical characteristics with perceived intelligence. It improved on the previous attempt, brain-to-body mass ratio , so it has persisted. Subsequent work, notably Roth, found EQ to be flawed and suggested brain size was a better predictor, but that has problems as well. Currently the best predictor for intelligence across all animals

126-403: A complex system leads to the phenomenon of intelligence. Neurons are the cells that transmit information in an animal 's nervous system so that it can sense stimuli from its environment and behave accordingly. Not all animals have neurons; Trichoplax and sponges lack nerve cells altogether. Neurons may be packed to form structures such as the brain of vertebrates or

189-520: A constant that depends on animal family (but close to 2/3 in many vertebrates ), and C is the cephalization factor. It has been argued that the animal's ecological niche , rather than its evolutionary family, is the main determinant of its encephalization factor C . 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

252-419: A higher level of encephalization equated to a higher ability to process information. A larger brain could mean a number of different things, including a larger cerebral cortex, a greater number of neuronal associations, or a greater number of neurons overall. Brain-to-body mass ratio Brain–body mass ratio , also known as the brain–body weight ratio , is the ratio of brain mass to body mass, which

315-739: A lower raw brain-to-body weight ratio. Mean EQs for reptiles are about one tenth of those of mammals. EQ in birds (and estimated EQ in other dinosaurs) generally also falls below that of mammals, possibly due to lower thermoregulation and/or motor control demands. Estimation of brain size in Archaeopteryx (one of the oldest known ancestors of birds), shows it had an EQ well above the reptilian range, and just below that of living birds. Biologist Stephen Jay Gould has noted that if one looks at vertebrates with very low encephalization quotients, their brains are slightly less massive than their spinal cords. Theoretically, intelligence might correlate with

378-438: A mirror to find food, show evidence of self-recognition when presented with their reflections and there is evidence suggesting that pigs are as socially complex as many other highly intelligent animals, possibly sharing a number of cognitive capacities related to social complexity. The concept of encephalization has been a key evolutionary trend throughout human evolution, and consequently an important area of study. Over

441-417: A much smaller one. Differing methods have been used to count neurons, and these may differ in degree of reliability. The primary methods are the optical fractionator, an application of stereology and the isotropic fractionator, a recent methodological innovation. Most numbers in the list are the result of studies using the newer isotropic fractionator. A variation of the optical fractionator

504-411: A negative reputation. However, with the advent of imaging techniques such as the fMRI and PET scan , several scientific studies were launched to suggest a relationship between encephalization and advanced cognitive abilities. Harry J. Jerison, who invented the formula for encephalization quotient, believed that brain size was proportional to the ability of humans to process information. With this belief,

567-465: A ranking of animals that better coincides with the observed complexity of animal behaviour. 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,

630-427: A relationship described by an allometric equation: the regression of the logarithms of brain size on body size. The distance of a species from the regression line is a measure of its encephalization. The scales are logarithmic, distance, or residual, is an encephalization quotient (EQ), the ratio of actual brain size to expected brain size. Encephalization is a characteristic of a species. Rules for brain size relates to

693-750: A relative constant size. Some brain functions, like the brain pathway responsible for a basic task like drawing breath, are basically similar in a mouse and an elephant. Thus, the same amount of brain matter can govern breathing in a large or a small body. While not all control functions are independent of body size, some are, and hence large animals need comparatively less brain than small animals. This phenomenon can be described by an equation C = E / S 2 / 3 , {\displaystyle C=E/S^{2/3},} where E {\displaystyle E} and S {\displaystyle S} are brain and body weights respectively, and C {\displaystyle C}

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756-420: A relatively large brain, giving a brain-to-body mass ratio similar to humans. One explanation could be that as an animal's brain gets larger, the size of the neural cells remains the same, and more nerve cells will cause the brain to increase in size to a lesser degree than the rest of the body. This phenomenon can be described by an equation of the form E = CS , where E and S are brain and body weights, r

819-422: A smaller brain. Both bird intelligence and brain anatomy are however very different from those of mammals, making direct comparison difficult. Manta rays have the highest EQ among fish , and either octopuses or jumping spiders have the highest among invertebrates . Despite the jumping spider having a huge brain for its size, it is minuscule in absolute terms, and humans have a much higher EQ despite having

882-411: A species as a whole. It is inherently biased given that the cranial volume of an obese and underweight individual would be roughly similar, but their body masses would be drastically different. Another difference of this nature is a lack of accounting for sexual dimorphism. For example, the female human generally has smaller cranial volume than the male; however, this does not mean that a female and male of

945-408: Is forebrain neuron count. 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 found a more reliable method of counting brain cells. It could have been anticipated that EQ might be superseded because of both the number of exceptions and the growing complexity of

1008-430: Is based on data from mammals, it should be applied to other animals with caution. For some of the other vertebrate classes the power of 3/4 rather than 2/3 is sometimes used, and for many groups of invertebrates the formula may give no meaningful results at all. Snell's equation of simple allometry is were E {\displaystyle E} is the weight of the brain, C {\displaystyle C}

1071-413: Is called the cephalization factor. To determine the value of this factor, the brain and body weights of various mammals were plotted against each other, and the curve of such formula chosen as the best fit to that data. The cephalization factor and the subsequent encephalization quotient was developed by H. J. Jerison in the late 1960s. The formula for the curve varies, but an empirical fitting of

1134-715: Is considered a pseudoscience . Among ancient Greek philosophers, Aristotle in particular believed that after the heart, the brain was the second most important organ of the body. He also focused on the size of the human brain, writing in 335 BCE that "of all the animals, man has the brain largest in proportion to his size." In 1861, French neurologist Paul Broca tried to make a connection between brain size and intelligence. Through observational studies, he noticed that people working in what he deemed to be more complex fields had larger brains than people working in less complex fields. Also, in 1871, Charles Darwin wrote in his book The Descent of Man : "No one, I presume, doubts that

1197-449: 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. List of animals by number of neurons#Forebrain (cerebrum or pallium) The following are two lists of animals ordered by the size of their nervous system . The first list shows number of neurons in their entire nervous system. The second list shows

1260-439: Is extremely costly in terms of energy needed to sustain it. Animals with nutrient rich diets tend to have higher EQs, which is necessary for the energetically costly tissue of brain matter. Not only is it metabolically demanding to grow throughout embryonic and postnatal development, it is costly to maintain as well. Arguments have been made that some carnivores may have higher EQ's due to their relatively enriched diets, as well as

1323-582: 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. Brain size usually increases with body size in animals (i.e. large animals usually have larger brains than smaller animals);

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1386-404: Is necessary for basic functions. Presumably these extra neurons are used for higher cognitive processes. Mean EQ for mammals is around 1, with carnivorans , cetaceans and primates above 1, and insectivores and herbivores below. Large mammals tend to have the highest EQs of all animals, while small mammals and avians have similar EQs. This reflects two major trends. One is that brain matter

1449-414: Is not linear, however. Generally, small mammals have relatively larger brains than big ones. Mice have a direct brain/body size ratio similar to humans (1/40), while elephants have a comparatively small brain/body size (1/560), despite being quite intelligent animals. Treeshrews have a brain/body mass ratio of (1/10). Several reasons for this trend are possible, one of which is that neural cells have

1512-489: Is only limited to when there are both cranial and post-cranial remains associated with individual fossils, to allow for brain to body size comparisons. For example, remains of one Middle Pleistocene human fossil from Jinniushan province in northern China has allowed scientists to study the relationship between brain and body size using the Encephalization Quotient. Researchers obtained an EQ of 4.150 for

1575-494: Is significant to inferring the pressures which drive higher EQ's. Specifically, frugivores must utilize a complex, trichromatic map of visual space to locate and pick ripe fruits and are able to provide for the high energetic demands of increased brain mass. Trophic level —"height" on the food chain —is yet another factor that has been correlated with EQ in mammals. Eutheria with either high AB (absolute brain-mass) or high EQ occupy positions at high trophic levels. Eutheria low on

1638-508: Is sociality and flock size. This was a long-standing theory until the correlation between frugivory and EQ was shown to be more statistically significant. While no longer the predominant inference as to selection pressure for high EQ, the social brain hypothesis still has some support. For example, dogs (a social species) have a higher EQ than cats (a mostly solitary species). Animals with very large flock size and/or complex social systems consistently score high EQ, with dolphins and orcas having

1701-438: Is swelling of the brain which, while resulting in greater surface area, does not alter the intelligence of those suffering from it. The relationship between brain weight and body weight of all living vertebrates follows two completely separate linear functions for cold-blooded and warm-blooded animals. Cold-blooded vertebrates have much smaller brains than warm-blooded vertebrates of the same size. However, if brain metabolism

1764-403: Is taken into account, the brain-to-body relationship of both warm and cold-blooded vertebrates becomes similar, with most using between 2 and 8 percent of their basal metabolism for the brain and spinal cord. Dolphins have the highest brain-to-body weight ratio of all cetaceans . Monitor lizards , tegus and anoles and some tortoise species have the largest among reptiles. Among birds,

1827-487: Is that smaller animals tend to have a higher proportional brain mass, but do not show the same indications of higher cognition as animals with a high EQ. The driving theorization behind the development of EQ is that an animal of a certain size requires a minimum number of neurons for basic functioning, sometimes referred to as a grey floor. There is also a limit to how large an animal's brain can grow given its body size – due to limitations like gestation period, energetics, and

1890-491: Is the cephalization factor, S {\displaystyle S} is body weight, and r {\displaystyle r} is the exponential constant. The "encephalization quotient" (EQ) is the coefficient C {\displaystyle C} in Snell's allometry equation, usually normalized with respect to a reference species. In the following table, the coefficients have been normalized with respect to

1953-504: The domestic pig may be significantly lower than would suggest for their apparent intelligence. According to Minervini et al (2016) the brain of the domestic pig is a rather small size compared to the mass of the animal. The tremendous increase in body weight imposed by industrial farming significantly influences brain-to-body weight measures, including the EQ. The EQ of the domestic adult pig is just 0.38, yet pigs can use visual information seen in

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2016-429: The endocast of the brain cavity and estimated body weight of an animal is all one has to work from. The behavior of extinct mammals and dinosaurs is typically investigated using EQ formulas. Encephalization quotient is also used in estimating evolution of intelligent behavior in human ancestors. This technique can help in mapping the development of behavioral complexities during human evolution. However, this technique

2079-407: The neural ganglions of insects . The number of neurons and their relative abundance in different parts of the brain is a determinant of neural function and, consequently, of behavior. All numbers for neurons (except Caenorhabditis and Ciona), and all numbers for synapses (except Ciona) are estimations. Proxies for animal intelligence have varied over the centuries. One early suggestion

2142-629: The Jinniushan fossil, and then compared this value with preceding Middle Pleistocene estimates of EQ at 3.7770. The difference in EQ estimates has been associated with a rapid increase in encephalization in Middle Pleistocene hominins. Paleo-neurological comparisons between Neanderthals and anatomically modern Homo sapiens (AMHS) via Encephalization quotient often rely on the use of endocasts, but this method has many drawbacks. For example, endocasts do not provide any information regarding

2205-437: 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. Behavioral complexity in living animals can to some degree be observed directly, making the predictive power of the encephalization quotient less relevant. It is however central in paleoneurology , where

2268-400: 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

2331-428: The authors, these inconsistencies were the result of the faulty assumption that N c 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 N c . According to Cairo (2011), EQ has flaws to its design when considering individual data points rather than

2394-421: The cognitive capacity required for effectively hunting prey. One example of this is brain size of a wolf ; about 30% larger than a similarly sized domestic dog, potentially derivative of different needs in their respective way of life. Of the animals demonstrating the highest EQ's (see associated table), many are primarily frugivores , including apes , macaques , and proboscideans . This dietary categorization

2457-445: The course of hominin evolution, brain size has seen an overall increase from 400 cm to 1400 cm. Furthermore, the genus Homo is specifically defined by a significant increase in brain size. The earliest Homo species were larger in brain size as compared to contemporary Australopithecus counterparts, with which they co-inhabited parts of Eastern and Southern Africa. Throughout modern history, humans have been fascinated by

2520-482: The electrogenic elephantfish has a ratio nearly 80 times higher—about 1/32, which is slightly higher than that for humans). Treeshrews have a higher brain to body mass ratio than any other mammal, including humans . Treeshrews hold about 10% of their body mass in their brain. Generally speaking, the larger the animal, the smaller the brain-to-body mass ratio is. Thus, large whales have very small brains compared to their weight, and small rodents like mice have

2583-399: The formula to a sample of mammals gives w ( brain ) 1   g = 0.12 ( w ( body ) 1   g ) 2 3 . {\displaystyle {\frac {w({\text{brain}})}{1~{\text{g}}}}=0.12\left({\frac {w({\text{body}})}{1~{\text{g}}}}\right)^{\frac {2}{3}}.} As this formula

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2646-404: The formulae it used. (See the rest of this article.) The simplicity of counting neurons has replaced it. The concept in EQ of comparing the brain capacity exceeding that required for body sense and motor activity may yet live on to provide an even better prediction of intelligence, but that work has not been done yet. Body size accounts for 80–90% of the variance in brain size, between species, and

2709-559: The highest EQ of all cetaceans , and humans with their extremely large societies and complex social life topping the list by a good margin. Birds generally have lower EQ than mammals, but parrots and particularly the corvids show remarkable complex behaviour and high learning ability. Their brains are at the high end of the bird spectrum, but low compared to mammals. Bird cell size is on the other hand generally smaller than that of mammals, which may mean more brain cells and hence synapses per volume, allowing for more complex behaviour from

2772-427: The highest brain-to-body ratios are found among parrots , crows , magpies , jays and ravens . Among amphibians, the studies are still limited. Either octopuses or jumping spiders have some of the highest for an invertebrate , although some ant species have 14–15% of their mass in their brains, the highest value known for any animal. Sharks have one of the highest for fish alongside manta rays (although

2835-505: The human brain and non-primate brains, larger or smaller, might simply be inadequate and uninformative – and our view of the human brain as outlier, a special oddity, may have been based on the mistaken assumption that all brains are made the same (Herculano-Houzel, 2012). 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

2898-404: The internal organization of the brain. Furthermore, endocasts are often unclear in terms of the preservation of their boundaries, and it becomes hard to measure where exactly a certain structure starts and ends. If endocasts themselves are not reliable, then the value for brain size used to calculate the EQ could also be unreliable. Additionally, previous studies have suggested that Neanderthals have

2961-509: The large proportion which the size of man's brain bears to his body, compared to the same proportion in the gorilla or orang, is closely connected with his mental powers." The concept of quantifying encephalization is also not a recent phenomenon. In 1889, Sir Francis Galton , through a study on college students, attempted to quantify the relationship between brain size and intelligence. Due to Hitler's racial policies during World War II , studies on brain size and intelligence temporarily gained

3024-585: The large relative size of our brains, trying to connect brain sizes to overall levels of intelligence. Early brain studies were focused in the field of phrenology, which was pioneered by Franz Joseph Gall in 1796 and remained a prevalent discipline throughout the early 19th century. Specifically, phrenologists paid attention to the external morphology of the skull, trying to relate certain lumps to corresponding aspects of personality. They further measured physical brain size in order to equate larger brain sizes to greater levels of intelligence. Today, however, phrenology

3087-404: The larger the brain is relative to the body, the more brain weight might be available for more complex cognitive tasks. The EQ formula, as opposed to the method of simply measuring raw brain weight or brain weight to body weight, makes for a ranking of animals that coincides better with observed complexity of behaviour. A primary reason for the use of EQ instead of a simple brain to body mass ratio

3150-422: The need to physically support the encephalized region throughout maturation. When normalizing a standard brain size for a group of animals, a slope can be determined to show what a species' expected brain to body mass ratio would be. Species with brain to body mass ratios below this standard are nearing the grey floor, and do not need extra grey matter. Species which fall above this standard have more grey matter than

3213-806: The network of food chains can only develop a high RB (relative brain-mass) so long as they have small body masses. This presents an interesting conundrum for intelligent small animals, who have behaviors radically different from intelligent large animals. According to Steinhausen et al .(2016): Animals with high RB [relative brain-mass] usually have (1) a short life span, (2) reach sexual maturity early, and (3) have short and frequent gestations. Moreover, males of species with high RB also have few potential sexual partners. In contrast, animals with high EQs have (1) a high number of potential sexual partners, (2) delayed sexual maturity, and (3) rare gestations with small litter sizes. Another factor previously thought to have great impact on brain size

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3276-436: 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. Size comparison between

3339-414: The number of neurons in the structure that has been found to be representative of animal intelligence. The human brain contains 86 billion neurons, with 16 billion neurons in the cerebral cortex . Neuron counts constitute an important source of insight on the topic of neuroscience and intelligence : the question of how the evolution of a set of components and parameters (~10 neurons, ~10 synapses) of

3402-401: The overall weight of the brain. Jerison (1973) has for this reason 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

3465-733: 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 absolute brain size (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 N c (the number of cortical neurons) correlated fairly well with cognitive abilities. However, there were inconsistencies found for N c . According to

3528-469: The relationship is not, however, linear. Small mammals such as mice may have a brain/body ratio similar to humans, while elephants have a comparatively lower brain/body ratio. In animals, it is thought that the larger the brain, the more brain weight will be available for more complex cognitive tasks. However, large animals need more neurons to represent their own bodies and control specific muscles; thus, relative rather than absolute brain size makes for

3591-553: The same body mass would have different cognitive abilities. Considering all of these flaws, EQ should not be viewed as a valid metric for intraspecies comparison. 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. According to Roth and Dicke (2012), mammals with relatively high cortex volume and neuron packing density (NPD) are more intelligent than mammals with

3654-625: 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 . Roth and Dicke consider syntactical language an "intelligence amplifier". Brain size usually increases with body size in animals (is positively correlated ), i.e. large animals usually have larger brains than smaller animals. The relationship

3717-423: The same encephalization quotient as modern humans, although their post-crania suggests that they weighed more than modern humans. Because EQ relies on values from both postcrania and crania, the margin for error increases in relying on this proxy in paleo-neurology because of the inherent difficulty in obtaining accurate brain and body mass measurements from the fossil record. The EQ of livestock farm animals such as

3780-467: 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

3843-408: The value for the cat, which is therefore attributed an EQ of 1. Another way to calculate encephalization quotient is by dividing the actual weight of an animal's brain with its predicted weight according to Jerison's formula. This measurement of approximate intelligence is more accurate for mammals than for other classes and phyla of Animalia . Intelligence in animals is hard to establish, but

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3906-531: Was brain size (or weight, which provides the same ordering.) A second proposal was brain-to-body-mass ratio , and a third was encephalization quotient , sometimes referred to as EQ. The current best predictor is number of neurons in the forebrain, based on Herculano-Houzel's improved neuron counts. This accounts for variation in the number of neurons in the rest of the brain, for which no link to intelligence has been established. Elephants, for example, have an exceptionally large cerebellum, while birds make do with

3969-446: Was responsible for the previous total human brain neuron count of 100,000,000,000 neurons, which has been revised down to 86,000,000,000 by the use of the isotropic fractionator. This is in part why it may be considered to be less reliable. Finally, some numbers are the result of estimations based on correlations observed between number of cortical neurons and brain mass within closely related taxa. The following table gives information on

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