Brain–body mass ratio
Brain–body mass ratio, also known as the brain–body weight ratio, is the ratio of brain mass to body mass, which is hypothesized to be a rough estimate of the
Brain–body size relationship
In animals, it is thought that the larger the brain, the more brain weight will be available for more complex cognitive tasks. However, large animals need more neurons to represent their own bodies and control specific muscles;[clarification needed][citation needed] thus, relative rather than absolute brain size makes for a ranking of animals that better coincides with the observed complexity of animal 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,[6] which increase the surface of the cortex, which is positively correlated in humans to intelligence. The noted exception to this, of course, is swelling of the brain which, while resulting in greater surface area, does not alter the intelligence of those suffering from it.[7]
Relation to metabolism
The relationship between brain weight and body weight of all living vertebrates follows two completely separate linear functions for cold-blooded and warm-blooded animals.[8] Cold-blooded vertebrates have much smaller brains than warm-blooded vertebrates of the same size. However, if brain metabolism is taken into account, the brain-to-body relationship of both warm and cold-blooded vertebrates becomes similar, with most using between 2 and 8 percent of their basal metabolism for the brain and spinal cord.[9]
Comparisons between groups
Species | Brain:body mass ratio (E:S)[4] |
---|---|
Small ants | 1:7[10] |
Tree shrew | 1:10 |
Small birds | 1:12 |
Elephantfish | 1:32 |
Mouse | 1:40 |
Human | 1:40 |
Cat | 1:100 |
Dog | 1:125 |
Frog | 1:172 |
Lion | 1:550 |
Elephant | 1:560 |
Horse | 1:600 |
Shark | 1:2496 |
Hippopotamus | 1:2789 |
It is a trend that the larger the animal gets, the smaller the brain-to-body mass ratio is. 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] 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 can be described by an equation of the form E = CSr, where E and S are brain and body weights, r a constant that depends on animal family (but close to 2/3 in many vertebrates[17]), and C is the cephalization factor.[12] It has been argued that the animal's ecological niche, rather than its evolutionary family, is the main determinant of its encephalization factor C.[17] In the essay "Bligh's Bounty",[18] 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.
Criticism
Recent research indicates that, in non-human 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 spatial relation.[19] The brain-to-body mass ratio was however found to be an excellent predictor of variation in problem solving abilities among carnivoran mammals.[20]
In humans, the brain to body weight ratio can vary greatly from person to person; it would be much higher in an underweight person than an overweight person, and higher in infants than adults. The same problem is encountered when dealing with marine mammals, which may have considerable body fat masses. Some researchers therefore prefer lean body weight to brain mass as a better predictor.[21]
See also
- Cranial capacity
- Craniometry
- Encephalization quotient
- List of animals by number of neurons
- Phrenology
- Schauenberg's index
References
- ^ "Development of Intelligence". Ircamera.as.arizona.edu. Archived from the original on 2014-12-31. Retrieved 2011-05-12.
- PMID 22065955.
- S2CID 19183523.
- ^ a b c d "Brain and Body Size... and Intelligence". SerendipStudio.org. 2003-03-07. Retrieved 2019-02-24.
- S2CID 53184282.
- ^ "Cortical Folding and Intelligence". Retrieved 2008-09-15.
- S2CID 29426973.
- ^ A graph of the relation between brain weight and body weight of living vertebrates Retrieved 10 February 2018.
- ^ A graph of the relation of CNS to body metabolism in vertebrates Retrieved 10 February 2018.
- S2CID 6177033.
- S2CID 14898849.
- ^ a b Gould (1977) Ever since Darwin, c7s1
- ^ "Jumping Spider Vision". Retrieved 2009-10-28.
- PMID 9318319.
- ^ http://genome.wustl.edu/genomes/view/tupaia_belangeri is an article on Tupaia belangeri from The Genome Institute published by Washington University, archived at https://web.archive.org/web/20100601201841/https://www.genome.wustl.edu/genomes/view/tupaia_belangeri
- ^ Feltman, Rachel (2018-03-15). "What does brain size have to do with intelligence?". Popular Science. Retrieved 2024-02-28.
- ^ PMID 2740904.
- ^ "Bligh's Bounty". Archived from the original on 2001-07-09. Retrieved 2011-05-12.
- S2CID 17107712.
- PMID 26811470. Retrieved 29 January 2016.
- S2CID 5885808.