Brain-to-body mass ratio

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Brain-body mass ratio relationship for mammals Brain-body mass ratio for some animals diagram.svg
Brain–body mass ratio relationship for mammals

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 hypothesized 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. [1] [2] 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 size usually increases with body size in animals (i.e. large animals usually have larger brains than smaller animals); [3] the relationship is not, however, linear. Small mammals such as mice may have a brain/body ratio similar to humans, while elephants have a comparatively lower brain/body ratio. [3] [4]

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, [5] 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. [6]

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. [7] 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. [8]

Comparisons between groups

mass ratio (E:S) [3]
small ants1:7 [9]
small birds1:12

Dolphins have the highest brain-to-body weight ratio of all cetaceans. [10] Monitor lizards, tegus and anoles and some tortoise species have the largest among reptiles. Among birds, the highest brain-to-body ratios are found among parrots, crows, magpies, jays and ravens. Among amphibians, the studies are still limited. Either octopuses [11] or jumping spiders [12] have some of the highest for an invertebrate, although some ant species have 14%-15% of their mass in their brains, the highest value known for any animal. Sharks have one of the highest for fish alongside manta rays (although the electrogenic elephantfish has a ratio nearly 80 times higher - about 1/32, which is slightly higher than that for humans). [13] Treeshrews have a higher brain to body mass ratio than any other mammal, including humans. [14] Shrews [ clarification needed ] hold about 10% of their body mass in their brain.[ citation needed ]

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. [3] 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 [15] ), and C is the cephalization factor. [11] It has been argued that the animal's ecological niche, rather than its evolutionary family, is the main determinant of its encephalization factor C. [15]

The Bony-eared assfish holds the record for the smallest brain-to-body weight ratio of all vertebrates. Acanthonus armatus.jpg
The Bony-eared assfish holds the record for the smallest brain-to-body weight ratio of all vertebrates.

In the essay "Bligh's Bounty", [16] 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 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. [17] The brain-to-body mass ratio was however found to be an excellent predictor of variation in problem solving abilities among carnivoran mammals. [18]

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. [19]

See also

Related Research Articles

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A brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It is located in the head, usually close to the sensory organs for senses such as vision. It is the most complex organ in a vertebrate's body. In a human, the cerebral cortex contains approximately 14–16 billion neurons, and the estimated number of neurons in the cerebellum is 55–70 billion. Each neuron is connected by synapses to several thousand other neurons. These neurons typically communicate with one another by means of long fibers called axons, which carry trains of signal pulses called action potentials to distant parts of the brain or body targeting specific recipient cells.

Central nervous system Brain and spinal cord

The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain and spinal cord. The CNS is so named because the brain integrates the received information and coordinates and influences the activity of all parts of the bodies of bilaterally symmetric animals—i.e., all multicellular animals except sponges and jellyfish. It consists of a large nerve running from the anterior to the posterior, with the anterior end is enlarged into the brain. Not all animals with a central nervous system have a brain, although the large majority do.

Treeshrew Order of mammals

The treeshrews are small mammals native to the tropical forests of Southeast Asia. They make up the entire order Scandentia, which split into two families: the Tupaiidae, and the Ptilocercidae.

Sexual dimorphism Condition where the two sexes of the same species exhibit different characteristics beyond the differences in their sexual organs

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Cetacean intelligence is the cognitive ability of the infraorder Cetacea of mammals. This order includes whales, porpoises, and dolphins.

Neuroanatomy Branch of neuroscience

Neuroanatomy is the study of the structure and organization of the nervous system. In contrast to animals with radial symmetry, whose nervous system consists of a distributed network of cells, animals with bilateral symmetry have segregated, defined nervous systems. Their neuroanatomy is therefore better understood. In vertebrates, the nervous system is segregated into the internal structure of the brain and spinal cord and the routes of the nerves that connect to the rest of the body. The delineation of distinct structures and regions of the nervous system has been critical in investigating how it works. For example, much of what neuroscientists have learned comes from observing how damage or "lesions" to specific brain areas affects behavior or other neural functions.

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Neocortex Mammalian structure involved in higher-order brain functions

The neocortex, also called the neopallium, isocortex, or the six-layered cortex, is a set of layers of the mammalian cerebral cortex involved in higher-order brain functions such as sensory perception, cognition, generation of motor commands, spatial reasoning and language. The neocortex is further subdivided into the true isocortex and the proisocortex.

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Northern treeshrew

The northern treeshrew is a treeshrew species native to Southeast Asia.

Largest body part

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Cat intelligence

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