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Brain-to-body mass ratio
Brain-to-body mass ratio, also known as the brain to body weight ratio, is the ratio of brain mass to body mass, which is hypothesised to be a rough estimate of the intelligence of an animal, although fairly inaccurate in many cases. A more complex measurement, encephalization quotient, takes into account allometric effects of widely divergent body sizes across several taxa.〔(【引用サイトリンク】title=Development of Intelligence )〕 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 (positive correlation) in animals (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.〔
In animals it's 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 neurons to represent their bodies and control specific muscles, 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,〔(【引用サイトリンク】title=Cortical Folding and Intelligence )〕 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.〔Haier, R.J., Jung, R.E., Yeo, R.C., Head, K. and Alkired, M.T. (2004): Structural brain variation and general intelligence. ''NeuroImage''
Vol. 23, Issue 1, September 2004, Pages 425-433 (summary )〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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