Brain to body mass ratio (also known as the Encephalisation Quotient) is a method to quickly get a rough estimate of the possible intelligence of an organism.
Roughly speaking, the larger an organism is, the more brain mass is required for housekeeping tasks, such as breathing, temperature control, senses, muscle control, etc. The larger the brain is relative to the body, the more brain mass might be available for more complex cognitive tasks.
Many people prefer this ratio, rather than simply measuring brain mass alone, because it puts humans closer to the top of the list.
Rather than simply dividing brain mass by body mass, some divide brain mass by the ⅔ power of body mass (in other words, divide brain mass by body surface area) or the ¾ power of body mass — whatever it takes to get humans even closer to the top of the list.
Dolphins have the highest brain to body mass ratio of all cetaceans. Sharks have the highest for a fish, and octopuses have the highest for an invertebrate. Humans have a higher brain to body mass ratio than any of these animals.
However, there seems to be some controversy over whether humans have the highest brain to body mass ratio (followed by dolphins)
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, or whether shrews are on the top of the list
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"shrews hold nearly 10% of their mass in their brain, making it one of the most encephalized animals." [6]
Since shrews are less intelligent than humans, many believe that intelligence correlates with the absolute brain-mass left over from when one subtracts the brain-mass for running the body. In the essay Blight's Bounty, 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.
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