View source for Encephalization quotient

{{Short description|Relative brain size measure}}
{{distinguish|cephalic index}}
{{cleanup|reason=multiple unresolved issues as per talk page|date=April 2016}}
{{Use dmy dates|date=November 2019}}
<!-- |-
|colspan="2" style="text-align: left;" |EQ relative to the cat as standard species: EQ(cat)=1 -->
'''Encephalization quotient''' ('''EQ'''), '''encephalization level''' ('''EL'''), or just '''encephalization''' is a relative [[brain size]] measure that is defined as the ratio between observed and predicted brain mass for an animal of a given size, based on [[nonlinear regression]] on a range of reference species.<ref>{{cite book|last1=Pontarotti|first1=Pierre|title=Evolutionary Biology: Convergent Evolution, Evolution of Complex Traits|date=2016|publisher=Springer|isbn=978-3-319-41324-2|pages=74}}</ref><ref>{{cite web |title=Natural Sciences 102: Lecture Notes: Emergence of Intelligence |author=G.Rieke |url=http://ircamera.as.arizona.edu/NatSci102/NatSci102/lectures/intelligence.htm |access-date=12 February 2011 |archive-date=7 April 2019 |archive-url=https://web.archive.org/web/20190407181204/http://ircamera.as.arizona.edu/NatSci102/NatSci102/lectures/intelligence.htm |url-status=dead }}</ref> It has been used as a proxy for [[intelligence]] and thus as a possible way of comparing the intelligence levels of different [[species]]. For this purpose, it is a more refined measurement than the raw [[brain-to-body mass ratio]], as it takes into account [[allometry|allometric]] effects. Expressed as a formula, the relationship has been developed for [[mammals]] and may not yield relevant results when applied outside this group.<ref name=Moore>{{cite web |author=Moore, J. |year=1999 |url=http://weber.ucsd.edu/~jmoore/courses/allometry/allometry.html |title=Allometry |publisher=[[University of California]], San Diego |access-date=5 January 2011 |archive-date=27 March 2019 |archive-url=https://web.archive.org/web/20190327101353/http://pages.ucsd.edu/~jmoore/courses/allometry/allometry.html |url-status=dead }}</ref>

== Perspective on intelligence measures ==
{{More citations needed section|date=May 2020}}
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,<ref name="Roth2005">{{cite journal|last1=Roth|first1=Gerhard|last2=Dicke|first2=Ursula|date=May 2005|title=Evolution of the brain and intelligence|journal=Trends in Cognitive Sciences|volume=9|issue=5|pages=250–7|doi=10.1016/j.tics.2005.03.005|pmid=15866152|s2cid=14758763}}</ref> found EQ to be flawed and suggested brain size was a better predictor, but that has problems as well.{{Unbalanced opinion|date=May 2020}}

Currently the best predictor for intelligence across all animals is [[forebrain]] neuron count.<ref name=houzel2017>{{cite journal |last1=Herculano-Houzel |first1=Suzana |title=Numbers of neurons as biological correlates of cognitive capability |journal=Current Opinion in Behavioral Sciences |year=2017 |volume=16 |pages=1–7 | doi=10.1016/j.cobeha.2017.02.004 |s2cid=53172110 }}</ref> 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 [[Suzana Herculano-Houzel|Herculano-Houzel]]<ref name=houzel2009>{{cite journal |last1=Herculano-Houzel |first1=Suzana |title=The human brain in numbers: a linearly scaled-up primate brain |journal=Frontiers in Human Neuroscience |year=2009 |volume=3 |pages=31 |doi=10.3389/neuro.09.031.2009 |pmc=2776484 |pmid=19915731 |doi-access=free }}</ref><ref>{{cite book |last1=Herculano-Houzel |first1=Suzana |title=The Human Advantage: How our brains became remarkable |year=2017 |publisher=MIT Press|isbn=978-0-262-53353-9}}{{pn|date=February 2020}}</ref> 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.){{Unbalanced opinion|date=May 2020}} The simplicity of counting neurons has replaced it.{{citation needed|date=May 2020}} 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.{{fact|date=February 2020}}{{Unbalanced opinion|date=May 2020}}

=== 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.<ref name=Finlay2009/> 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).<ref name=Herculano2012/>{{citation needed |reason=Entire paragraph is full of spelling/grammatical errors. Source material should be referenced to obtain the correct verbiage. |date=December 2019}}

=== 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.<ref>Jerison, H.J., 1973. Evolution of the brain and [[intelligence]] Academic Press.[HTE].{{pn|date=February 2020}}</ref><ref name="roth">{{cite book |doi=10.1016/B978-0-444-53860-4.00020-9 |pmid=22230639 |isbn=9780444538604 |chapter=Evolution of the brain and intelligence in primates |title=Evolution of the Primate Brain |series=Progress in Brain Research |year=2012 |last1=Roth |first1=Gerhard |last2=Dicke |first2=Ursula |volume=195 |pages=413–430 }}</ref>

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 [[correlation|positively correlated]] to intelligence in humans.<ref>{{cite journal |last1=Haier |first1=Richard J. |last2=Jung |first2=Rex E. |last3=Yeo |first3=Ronald A. |last4=Head |first4=Kevin |last5=Alkire |first5=Michael T. |title=Structural brain variation and general intelligence |journal=NeuroImage |date=September 2004 |volume=23 |issue=1 |pages=425–433 |doi=10.1016/j.neuroimage.2004.04.025 |pmid=15325390 |s2cid=29426973 }}</ref><ref>{{cite journal |last1=Gregory |first1=Michael D. |last2=Kippenhan |first2=J. Shane |last3=Dickinson |first3=Dwight |last4=Carrasco |first4=Jessica |last5=Mattay |first5=Venkata S. |last6=Weinberger |first6=Daniel R. |last7=Berman |first7=Karen F. |title=Regional Variations in Brain Gyrification Are Associated with General Cognitive Ability in Humans |journal=Current Biology |date=May 2016 |volume=26 |issue=10 |pages=1301–1305 |doi=10.1016/j.cub.2016.03.021 |pmid=27133866 |pmc=4879055 |bibcode=2016CBio...26.1301G }}</ref>

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<small>c</small> (the number of cortical neurons) correlated fairly well with cognitive abilities. However, there were inconsistencies found for N<small>c</small>. According to the authors, these inconsistencies were the result of the faulty assumption that N<small>c</small> increases linearly with the size of the cortical surface. This notion is incorrect because the assumption does not take into account the variability in [[Cerebral cortex#Thickness|cortical thickness]] and cortical neuron density, which should influence N<small>c</small>.<ref>{{cite journal |title=Overall Brain Size, and Not Encephalization Quotient, Best Predicts Cognitive Ability across Non-Human Primates |journal=Brain Behav Evol |year=2007 |volume=70 |issue=2 |pages=115–124 |doi=10.1159/000102973 |pmid=17510549 |last1=Deaner |first1=Robert O. |last2=Isler |first2=Karin |last3=Burkart |first3=Judith |last4=Van Schaik |first4=Carel |citeseerx=10.1.1.570.7146 |s2cid=17107712 }}</ref><ref name="roth"/>

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.<ref name="Cairo O. 2011 108"/>

The notion that encephalization quotient corresponds to intelligence has been disputed by Roth and Dicke (2012). They consider the absolute [[List of animals by number of neurons#Forebrain (cerebrum or pallium)|number of cortical neurons]] and [[Neural circuit|neural connection]]s as better correlates of cognitive ability.<ref>{{cite book |doi=10.1016/B978-0-12-374236-0.10001-X |isbn=9780123742360 |chapter=Brain Evolution |title=The Human Nervous System |year=2012 |last1=Herculano-Houzel |first1=Suzana |pages=2–13 }}</ref> 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, [[Nerve conduction velocity|conduction velocity]], and [[Connectome#Primary challenge for macroscale connectomics%3A determining parcellations of the brain|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|syntactical language]]. Roth and Dicke consider syntactical language an "intelligence amplifier".<ref name="roth"/>

== Brain-body size relationship ==
{| class="wikitable sortable" style="float: right; clear: right; margin-left:1em;"
!Species
!Simple brain-to-body <br/>ratio (''E''/''S''){{citation needed|date=February 2020}}
|-
|[[Treeshrew]] || 1/10
|-
|Small [[birds]] || 1/12
|-
|[[Human]] || 1/40
|-
|[[Mouse]] || 1/40
|-
|[[Dolphin]] || 1/50
|-
|[[Cat]] || 1/100
|-
|[[Chimpanzee]] || 1/113{{Undue precision}}
|-
|[[Dog]] || 1/125{{Undue precision}}
|-
|[[Frog]] || 1/172{{Undue precision}}
|-
|[[Lion]] || 1/550
|-
|[[Elephant]] || 1/560
|-
|[[Horse]] || 1/600
|-
|[[Shark]] || 1/2496{{Undue precision}}
|-
|[[Hippopotamus]] || 1/2789{{Undue precision}}
|}
Brain size usually increases with body size in animals (is [[correlation|positively correlated]]), i.e. large animals usually have larger brains than smaller animals.<ref name=Serendip /> The relationship 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 [[elephant]]s have a comparatively small brain/body size (1/560), despite being quite intelligent animals.<ref>{{cite journal |last1=Hart |first1=Benjamin L. |last2=Hart |first2=Lynette A. |last3=McCoy |first3=Michael |last4=Sarath |first4=C.R. |title=Cognitive behaviour in Asian elephants: use and modification of branches for fly switching |journal=Animal Behaviour |date=November 2001 |volume=62 |issue=5 |pages=839–847 |doi=10.1006/anbe.2001.1815 |s2cid=53184282 }}</ref> [[Treeshrew|Treeshrews]] have a brain/body mass ratio of (1/10).<ref>{{Cite web |last=Feltman |first=Rachel |date=2018-03-15 |title=What does brain size have to do with intelligence? |url=https://www.popsci.com/what-does-brain-size-have-to-do-with-intelligence/ |access-date=2024-02-28 |website=Popular Science |language=en-US}}</ref>

Several reasons for this trend are possible, one of which is that [[neuron|neural cells]] have a relative constant size.<ref>{{cite book |last1=Oxnard |first1=C. |last2=Cartmill |first2=M. |last3=Brown |first3=K.B. |title=The human body: Developmental, functional and evolutionary bases |year=2008 |publisher=Wiley |location=Hoboken, NJ |isbn=978-0471235996 |page=274 |url=https://books.google.com/books?id=6W7gCAAAQBAJ&pg=PA274}}</ref> 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 <math>C = E/S^{2/3},</math> 
where <math>E</math> and <math>S</math> are brain and body weights respectively, and <math>C</math> is called the cephalization factor.<ref name="Gould 1977 Ever since Darwin, c7s1"/> 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.<ref name=Jerison>{{cite book |title=Advances in the Study of Mammalian Behavior |url=https://archive.org/details/advancesinstudyo00eise |year=1983 |series=Special Publication |publisher=[[American Society of Mammalogists]] |volume=7 |location=Pittsburgh, PA |author=Jerison, H. J. |author-link=The evolution of the mammalian brain as an information-processing system |editor1=Eisenberg, J. F. |editor2=Kleiman, D. G. |pages=[https://archive.org/details/advancesinstudyo00eise/page/113 113]–146}}</ref>

The cephalization factor and the subsequent encephalization quotient was developed by H.&nbsp;J.&nbsp;Jerison in the late 1960s.<ref name=Dinosaur_Paleoneurology>{{cite book |editor1-last=Brett-Surman |editor1-first=Michael K. |editor2-first=Thomas R. |editor2-last=Holtz |editor3-first=James O. |editor3-last=Farlow |others=Illustrated by Bob Walters |title=The complete dinosaur |publisher=Indiana University Press |location=Bloomington, Ind. |isbn=978-0-253-00849-7 |pages=191–208 |edition=2nd |date=27 June 2012 }}</ref> The formula for the curve varies, but an empirical fitting of the formula to a sample of mammals gives<ref name=Moore/>
<math display="block">
 \frac{w(\text{brain})}{1~\text{g}} = 0.12 \left(\frac{w(\text{body})}{1~\text{g}}\right)^{\frac{2}{3}}.
</math>
As this formula 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.<ref name=Moore/>

==Calculation==
Snell's equation of simple [[allometry]] is<ref>{{Cite journal |last=Williams |first=M. F. |date=2002-04-01 |title=Primate encephalization and intelligence |url=https://www.sciencedirect.com/science/article/pii/S0306987701915166 |journal=Medical Hypotheses |volume=58 |issue=4 |pages=284–290 |doi=10.1054/mehy.2001.1516 |pmid=12027521 |issn=0306-9877}}</ref>
:<math>E = CS^r,</math>
were <math>E</math> is the weight of the brain, <math>C</math> is the [[cephalization]] factor, <math>S</math> is body weight, and <math>r</math> is the exponential constant.

The "encephalization quotient" (EQ) is the coefficient <math>C</math> 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.<ref name=Serendip>{{Cite web |title=Thinking about Brain Size |url=https://serendipstudio.org/bb/kinser/Int3.html |access-date=2020-11-23}}</ref>

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.<ref name="roth"/>

{| class="wikitable sortable"
! Species
! EQ<ref name="Roth2005"/>
|-
|[[Human]]
|align="center"|7.4&ndash;7.8
|-
|[[Northern right whale dolphin]]<ref>{{cite journal |last=Osvaldo |first=Cairo |title=External Measures of Cognition |journal=Frontiers in Human Neuroscience |volume=5 |issue=108 |date=2011-10-04 |page=108 |doi= 10.3389/fnhum.2011.00108 |pmid=22065955 |pmc=3207484 |quote=Table 1 |doi-access=free }}</ref>
|align="center"|5.55
|-
|[[Bottlenose dolphin]]
|align="center"|5.3
|-
|[[Chimpanzee]]
|align="center"|2.2&ndash;2.5
|-
|[[Raven]]<ref name=pmid16553307>{{cite journal |last1=Emery |first1=Nathan J. |title=Cognitive ornithology: The evolution of avian intelligence |journal=Philosophical Transactions of the Royal Society B: Biological Sciences |date=29 January 2006 |volume=361 |issue=1465 |pages=23–43 |doi=10.1098/rstb.2005.1736 |pmc=1626540 |pmid=16553307}}</ref>
|align="center"|2.49
|-
|[[Rhesus monkey]]
|align="center"|2.1
|-
|[[African elephant]]
|align="center"|1.3
|-
|[[Dog]]
|align="center"|1.2
|-
|[[Cat]]
|align="center"|1.0
|-
|[[Horse]]
|align="center"|0.9
|-
|[[Sheep]]
|align="center"|0.8
|-
|[[Mouse]]
|align="center"|0.5
|-
|[[Rat]]
|align="center"|0.4
|-
|[[Rabbit]]
|align="center"|0.4
|-
|[[Opossum]]
|align="center"|0.2
|}

This measurement of approximate intelligence is more accurate for mammals than for other [[Class (biology)|classes]] and [[phylum|phyla]] of [[Animalia]].

==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 [[cognition|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.<ref name="Cairo O. 2011 108">{{cite journal |author=Cairo O. |title=External measures of cognition |journal=Frontiers in Human Neuroscience |volume=5 |pages=108 |doi=10.3389/fnhum.2011.00108 |pmid=22065955 |pmc=3207484 |year=2011 |doi-access=free }}</ref>

=== 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.<ref name="Dunbar R.I. 2007 1344–1347">{{citation |author=Dunbar R.I. |title=Evolution in the social brain |journal=Science |volume=317 |issue=5843 |pages=1344–1347 |publisher=science magazine |bibcode=2007Sci...317.1344D |year=2007 |doi=10.1126/science.1145463 |pmid=17823343 |s2cid=1516792 }}</ref>

=== Taxonomic trends ===
Mean EQ for mammals is around 1, with [[carnivora]]ns, [[cetacea]]ns and [[primates]] above 1, and [[insectivora|insectivores]] and [[herbivore]]s below. Large mammals tend to have the highest EQs of all animals, while small mammals and avians have similar EQs.<ref name="Dunbar R.I. 2007 1344–1347"/> This reflects two major trends. One is that brain matter is extremely costly in terms of energy needed to sustain it.<ref>{{cite journal |last=Isler |first=K. |author2=van Schaik |author3=C. P  |title=Metabolic costs of brain size evolution |journal=Biology Letters |date=22 December 2006 |volume=2 |issue=4 |pages=557–560 |doi=10.1098/rsbl.2006.0538 |pmid=17148287 |pmc=1834002}}</ref> 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.<ref>{{cite journal |last=Savage |first=J.G. |title=Evolution in carnivorous mammals |journal=Palaeontology |year=1977 |volume=20 |issue=2 |pages=237–271 |url=https://www.palass.org/publications/palaeontology-journal/archive/20/2/article_pp237-271 }}</ref><ref>{{cite journal |last1=Lefebvre |first1=Louis |last2=Reader |first2=Simon M. |last3=Sol |first3=Daniel |title=Brains, Innovations and Evolution in Birds and Primates |journal=Brain, Behavior and Evolution |date=2004 |volume=63 |issue=4 |pages=233–246 |doi=10.1159/000076784 |pmid=15084816 |doi-access=free }}</ref> 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.<ref>{{cite news |title=Why Brain Size Doesn't Correlate With Intelligence |url=https://www.smithsonianmag.com/science-nature/why-brain-size-doesnt-correlate-with-intelligence-180947627/ |work=Smithsonian Magazine }}</ref>

=== Dietary trends ===
Of the animals demonstrating the highest EQ's (see associated table), many are primarily [[Frugivore|frugivores]], including [[apes]], [[macaque]]s, and [[proboscidea]]ns. This dietary categorization 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.<ref name=DecasienPrimate>{{cite journal |last1=DeCasien |first1=Alex R. |last2=Williams |first2=Scott A. |last3=Higham |first3=James P. |title=Primate brain size is predicted by diet but not sociality |journal=Nature Ecology & Evolution |date=27 March 2017 |volume=1 |issue=5 |pages=112 |doi=10.1038/s41559-017-0112 |pmid=28812699 |bibcode=2017NatEE...1..112D |s2cid=205564046 }}</ref>

[[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.<ref name=Steinhausen2016>{{cite journal |last1=Steinhausen |first1=Charlene |last2=Zehl |first2=Lyuba |last3=Haas-Rioth |first3=Michaela |last4=Morcinek |first4=Kerstin |last5=Walkowiak |first5=Wolfgang |last6=Huggenberger |first6=Stefan |title=Multivariate Meta-Analysis of Brain-Mass Correlations in Eutherian Mammals |journal=Frontiers in Neuroanatomy |date=30 September 2016 |volume=10 |pages=91 |doi=10.3389/fnana.2016.00091 |pmid=27746724 |pmc=5043137 |doi-access=free }}</ref> This presents an interesting conundrum for intelligent small animals, who have behaviors radically different from intelligent large animals. 

According to Steinhausen ''et al''.(2016): <blockquote>Animals with high RB [relative brain-mass] usually have (1)&nbsp;a short life span, (2)&nbsp;reach sexual maturity early, and (3)&nbsp;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)&nbsp;a high number of potential sexual partners, (2)&nbsp;delayed sexual maturity, and (3)&nbsp;rare gestations with small litter sizes.<ref name=Steinhausen2016/></blockquote>

=== Sociality ===
Another factor previously thought to have great impact on brain size is sociality and flock size.<ref>{{cite journal |doi=10.1098/rspb.2005.3283 |title=Both social and ecological factors predict ungulate brain size |first1=Susanne |last1=Shultz |first2=R.I.M. |last2=Dunbar |volume=273 |issue=1583 |journal=Proceedings of the Royal Society B: Biological Sciences |pages=207–215 |pmid=16555789 |pmc=1560022 |year=2006}}</ref> 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.<ref name=DecasienPrimate/> 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 [[dolphin]]s and [[orca]]s having the highest EQ of all [[cetaceans]],<ref name="Marino2006">{{cite journal|last1=Marino|first1=Lori|last2=Sol|first2=Daniel|last3=Toren|first3=Kristen|last4=Lefebvre|first4=Louis|date=April 2006|title=Does diving limit brain size in cetaceans?|url=https://animalstudiesrepository.org/cgi/viewcontent.cgi?article=1011&context=acwp_vsm|journal=Marine Mammal Science|volume=22|issue=2|pages=413–425|doi=10.1111/j.1748-7692.2006.00042.x|bibcode=2006MMamS..22..413M |s2cid=14898849 }}</ref> and humans with their extremely large societies and complex social life topping the list by a good margin.<ref name="Roth2005" />

== Comparisons with non-mammalian animals ==
Birds generally have lower EQ than mammals, but [[parrots]] and particularly the [[corvid]]s 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 [[synapse]]s per volume, allowing for more complex behaviour from a smaller brain.<ref name="Roth2005"/> Both [[bird intelligence]] and brain anatomy are however very different from those of mammals, making direct comparison difficult.<ref name=pmid16553307/>

[[Manta ray]]s have the highest EQ among [[fish]],<ref>{{cite book |last=Striedter |first=Georg F.|title=Principles of brain evolution |year=2005 |publisher=Sinauer |location=Sunderland, Mass. |isbn=978-0-87893-820-9}}{{page needed|date=April 2019}}</ref> and either [[octopus]]es<ref name="Gould 1977 Ever since Darwin, c7s1">Gould (1977) Ever since Darwin, c7s1</ref> or [[jumping spider]]s<ref>{{cite web |url=http://tolweb.org/accessory/Jumping_Spider_Vision?acc_id=1946 |title=Jumping Spider Vision |access-date=28 October 2009}}</ref> have the highest among [[invertebrate]]s. 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 a lower raw brain-to-body weight ratio.<ref>{{cite journal |last1=Meyer |first1=Wilfried |last2=Schlesinger |first2=Christa |last3=Poehling |first3=Hans Michael |last4=Ruge |first4=Wolfgang |title=Comparative quantitative aspects of putative neurotransmitters in the central nervous system of spiders (Arachnida: Araneida) |journal=Comparative Biochemistry and Physiology Part C: Comparative Pharmacology |date=January 1984 |volume=78 |issue=2 |pages=357–362 |doi=10.1016/0742-8413(84)90098-7 |pmid=6149080 }}</ref><ref>{{cite journal |last1=Rilling |first1=James K. |last2=Insel |first2=Thomas R. |title=The primate neocortex in comparative perspective using magnetic resonance imaging |journal=Journal of Human Evolution |date=August 1999 |volume=37 |issue=2 |pages=191–223 |doi=10.1006/jhev.1999.0313 |pmid=10444351 |bibcode=1999JHumE..37..191R }}</ref><ref name=houzel2009 /> Mean EQs for [[reptiles]] are about one tenth of those of mammals. EQ in birds (and estimated EQ in [[non-avian dinosaur|other]] dinosaurs) generally also falls below that of mammals, possibly due to lower thermoregulation and/or motor control demands.<ref>Paul, Gregory S. (1988) Predatory dinosaurs of the world. Simon and Schuster. {{ISBN|0-671-61946-2}}{{page needed|date=April 2019}}</ref> 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.<ref>{{cite journal |last1=Hopson |first1=J A |title=Relative Brain Size and Behavior in Archosaurian Reptiles |journal=Annual Review of Ecology and Systematics |date=November 1977 |volume=8 |issue=1 |pages=429–448 |doi=10.1146/annurev.es.08.110177.002241 |bibcode=1977AnRES...8..429H }}</ref>

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.<ref>{{cite web |url=http://yoyo.cc.monash.edu.au/~tzvi/GOULD.html |title=Bligh's Bounty |access-date=12 May 2011 |url-status=dead |archive-url=https://web.archive.org/web/20010709234346/http://yoyo.cc.monash.edu.au/~tzvi/GOULD.html |archive-date=9 July 2001 }}</ref> 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 [[paleoneurology]], where 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 [[dinosaur]]s is typically investigated using EQ formulas.<ref name=Dinosaur_Paleoneurology/>

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.<ref name=pmid16505378>{{cite journal |last1=Rosenberg |first1=K. R. |last2=Zune |first2=L. |last3=Ruff |first3=C. B. |title=Body size, body proportions, and encephalization in a Middle Pleistocene archaic human from northern China |journal=Proceedings of the National Academy of Sciences |date=27 February 2006 |volume=103 |issue=10 |pages=3552–3556 |doi=10.1073/pnas.0508681103 |pmid=16505378 |pmc=1450121 |bibcode=2006PNAS..103.3552R |doi-access=free }}</ref> 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.<ref name=pmid16505378/> 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 [[Neanderthal]]s and anatomically modern ''Homo sapiens'' (AMHS) via Encephalization quotient often rely on the use of endocasts, but this method has many drawbacks.<ref name=Cairo2011>{{cite journal |last1=Cairό |first1=Osvaldo |title=External measures of cognition |journal=Frontiers in Human Neuroscience |date=2011 |volume=5 |pages=108 |doi=10.3389/fnhum.2011.00108 |pmc=3207484 |pmid=22065955 |doi-access=free }}</ref> 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.<ref name=":0">{{cite journal |last1=Schoenemann |first1=P. Thomas |title=Brain Size Scaling and Body Composition in Mammals |journal=Brain, Behavior and Evolution |date=2004 |volume=63 |issue=1 |pages=47–60 |doi=10.1159/000073759 |pmid=14673198 |s2cid=5885808 }}</ref> 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 [[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.<ref name="Minervini_2016">{{cite journal |last1=Minervini |first1=Serena |last2=Accogli |first2=Gianluca |last3=Pirone |first3=Andrea |last4=Graïc |first4=Jean-Marie |last5=Cozzi |first5=Bruno |last6=Desantis |first6=Salvatore |title=Brain Mass and Encephalization Quotients in the Domestic Industrial Pig (Sus scrofa) |journal=PLOS ONE |date=28 June 2016 |volume=11 |issue=6 |pages=e0157378 |doi=10.1371/journal.pone.0157378 |pmid=27351807 |pmc=4924858 |bibcode=2016PLoSO..1157378M |doi-access=free }}</ref> The tremendous increase in body weight imposed by industrial farming significantly influences brain-to-body weight measures, including the EQ.<ref name="Minervini_2016" /> The EQ of the domestic adult pig is just 0.38, yet pigs can use visual information seen in a mirror to find food, show evidence of self-recognition when presented with their reflections<ref>{{Cite journal|last1=Broom|first1=D. M.|last2=Sena|first2=H.|last3=Moynihan|first3=K. L.|year=2009|title=Pigs learn what a mirror image represents and use it to obtain information|journal=Animal Behaviour|volume=78|issue=5|pages=1037–1041|doi=10.1016/j.anbehav.2009.07.027|s2cid=53175225}}</ref> 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.<ref>{{cite journal |last1=Marino |first1=Lori |last2=Colvin |first2=Christina M. |title=Thinking Pigs: A Comparative Review of Cognition, Emotion, and Personality in Sus domesticus |journal=International Journal of Comparative Psychology |date=2015 |volume=28 |issue=1 |doi=10.46867/ijcp.2015.28.00.04 |url=https://escholarship.org/uc/item/8sx4s79c |language=en |issn=0889-3667|doi-access=free }}</ref>

==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&nbsp;cm<sup>3</sup> to 1400&nbsp;cm<sup>3</sup>.<ref name=pmid16505378/> Furthermore, the genus ''Homo'' is specifically defined by a significant increase in brain size.<ref name=Cairo2011/> 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.<ref name=Cairo2011/> 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]].<ref>{{cite book |last=Graham |first=Patrick |title=Phrenology: [revealing the mysteries of the mind |year=2001 |publisher=Distributed by American Home Treasures |language=en|isbn=9780779251353 |oclc=51477257}}</ref>

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&nbsp;BCE that "of all the animals, man has the brain largest in proportion to his size."<ref name="RussellNorvigAI">{{cite book |author1=Russell, Stuart |author2=Norvig, Peter |title=Artificial Intelligence: A modern approach |year=2003 |place=Upper Saddle River, N.J. |publisher=Prentice Hall/Pearson Education |isbn=978-0-13-790395-5}}</ref> In 1861, French neurologist [[Paul Broca]] tried to make a connection between brain size and intelligence.<ref name=Cairo2011/> 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 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."<ref name="Darwin1871reprintp145">{{cite book |author=Darwin, Charles |title=The Descent of Man, and Selection in Relation to Sex |page=[https://archive.org/details/descentofmans00darw/page/145 145] |place=Princeton, New Jersey |publisher=Princeton University Press |isbn=978-0-691-02369-4 |edition=reprint |orig-year=1871 |year=1981 |url=https://archive.org/details/descentofmans00darw|url-access=registration }}</ref>{{refn|See also {{cite web |author=Darwin, Charles |title=The Descent of Man, and Selection in Relation to Sex |year=1874 |edition=reprint |url=https://archive.org/stream/descentofmansele00darwiala#page/60/mode/2up |page=60}} same quote as Darwin (1871)<ref name="Darwin1871reprintp145"/> cited above, on p.&nbsp;60 in online text of earlier reprint of second (1874) edition.}} The concept of quantifying encephalization is also not a recent phenomenon. In 1889, [[Francis Galton|Sir Francis Galton]], through a study on college students, attempted to quantify the relationship between brain size and intelligence.<ref name=Cairo2011/>

Due to [[Adolf Hitler|Hitler's]] racial policies during [[World War II]], studies on brain size and intelligence temporarily gained a negative reputation.<ref name=Cairo2011/> However, with the advent of imaging techniques such as the [[Functional magnetic resonance imaging|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.<ref>{{Cite journal|last1=Jerison H. J.|last2=Barlow Horace Basil|last3=Weiskrantz Lawrence|date=13 February 1985|title=Animal intelligence as encephalization|journal= Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences|volume=308|issue=1135|pages=21–35|doi=10.1098/rstb.1985.0007|pmid=2858875|bibcode=1985RSPTB.308...21J|doi-access=free}}</ref> With this belief, 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.<ref name=Cairo2011/>

==See also==
{{Portal|Evolutionary biology}}
*[[Brain–body mass ratio]]
*[[Brain development timelines]]
*[[Cephalization]]
*[[Cranial capacity]]
*[[Craniometry]]
*[[Evolutionary biology]]
*[[Human brain]]
*[[Human evolution]]
*[[Neuroscience and intelligence]]
*[[Phrenology]]
*[[Evolutionary neuroscience]]

==References==
{{reflist|25em |refs=

<ref name=Herculano2012>{{cite book |author=Herculano-Houzel, Suzana |article=The Human Nervous System |edition=Third |orig-year=2012 |title=Reference Module in Neuroscience and Biobehavioral Psychology |year=2017}}</ref>{{full citation |reason=This and Finlay (2009), below, lumped together in an ill-formed double(?) citation. Needs to be checked at source. |date=January 2020}}

<ref name=Finlay2009>{{cite book |doi=10.1016/B978-008045046-9.00939-6 |isbn=978-0-08-045046-9 |chapter=Brain Evolution: Developmental Constraints and Relative Developmental Growth |title=Encyclopedia of Neuroscience |year=2009 |last1=Finlay |first1=B.L. |pages=337–345 }}</ref>{{full citation |reason=This and Herculano (2012), above, lumped together in an ill-formed double(?) citation. Needs to be checked at source. |date=January 2020}}

}}

<!-- From Reference Module in Neuroscience and Biobehavioral Psychology, 2017. -->

==Bibliography==
* {{cite book |author=Allman, John Morgan |author-link=John Allman |year=1999 |title=Evolving Brains |place=New York |publisher=Scientific American Library |isbn=978-0-7167-5076-5 |url-access=registration |url=https://archive.org/details/evolvingbrains0000allm}}
* {{cite journal |doi=10.1098/rstb.1991.0111 |pmid=1685580 |last1=Foley|first1=R.A |author-link=Robert Foley (academic) |last2=Lee |first2=P.C |last3=Widdowson |year=1991 |first3=E. M. |last4=Knight |first4=C. D. |last5=Jonxis |first5=J. H. P. |title=Ecology and energies of encephalization in hominid evolution |journal= Philosophical Transactions of the Royal Society of London. Series B: Biological Sciences |volume=334 |issue=1270 |pages=223–232}}
* {{cite journal |last1=Jerison |first1=Harry J. |title=Paleoneurology and the Evolution of Mind |journal=Scientific American |date=January 1976 |volume=234 |issue=1 |pages=90–101 |doi=10.1038/scientificamerican0176-90 |pmid=1251178 |bibcode=1976SciAm.234a..90J }}
* {{cite book |year=2004 |editor1=Russon, Ann E. |editor2=Begun, David R. |title=The Evolution of Thought: Evolutionary Origins of Great Ape Intelligence |place=Cambridge |publisher=Cambridge University Press|isbn=978-0-521-78335-4 |url=https://books.google.com/books?id=e9RgrV7QnhEC&q=Russon+2004+%22evolution+of+thought%22 }}
* {{cite book |author=Tobias P.V. |title=The Brain in Hominid Evolution |publisher=Columbia University Press |location=New York and London |year=1971 |isbn=978-0-231-03518-7 |url=https://archive.org/details/braininhominidev00tob_f8p }}
* {{cite journal |last1=Van Schaik |first1=Carel |title=Why Are Some Animals So Smart? |journal=Scientific American |date=April 2006 |volume=294 |issue=4 |pages=64–71 |doi=10.1038/scientificamerican0406-64 |pmid=16596881 |bibcode=2006SciAm.294d..64V }} (Also cited in various publications as volume 16, issue 2, pp.&nbsp;30–37. [http://nb.vse.cz/kfil/elogos/biocosmology/saniot07b.pdf For example] {{Webarchive|url=https://web.archive.org/web/20210224121626/https://nb.vse.cz/kfil/elogos/biocosmology/saniot07b.pdf |date=24 February 2021 }})
* {{cite journal |last1=Boddy |first1=AM |last2=McGowen |first2=MR |last3=Sherwood |first3=CC |last4=Grossman |first4=LI |last5=Goodman |first5=M |last6=Wildman |first6=DE |title=Comparative analysis of encephalization in mammals reveals relaxed constraints on anthropoid primate and cetacean brain scaling. |journal=Journal of Evolutionary Biology |date=May 2012 |volume=25 |issue=5 |pages=981–94 |doi=10.1111/j.1420-9101.2012.02491.x |pmid=22435703 |s2cid=35368663 |doi-access=free }}
* {{cite journal |author=Dunbar R.I. |title=Evolution in the social brain |journal=Science |volume=317 |issue=5843 |pages=1344–1347 |publisher=science magazine |bibcode=2007Sci...317.1344D |year=2007 |doi=10.1126/science.1145463 |pmid=17823343 |s2cid=1516792 }}
* {{cite journal |last=Decasien |first=A.R. |title=Primate brain size is predicted by diet but not sociality |journal=Nature|volume=1 |issue=5 |pages=112 |pmid=28812699 |year=2017 |doi=10.1038/s41559-017-0112 |bibcode=2017NatEE...1..112D |s2cid=205564046 }}
* {{cite journal |author=Cairo O. |title=External measures of cognition |journal=Frontiers in Human Neuroscience |volume=5 |pages=108 |doi=10.3389/fnhum.2011.00108 |pmid=22065955 |pmc=3207484 |year=2011 |doi-access=free }}
* {{cite book |author=Herculano-Houzel S. |title=The Human Advantage: How our brains became remarkable |publisher=MIT Press |place=Cambridge, MA |year=2017 |isbn=978-0-262-53353-9}}

==External links==
* {{cite web |url=http://www.wsu.edu:8080/~taflinge/mindwork/mawint1.html |archive-url=https://web.archive.org/web/20110104114958/http://www.wsu.edu:8080/~taflinge/mindwork/mawint1.html |archive-date=2011-01-04 |df=dmy-all |title=mawint1}}
* {{cite web |url=http://brainmuseum.org/evolution/paleo/index.html |title=A graph of body mass vs. brain mass |website=brainmuseum.org}}
* {{cite web |url=http://yoyo.cc.monash.edu.au/~tzvi/GOULD.html |archive-url=https://web.archive.org/web/20010709234346/http://yoyo.cc.monash.edu.au/~tzvi/GOULD.html |archive-date=2001-07-09 |df=dmy-all |title=Bligh's Bounty |author-link=Stephen Jay Gould |first=Stephen Jay |last=Gould |website=monash.edu.au}}
* {{cite web |url=http://www.beyondveg.com/billings-t/comp-anat/comp-anat-appx2.shtml |title=Encephalization quotients, Kleiber's Law, and statistical methods}}
* {{cite AV media |url=https://www.ted.com/talks/suzana_herculano_houzel_what_is_so_special_about_the_human_brain |title=What is so special about the human brain |first=Suzana |last=Herculano-Houzel |year=2013 |publisher=TED Talk |medium=video}}

{{animal cognition}}

{{DEFAULTSORT:Brain-To-Body Mass Ratio}}
[[Category:Animal nervous system]]
[[Category:Mass]]
[[Category:Animal intelligence]]