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|Species||Simple brain-to body ratio (E:S)|
Brain-to-body mass ratio, also known as the brain to body weight ratio, is the ratio of brain weight to body weight, which is hypothesised to be a rough estimate of the intelligence of an animal. A more complex measurement, encephalization quotient, takes into account allometric effects of widely divergent body sizes across several taxa. The raw brain-to-body mass ratio is however simpler to come by, and is still a useful tool for comparing encephalization within species or between fairly closely related species.
Brain-body size relationship
Brain size usually increases with body size in animals (is positively correlated), i.e. large animals usually have larger brains than smaller animals. The relationship is not linear however. Small mammals like mice have a direct brain/body size similar to humans, while elephants have comparatively small brain/body size, despite elephants being obviously intelligent animals.
Intelligence in animals is hard to establish, but the larger the brain the more brain weight might be available for more complex cognitive tasks. However, large animals need more brain to control their body, so that relative rather than absolute brain size makes for a ranking of animals that coincide better with observed complexity of behaviour. The relationship between brain-to-body mass ratio and complexity of behaviour is not perfect as other factors also influence intelligence, like the evolution of the recent cerebral cortex and different degrees of brain folding, which increase the surface of the cortex, which is positively correlated in humans to intelligence. The noted exception to this, of course, are those suffering from swelling of the brain which, while resulting in greater surface area, does not alter intelligence. 
Comparisons between groups
Dolphins have the highest brain-to-body weight ratio of all cetaceans. Either octopuses or jumping spiders have the highest for an invertebrate. Humans have a higher brain-to-body weight ratio than any of these animals. Sharks have one of the highest for fish (although the electrogenic elephantfish has a ratio nearly 100 times higher - about 1/34, which is slightly higher than that for humans). The tiny shrew, which holds nearly 10% of its body mass in its brain, has the highest brain-to-body mass ratio of any known animal. Mean EQ for reptiles are about one tenth of the EQ for mammals. EQ in birds (and estimated EQ in dinosaurs) generally also falls below that of mammals, partly due to lower thermoregulation and/or motor control demands.
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 relatively large 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. 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.
In the essay "Bligh's Bounty", Stephen Jay Gould noted that if one looks at vertebrates with very low encephalization quotient, 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.
Recent research indicates that, in primates, whole brain size is a better measure of cognitive abilities than brain-to-body mass ratio. The total weight of the species is greater than the predicted sample only if the frontal lobe is adjusted for spacial relation. 
The brain to body weight ratio also varies greatly from person to person; it would be much higher in an underweight person than an overweight person, and higher in infants than adults.
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- Cortical Folding and Intelligence. URL accessed on 2008-09-15.
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- Gould (1977)Ever since Darwin, c7s1
- Jumping Spider Vision. URL accessed on 2009-10-28.
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- Bligh's Bounty. URL accessed on 2011-05-12.
- Overall Brain Size, and Not Encephalization Quotient, Best Predicts Cognitive Ability across Non-Human Primates. Brain Behav Evol 2007;70:115-124 (DOI: 10.1159/000102973)
- a graph of body mass vs. brain mass
- "Bligh's Bounty" — Stephen Jay Gould