Brain to body mass ratio

Brain to body mass ratio (also known as the Encephalization Quotient or EQ) is a rough estimate of the possible intelligence of an organism.

It is defined as the ratio of the actual brain mass to the expected brain mass of a typical animal that size, EQ=m(brain)/Em(brain).Fact|date=July 2008 The formula for the expected mass of the brain varies, but is usually $Em(brain)=0.12\; m(body)^\{2/3\}$, though for some classes of animals the power is 3/4 rather than 2/3. [cite web | url= http://weber.ucsd.edu/~jmoore/courses/allometry/allometry.html |title=Allometry | accessdate=2008-09-15]

Roughly speaking, the larger an organism is, the more brain mass is required for basic survival tasks, such as breathing, thermoregulation, senses, motor skill, etc. The larger the brain is relative to the body, the more brain mass might be available for more complex cognitive tasks. This method, as opposed to the method of simply measuring brain mass alone, puts humans closer to the top of the list. Also, reflecting the evolution of the recent cerebral cortex, different animals have different degrees of brain folding [cite web | url=http://serendip.brynmawr.edu/bb/kinser/Int4.html |title=Cortical Folding and Intelligence | accessdate=2008-09-15] , which increase the surface of the cortex, which is positively correlated in humans to intelligence (Duncan et al. 1995).

Dolphins have the highest brain to body mass ratio of all cetaceans.Fact|date=July 2008 Sharks have the highest for a fish, and octopuses have the highest for an invertebrate. [Gould (1977)Ever since Darwin, c7s1] Humans have a higher brain to body mass ratio than any of these animals. Birds and dinosaurs generally have a relatively smaller encephalization quotient, partly due to lower thermoregulation and/or motor control demands compared to mammals. [ Paul, Gregory S. (1988) Predatory dinosaurs of the world. Simon and Schuster. ISBN: 0671619462]

It is a trend that the larger the animal gets, the smaller the relative brain size gets. Large whales have very small brains compared to their weight, and small rodents have huge brains. One explanation could beOr|date=July 2008 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 has been called the cephalization factor; "E = CS²", where E and S are body and brain weights and C is the cephalization factor. [Gould (1977)Ever since Darwin, c7s1] Just focusing on the relationship between the body and the brain is not enough; one also has to consider the total size of the animal.Or|date=July 2008

In the essay "Bligh's Bounty", [ [http://web.archive.org/web/20010709234346/http://yoyo.cc.monash.edu.au/~tzvi/GOULD.html web archive of monash.edu.au] ] Stephen Jay Gould 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 mass 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.

The brain to LBM (Lean Body Mass) ratio is a better indicator than the brain to gross body mass ratio.Fact|date=July 2008 Cetaceans have a much higher percentage of body fat compared to non-obese humans (30-40%)Fact|date=July 2008, as the average fat percentage of non-obese humans is 15% for men and 25% for women, increasing marginally with age.Fact|date=July 2008 If we estimate the gross body mass of a bottlenose dolphin at 250 kg and the percentage of body fat at 30 and deduct the 75 kg of fat mass from gross body mass, the LBM will be approximately 175 kg, brain mass approximately 1700 gram (1.7 kilograms), which lifts the percentage of brain mass very close to 1% of LBM.Or|date=July 2008These figures are just an example because the gross body mass of bottlenose dolphins can be anywhere between 200 and 500 kg. There is however another argument for this thesis, based on the brain to body ratio of men & women. Females generally have a somewhat smaller brain volume than males, but if you correct for the higher percentage of body fat in women the ratio/EQ will be the same as in males. This correlates with the result of IQ testing, the same in average for males and females. [See: Zon, Peter van der, Waar spieren zitten, zitten geen hersens (where there are muscles, there are no brains) SPORT & FITNESS (Dutch) nr. 146, may 2008.]

ee also

*Cranial capacity
*Encephalization
*Neuroscience and intelligence

References

External links

*http://www.tufts.edu/as/wright_center/cosmic_evolution/docs/text/text_cult_3.html
*http://www.wsu.edu:8080/~taflinge/mindwork/mawint1.html
* [http://brainmuseum.org/evolution/paleo/index.html a graph of body mass vs. brain mass]
* [http://web.archive.org/web/20010709234346/http://yoyo.cc.monash.edu.au/~tzvi/GOULD.html "Bligh's Bounty"] — Stephen Jay Gould