Encephalization quotient

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Encephalization quotient (EQ), encephalization level (EL), or just encephalization is a relative

Perspective on intelligence measures

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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,

unbalanced opinion?

]

Currently the best predictor for intelligence across all animals is forebrain neuron count.^[5] 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^[6][7] 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It could have been anticipated that EQ might be superseded because of both the number of exceptions and the growing complexity of the formulae it used. (See the rest of this article.)^[

]

Variance in brain sizes

Body size accounts for 80–90% of the variance in brain size, between species, and 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.^[8] 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 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 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).^{[9][citation needed]}

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) 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.^[10][11]

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.^[12][13]

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 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.^[14][11]

According to Cairo (2011), EQ has flaws to its design when considering individual data points rather than 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 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.^[15]

The notion that encephalization quotient corresponds to intelligence has been disputed by Roth and Dicke (2012). They consider the absolute

Brain-body size relationship

Species	Simple brain-to-body ratio (E/S)[citation needed]
Treeshrew	1⁄10
Small birds	1⁄12
Human	1⁄40
Mouse	1⁄40
Dolphin	1⁄50
Cat	1⁄100
Chimpanzee	1⁄113
Dog	1⁄125
Frog	1⁄172
Lion	1⁄550
Elephant	1⁄560
Horse	1⁄600
Shark	1⁄2496
Hippopotamus	1⁄2789

Brain size usually increases with body size in animals (is

Several reasons for this trend are possible, one of which is that neural cells have a relative constant size.^[20] 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: ${\displaystyle C={{E} \over {S^{\frac {2}{3}}}}}$, where ${\displaystyle E}$ and ${\displaystyle S}$ are brain and body weights respectively, and ${\displaystyle C}$ is called the cephalization factor.^[21] 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.^[22]

The cephalization factor and the subsequent encephalization quotient was developed by H.J. Jerison in the late 1960s.^[23] The formula for the curve varies, but an empirical fitting of the formula to a sample of mammals gives ${\displaystyle {{Ew(brain)} \over {1g}}=0.12{\left({{w(body)} \over {1g}}\right)^{\frac {2}{3}}}}$.

Calculation

Snell's equation of simple allometry is:^[24]

${\displaystyle E=CS^{r}}$

Here ${\displaystyle E}$ is the weight of the brain, ${\displaystyle C}$ is the cephalization factor, ${\displaystyle S}$ is body weight and ${\displaystyle r}$ is the exponential constant.

The "encephalization quotient" (EQ) is the coefficient ${\displaystyle C}$ 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 the value for the cat, which is therefore attributed an EQ of 1.^[17]

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.^[11]

Species	EQ[4]
Human	7.4–7.8
Bottlenose dolphin	5.3
Chimpanzee	2.2–2.5
Raven[25]	2.49
Rhesus monkey	2.1
African elephant	1.3
Dog	1.2
Cat	1.0
Horse	0.9
Sheep	0.8
Mouse	0.5
Rat	0.4
Rabbit	0.4
Opossum	0.2

This measurement of approximate intelligence is more accurate for mammals than for other

.

EQ and intelligence in mammals

Intelligence in animals is hard to establish, but 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 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.^[15]

Grey floor

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 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 is necessary for basic functions. Presumably these extra neurons are used for higher cognitive processes.^[26]

Taxonomic trends

Mean EQ for mammals is around 1, with

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 the cognitive capacity required for effectively hunting prey.[28]^[29] 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.^[30]

Dietary trends

Of the animals demonstrating the highest EQ's (see associated table), many are primarily

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 the network of food chains can only develop a high RB (relative brain-mass) so long as they have small body masses.^[32] 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.^[32]

Sociality

Another factor previously thought to have great impact on brain size is sociality and flock size.

Comparisons with non-mammalian animals

Birds generally have lower EQ than mammals, but

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 the absolute amount of brain an animal has after subtracting the weight of the spinal cord from the brain.^[41] This formula is useless for invertebrates because they do not have spinal cords or, in some cases, central nervous systems.

EQ in paleoneurology

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

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 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.

Researchers obtained an EQ of 4.150 for 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.^[43] For example, endocasts do not provide any information regarding 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 the same encephalization quotient as modern humans, although their post-crania suggests that they weighed more than modern humans.^[44] 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.

EQ of livestock animals

The EQ of livestock farm animals such as the

History

The concept of encephalization has been a key evolutionary trend throughout human evolution, and consequently an important area of study. Over the course of hominin evolution, brain size has seen an overall increase from 400 cm³ to 1400 cm³.[42] Furthermore, the genus Homo is specifically defined by a significant increase in brain size.^[43] 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 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.^[43] 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 is considered a pseudoscience.^[48]

Among ancient Greek philosophers,

Due to

See also

• Brain–body mass ratio
• Brain development timelines
• Cephalization
• Cranial capacity
• Craniometry
• Evolutionary biology
• Human brain
• Human evolution
• Neuroscience and intelligence
• Phrenology
• Evolutionary neuroscience

References

Bibliography

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• Russon, Ann E.; Begun, David R., eds. (2004). The Evolution of Thought: Evolutionary Origins of Great Ape Intelligence. Cambridge: Cambridge University Press.
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• Tobias P.V. (1971). The Brain in Hominid Evolution. New York and London: Columbia University Press.
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• Van Schaik, Carel (April 2006). "Why Are Some Animals So Smart?". Scientific American. 294 (4): 64–71.
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• Boddy, AM; McGowen, MR; Sherwood, CC; Grossman, LI; Goodman, M; Wildman, DE (May 2012). "Comparative analysis of encephalization in mammals reveals relaxed constraints on anthropoid primate and cetacean brain scaling". Journal of Evolutionary Biology. 25 (5): 981–94.
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• Dunbar R.I. (2007). "Evolution in the social brain". Science. 317 (5843). science magazine: 1344–1347.
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• Decasien, A.R. (2017). "Primate brain size is predicted by diet but not sociality". Nature. 1 (5): 112.
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• Cairo O. (2011). "External measures of cognition". Frontiers in Human Neuroscience. 5: 108.
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• Herculano-Houzel S. (2017). The Human Advantage: How our brains became remarkable. Cambridge, MA: MIT Press.
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External links

• "mawint1". Archived from the original on 4 January 2011.
• "A graph of body mass vs. brain mass". brainmuseum.org.
• Gould, Stephen Jay. "Bligh's Bounty". monash.edu.au. Archived from the original on 9 July 2001.
• "Encephalization quotients, Kleiber's Law, and statistical methods".
• Herculano-Houzel, Suzana (2013). What is so special about the human brain (video). TED Talk.