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One early suggestion was brain size (or weight, which provides the same ordering.) A second proposal was brain-to-body-mass ratio, and a third was encephalization quotient, sometimes referred to as EQ. The current best predictor is number of neurons in the forebrain, based on Herculano-Houzel's improved neuron counts. [1]
Treeshrews hold about 10% of their body mass in their brain. [16] Generally speaking, the larger the animal, the smaller the brain-to-body mass ratio is. Thus, large whales have very small brains compared to their weight, and small rodents like mice have a relatively large brain, giving a brain-to-body mass ratio similar to humans. [4]
Elephants have a higher number of total neurons (257 billion) [61] compared to humans (100 billion). [62] [63] Relative brain size, overall mass, and total number of neurons are only a few metrics that help scientists follow the evolutionary trend of increased brain to body ratio through the hominin phylogeny.
Since most of the brain is used for maintaining bodily functions, greater ratios of brain to body mass may increase the amount of brain mass available for more complex cognitive tasks. [3] Allometric analysis indicates that in general, mammalian brain size scales at approximately the 2 ⁄ 3 or 3 ⁄ 4 exponent of body mass. [4]
Mice have a direct brain/body size ratio similar to humans (1/40), while elephants have a comparatively small brain/body size (1/560), despite being quite intelligent animals. [18] Treeshrews have a brain/body mass ratio of (1/10). [19] Several reasons for this trend are possible, one of which is that neural cells have a relative constant size ...
A standard measure for assessing an animal's brain size compared to what would be expected from its body size is known as the encephalization quotient. The encephalization quotient for humans is between 7.4-7.8. [61] When the mammalian brain increases in size, not all parts increase at the same rate. [62]
Loss of muscle mass. Increased body fat. Decreased bone density. Mood changes. Memory trouble. Reduced red blood cell count (anemia) Less body hair. Gynecomastia (male breast development) Reduced ...
The number of neurons in the brain varies dramatically from species to species. [63] In a human, there are an estimated 10–20 billion neurons in the cerebral cortex and 55–70 billion neurons in the cerebellum . [ 64 ]