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Encephalization is defined as the amount of brain mass related to an animal's total body mass. Quantifying an animal's encephalization has been argued to be directly proportional, although not equal, to that animal's level of intelligence. Aristotle wrote in 335 BCE: "Of all the animals, man has the brain largest in proportion to his size."[1] 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."[2]

In 2004, Dennis Bramble and Daniel Lieberman proposed that early Homo were scavengers who used stone tools to harvest meat off carcasses and to open bones. They proposed that humans specialized in long-distance running to compete with other scavengers in reaching carcasses.[3] It has been suggested that such an adaptation ensured a food supply that made large brains possible.[4] More encephalized species tend to have longer spinal shock duration.[citation needed]

Encephalization can also refer to the tendency for a species to evolve larger brains through time. Anthropological studies indicate that bipedalism preceded encephalization in the human evolutionary lineage after divergence from the chimpanzee lineage. Compared to the chimpanzee brain, the human brain is larger and certain brain regions have been particularly altered during human evolution.[5] Most brain growth of chimpanzees happens before birth while most human brain growth happens after birth.[6]

Encephalization quotient

Snell's equation of simple allometry[7] is:

Here "E" is the weight of the brain, "C" is the cephalization factor, "S" is body weight, and "r" is the exponential constant. The exponential constant for primates is 0.28[7] and either 0.56 or 0.66 for mammals in general.[8]

The "Encephalization Quotient" (EQ) is the ratio of "C" over the expected value for "C" of an animal of given weight "S".[8]

Species EQ[8] Species EQ[8]
Human 7.44 Dog 1.17
Dolphin 5.31 Cat 1.00
Chimpanzee 2.49 Horse 0.86
Raven[9] 2.49 Sheep 0.81
Rhesus monkey 2.09 Mouse 0.50
Elephant 1.87 Rat 0.40
Whale[clarification needed] 1.76 Rabbit 0.40

This measurement of approximate intelligence is more accurate for mammals than for other phyla of Animalia.[citation needed]

See also


  1. ^ Russell, Stuart; Norvig, Peter (2003), Artificial Intelligence: A Modern Approach, Upper Saddle River, N.J.: Prentice Hall/Pearson Education, ISBN 0-13-790395-2 
  2. ^ Darwin, Charles, The Descent of Man, and Selection in Relation to Sex (1981 reprint of 1871 ed.), Princeton, New Jersey: Princeton University Press, p. 145, ISBN 0-691-02369-7  See also quote, p.60, in online text of earlier reprint of second (1874) edition.
  3. ^ Bramble DM, Lieberman DE (November 2004), "Endurance running and the evolution of Homo" (PDF), Nature, 432 (7015): 345–52, doi:10.1038/nature03052, PMID 15549097. 
  4. ^ Aiello LC, Wheeler P (April 1995), "The Expensive-Tissue Hypothesis: The Brain and the Digestive System in Human and Primate Evolution" (PDF), Current Anthropology, 36 (2): 199–221, doi:10.1086/204350. 
  5. ^ See Figures 1 and 2 of Bradbury J (March 2005), "Molecular insights into human brain evolution", PLoS Biol., 3 (3): e50, doi:10.1371/journal.pbio.0030050, PMC 1065704Freely accessible, PMID 15760271. 
  6. ^ Penin, X; Berge, C; Baylac, M (2002). "Ontogenetic study of the skull in modern humans and the common chimpanzees: Neotenic hypothesis reconsidered with a tridimensional Procrustes analysis". American Journal of Physical Anthropology. 118 (1): 50–62. doi:10.1002/ajpa.10044. PMID 11953945. 
  7. ^ a b Williams, M.F. (April 2002), "Primate encephalization and intelligence", Medical Hypotheses, 58 (4): 284–290, doi:10.1054/mehy.2001.1516, PMID 12027521 
  8. ^ a b c d "Thinking about brain size". Serendip Studeio. Retrieved 2011-05-21. 
  9. ^ Emery, N. J. (2006). "Cognitive ornithology: The evolution of avian intelligence". Philosophical Transactions of the Royal Society B: Biological Sciences. 361 (1465): 23–43. doi:10.1098/rstb.2005.1736. PMC 1626540Freely accessible. PMID 16553307. 


  • Allman, John Morgan (1999), Evolving Brains, New York: Scientific American Library, ISBN 0-7167-5076-7 
  • Foley, R.A; Lee, P.C; Widdowson, E. M.; Knight, C. D.; Jonxis, J. H. P. (1991), "Ecology and energies of encephalization in hominid evolution", Philosophical Transactions of the Royal Society B: Biological Sciences, 334: 223–232, doi:10.1098/rstb.1991.0111 
  • Jerison H.J. (1976), "Paleoneurology and the evolution of the mind", Scientific American, 234 (1): 90–101, doi:10.1038/scientificamerican0176-90. 
  • Ann E. Russon; David R. Begun, eds. (2004), The Evolution of Thought: Evolutionary Origins of Great Ape Intelligence, Cambridge: Cambridge University Press, ISBN 0-521-78335-6 
  • Tobias P.V. (1971), The Brain in Hominid Evolution, New York and London: Columbia University Press, ISBN 0-231-03518-7 
  • Carel van Schaik (April 2006), "Why Are Some Animals So Smart?", Scientific American, 294 (4): 64–71, doi:10.1038/scientificamerican0406-64, PMID 16596881  (Also cited in various publications as volume 16, issue 2, pp. 30–37. For example)

External links

  • Encephalization quotients, Kleiber's Law, and statistical methods
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