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Species Encephalization quotient (EQ)[1]
Human 7.44
Dolphin 5.31
Chimpanzee 2.49
Rhesus monkey 2.09
Elephant 1.87
Whale 1.76
Dog 1.17
Cat 1.00
Horse 0.86
Sheep 0.81
Mouse 0.50
Rat 0.40
Rabbit 0.40

Brain-to-body weight ratio (also known as the encephalization quotient or EQ) is a rough estimate of the possible intelligence of an organism. It can be defined as the ratio of the actual brain weight to the expected brain weight of a typical animal that size, EQ=w(brain)/Ew(brain).[citation needed]

The formula for the expected weight of the brain varies, but is usually Ew(brain) = 0.12w(body)2 / 3, though for some classes of animals the power is 3/4 rather than 2/3.[2] The idea behind EQ is that the larger an organism is, the more brain weight 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 weight might be available for more complex cognitive tasks. This method, as opposed to the method of simply measuring brain weight 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,[3] which increase the surface of the cortex, which is positively correlated in humans to intelligence.[4]


Comparisons between species

Dolphins have the highest brain-to-body weight ratio of all cetaceans.[5] Sharks have the highest for a fish, and either octopuses[6] or jumping spiders[7] have the highest for an invertebrate. Humans have a higher brain-to-body weight ratio than any of these animals[8][9]. Birds and dinosaurs generally have a smaller encephalization quotient, partly due to lower thermoregulation and/or motor control demands compared to mammals.[10]

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 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 has been called the cephalization factor; E = CS2, where E and S are body and brain weights and C is the cephalization factor.[6] 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.

Species Simple brain-to body ratio (E/S)[1]
small birds 1/12
human 1/40
mouse 1/40
cat 1/100
dog 1/125
frog 1/172
lion 1/550
elephant 1/560
horse 1/600
shark 1/2496
hippopotamus 1/2789

In the essay "Bligh's Bounty",[11] 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 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.

Brain to Lean-Body Mass ratio

The brain to LBM (lean body mass) ratio is a better indicator than the brain to gross body mass ratio.[citation needed] Cetaceans have a much higher percentage of body fat compared to non-obese humans (30-40%)[citation needed], as the average fat percentage of non-obese humans is 15% for men and 25% for women, increasing marginally with age.[citation needed] 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 1,700 gram (1.7 kilograms), which lifts the percentage of brain mass very close to 1% of LBM. These 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 and 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, however, does not correlate with the results of IQ testing where the majority of studies have shown higher average IQ-scores for males than for females.[12][13][14]

See also


  1. ^ a b
  2. ^ "Allometry". Retrieved 2008-09-15. 
  3. ^ "Cortical Folding and Intelligence". Retrieved 2008-09-15. 
  4. ^ Duncan et al. 1995
  5. ^ Marino, L. and Sol, D. and Toren, K. and Lefebvre, L. (2006). "Does diving limit brain size in cetaceans?". Marine Mammal Science 22 (2): 413–425. doi:10.1111/j.1748-7692.2006.00042.x. 
  6. ^ a b Gould (1977)Ever since Darwin, c7s1
  7. ^ "Jumping Spider Vision". Retrieved 2009-10-28. 
  8. ^ James K. Riling (1999). "The Primate Neocortex in Comparative Perspective using Magnetic Resonance Imaging". Journal of Human Evolution 37 (2): 191–223. 
  9. ^ Suzana Herculano-Houzel (2009). "The Human Brain in Numbers- A Linearly Scaled-Up Primae Brain". Frontiers in Human Neuroscience 2: 1–11 (2). doi:10.3389/neuro.09.031.2009. 
  10. ^ Paul, Gregory S. (1988) Predatory dinosaurs of the world. Simon and Schuster. ISBN 0671619462
  11. ^ web archive of
  12. ^ Nyborg (2005). Sex related differences in general intelligence g, brain size, and social status. Personality & Individual Differences, 39, 497−510.
  13. ^ Lynn, R., & Irwing, P. (2004). Sex differences on the progressive matrices: A meta-analysis. Intelligence, 32, 481−498.
  14. ^ Jackson, D. N., & Rushton, J. P. (2006). Males have greater g: Sex differences in general mental ability from 100,000 17–18 year olds on the scholastic assessment test. Intelligence, 34, 479−486.

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