Talk:Brain to body mass ratio

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Placed a stub and some basic articles to research. Plenty of room to expand this article with tables and graphs (see refs). :-) Kim Bruning 22:36, 13 Oct 2004 (UTC)

  • As "Encephalisation Quotient" seems to be a far less common term have redirected (UK and US spellings) here. CheekyMonkey 23:01, 13 Oct 2004 (UTC)

Encephalisation Quotient is not the same as brain to body mass ratio, and the current arrangement is not acceptable. The page on Homo floresiensis, for example, now says that H. floresiensis has a similar brain to body mass ratio to H. erectus. This is false: the ratio is dissimilar (and generally considered irrelevant), it is the EQ that is similar, and relevant, and discussed with interest on the talk page.

I think we need a few short articles to define EQ, brain to body mass ratio, brain to spinal cord mass, and any other measures that are used. Then other articles can talk about e.g. the EQs of hominids and the reader will know what the author means. These pages can then reference this one, which should be retitled something like 'relative brain size' (or encephalization, but I think it's worth keeping the existing article with that name). This article can explain current thinking on how to compare brain size between species, and the significance of relative brain size. Securiger's comments below are a good start.

2 other notes:

  • The Gould article linked suggests dividing brain mass by spinal cord mass, not subtracting. Was that what you mean, Ŭalabio?
  • The "Cosmic Evolution" link has a plot of log(brain mass) vs. log(body mass), labelled "A plot of brain mass versus body mass". Since this is confusing, and the relevant parts of the text are rather vague, I favour removing this link, interesting though it is.

Townmouse 13:24, 5 Mar 2005 (UTC)

I second the call to remove the incorrect identification between "encephalization quotient" and "brain to body mass ratio". Let's rename this article "encephalization quotient", and create a stub for "brain to body mass ratio" to caution against merging them again. Perhaps a more general article stub could be created to relate them, perhaps "inter-species scaling laws and intelligence". Bayle Shanks 06:25, 10 August 2007 (UTC)


Contents

[edit] Some problems with this article

This article is way too oversimplified. Some comments:

  • Encephalization Quotient (or Encephalisation Quotient) is not "brain to body mass ratio". It is defined as the ratio between actual brain mass and predicted brain mass—predicted from a certain logarithmic regression curve. If brain mass and body mass of many different vertebrates are plotted on a log-log chart, we find that they mostly lie on two rough lines - an upper line for birds and mammals, and a lower line for reptiles and fish. The slope of this line is approximately 2/3, suggesting that typically M_{brain(est.)} = \alpha M_{body}^{\frac23} where α depends on whether your critter is warm or cold blooded. EQ is the ratio between actual brain mass and the value predicted by this equation. If the exponent was 1, that would be proportional (but not equal) to "brain to body mass ratio", since EQ = Mbrain / Mbrain(est.) = Mbrain / (αMbody). But with a 2/3 exponent, it isn't even proportional.
  • Here's a big "bzzzzt" - that 2/3 exponent is fudged. Jerison, the guy who invented EQ, fitted the curve by eye after already deciding by hypothesis that that's what it should be. In fact, the actual slope of his log-log plot is more like 3/4 [1]. So EQ, as originally defined, is horse puckey. Many people still seem to use this fictitious exponent. Well, we'll just fix up the exponent and ignore that.
  • Let r = Mbrain(est.) / Mbody Because - approximately speaking - for most species it happens that M_{brain(est.)} = \alpha M_{body}^{\frac34}, it follows that

\begin{matrix}r & = & \alpha M_{body}^{\frac34}/M_{body} \\ \ & = & \alpha M_{body}^{-\frac14} \\ \ & = & \alpha \sqrt[3]\frac{\alpha}{M_{brain}} \\ \Rightarrow M_{brain} & = & \alpha^4/r^3 \end{matrix} Hmm. If we assume that brain to body mass ratio is a measure of intelligence, it turns out that the smarter a creature is, the smaller it's brain is. Clearly, that isn't right. What gives?

In the penultimate line you assume Mbrain = Mbrain(est), i.e. that all animals have an EQ of 1.
You have shown that if brain size is proportional to the 3/4 power of body size, then animals with absolutely smaller brains (smaller animals) have proportionally larger brains. This will be so for any power greater than 0 and less than 1. Townmouse
  • It gets worse. The fit to this line is really pretty rough. We can get lots of completely different slopes by varying our data set a bit. In fact it turns out that the smaller the taxon from which your data comes, the smaller the correlation. That is, the more closely related the species within your sample, the less relevance body mass has to predicting brain size. And within one species, there is practically no relationship.
  • But the really big problem: while it is true that EQ is often used as if it were an estimate for the intelligence of a species, there is no scientific justification for this! Remember, for EQ=1 we have a line running from very small bodied, very small brained creatures at the lower left of the curve, up to very large brained, very large bodied creatures at the top right of the curve. All those species lying on the line have the same EQ. If EQ was a measure of intelligence, we would be claiming that all creatures on this line are of equal intelligence. This is plainly complete nonsense; the line has been statistically constructed so that as many creatures as possible lie as close as possible to it, without any regard to their intelligence (which in most cases has never even been measured).
  • There does seem to be a weak correlation between absolute brain size and intelligence within a species; a rank correlation of about 0.4 has been reported for rat maze solving times and brain mass. That means (more or less) that about 40% of the variation in rat maze solving ability is attributable to brain size—and 60% to "something" else. Between species, there is generally a feeling that there is some sort of relationship, but there are obvious exceptions, and efforts to pin it down to numbers run afoul of the difficulty of numerically comparing intelligence between species.

Securiger 16:55, 8 Nov 2004 (UTC)

[edit] Brain-mass minus spinal-cord-mass versus encephalization quotient

The discovery that the brain-size increases with the surface-area of an animal is great and all, but I remember reading in an Issue of Natural History from the 1980s an article by Stephen Jay Gould where he noted encephalization quotient is good and all, but he observed that animals with small brains for the size of their bodies have brains with about the same size as their spinal cords. He wrote about this in "Bligh's Bounty." http://web.archive.org/web/20010709234346/http://yoyo.cc.monash.edu.au/~tzvi/GOULD.html He figures that all other things being equal (which they never all), the intelligence of an animal should be proportional to the absolute mass of the brain minus the mass of spinal cord.

-- Ŭalabio 22:27, 2004 Dec 4 (UTC)

I have nothing much to say, other than that I agree with the previous posters that this article needs a major overhauling. The article seems to give a weak description of what EQ is not. It would be nice if we could give a short description of the history of EQ, the problems it was attempting to solve, its current acceptence in evolutionary biology, along with an accurate description of how it is found and what value it has.

I also think it would be appropriate to give a few lines as to why simple brain to body mass ratios fail, along with why brain size is not very valuable for determining intelligence.

Another poster knowing this article is bad, but without the motivation and confidence to fix it himself.

-b

[edit] ============

Furthermore, what can we say about people who lost half of their brains, and yet still perform just as well on IQ tests as people who have not lost half of their brains? 24.16.15.81 22:30, 16 July 2006 (UTC)

What we can say is that it is an urban legend, probably widely distributed by self-improvement promoters, although the origin of the story is uncertain (see, e.g. Snopes). People can receive severe head injuries or strokes and yet still (sometimes, eventually) recover many of their faculties (sometimes at a cost of losing others), but even ~30% loss causes crippling debility (see, e.g. Scans show dramatic brain cell loss in Alzheimer's). -- Securiger 07:29, 20 July 2006 (UTC)

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[edit] Discussion

[edit] The cephalization factor

Found on this phage: http://www.ccwu.edu/Thesis_Moynihan/Chapter%205.doc.

"Starting with the largest brains where we have the most experience, one would assume to a first order that a big brain would imply greater intelligence. But when comparing among species, one must be careful lest one obtain skewed results. For example, very early research in this area compared the ratio of brain weight to body weight as a possible way of deriving relative intelligence among species. But this simplistic approach revealed that both mice and humans have a body weight that is 40 times the brain weight – and small birds have a body weight of only 12 times the brain weight. These results would seem to imply that mice and men are of equal intelligence (many women would agree) and that birds were considerably smarter. But brain weight does not increase linearly with body weight. Instead, the increase in brain size with body size follows a specific exponential rule. In the late Nineteenth Century, Snell derived the following equation relating brain and body weight:


                       E = C S r

where E and S are body and brain weights, respectively, and C is a constant referred to as a “cephalization factor.” The r is an empirically determined generally accepted exponent that was derived initially by Bonin and is approximately two-thirds for most mammals. The rationale for the two-thirds exponent is based on the premise that the major determinant for required brain size is the body surface. Given a characteristic length of an animal, its surface increases with the square of that length, while its volume increases with the length cubed. If brain size growth were then taken as the animal's surface per volume, then the two-thirds exponent would result. Figure 5.4 shows the brain and body weights for various species with Snell’s equation overlaid representing a best fit of the data. One might note that the polygon connecting the outer limits of the data points shows the elephant and blue whale at the extreme points. Although elephants are generally considered to be fairly intelligent, there is no evidence that they are more intelligent than humans. Therefore, interpretation of this figure requires a methodology for establishing the relative brain capacities of the different species independently of body size. One method commonly used for factoring out body size is to calculate the displacement of each data point from the best-fit line in the polygon and use these residuals to estimate an index of brain size relative to that which would be predicted for a particular animal. Relative brain capacities for different species could then be compared. By entering the brain and body weights for different species into Snell’s equation and solving for C, one can then determine the ratio of the calculated C with that of the average mammalian value. The result is the index referred to as the “encephalization quotient” or EQ. The EQ index enables one to compare the brain capacity of a given species with what would be expected of an animal of comparable weight with average encephalization."

This would mean that each time an animal's mass is doubled, the brain would be for instance 5% smaller to adapt to the new body. This is also seen in other ways. The head of a mouse ir very large compared to its body. Imaging a hore with the same relationship between head and body. A giraffe the size of a rat would have a totally different anatomy, and the different sections of the body would change to adapt. The brain is no exeption, actually this especially goes for the brain. Bt of course, the larger the brain, the bigger the potential, and this in turn can make it grow bigger than one would expect from the rest of the body. Rhynchosaur 23:02, 8 February 2007 (UTC)


When it says "dolphins" have highest EQ, it should state what species. river dolphins have an EQ of only 1.5. Orca between 1.7 and 2...and Orca are considered the most intelligent dolphin of them all. —Preceding unsigned comment added by 60.234.140.44 (talk) 06:55, 10 November 2007 (UTC)

[edit] "ocotpuses" is more properly written as "ocotopi"

I do not know enough about the subject matter to criticize the content, but I did find this spelling/grammatical error distracting. 71.132.86.150 07:43, 13 June 2007 (UTC)

Actually I think it's "octopuses". Bayle Shanks 06:21, 10 August 2007 (UTC)

[edit] Peters' elephantnose fish

I think that Peters' elephantnose fish should be included in this article. They have a higher brain-to-body mass ratio than humans as well, and are apparently fairly intelligent. Also they can do really neat metal detecting things in total darkness! Here's a blog entry about them:

http://scienceblogs.com/zooillogix/2007/08/hilariouslooking_fish_uses_chi.php Gary 00:33, 23 August 2007 (UTC)

[edit] Problem with units of the factor 0.12

There is a problem with the physical units of the coefficient 0.12 in the formula of the article. This number only works if brain mass is in grams while body mass is in kilograms. Not very beautyful. The true value of this coefficient, with proper units, is

0.12\ \frac{\mbox{kg}}{\mbox{g}^{2/3}} = 120\ \frac{\mbox{g}}{\mbox{g}^{2/3}} = 120\ \mbox{g}^{1/3} = 120 \cdot (0.001\ \mbox{kg})^{1/3} = 12\ \mbox{kg}^{1/3}

I will correct the article accordingly. 83.221.140.244 13:46, 29 August 2007 (UTC)

Sorry, I was wrong. I think the constant was correct before

0.12\ \mbox{g}^{1/3} = 0.012\ \mbox{kg}^{1/3}

Sorry for the confusion. 83.221.140.244 14:49, 29 August 2007 (UTC)

Brain/body masses don't have to be grams, or any particular unit, the equation is unit-balanced, as long as you keep your units consistant, the equation doesn't care if they are grams, ounces, tons, or double-decker buses. I've deleted the reference to grams. -- PaulxSA (talk) 11:11, 12 January 2008 (UTC)