Brain to body mass ratio - AbsoluteAstronomy.com

Species	Simple brain-to body ratio (E:S)
small birds	1:12
human	1:40
mouse	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

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

Animal

Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...

. A more complex measurement

Measurement

Measurement is the process or the result of determining the ratio of a physical quantity, such as a length, time, temperature etc., to a unit of measurement, such as the metre, second or degree Celsius...

, encephalization quotient

Encephalization quotient

Encephalization Quotient , or encephalization level is a measure of relative brain size defined as the ratio between actual brain mass and predicted brain mass for an animal of a given size, which is hypothesized to be a rough estimate of the intelligence of the animal.This is a more refined...

, takes into account allometric effects of widely divergent body sizes across several taxa

Taxon

|thumb|270px|[[African elephants]] form a widely-accepted taxon, the [[genus]] LoxodontaA taxon is a group of organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement...

. The raw brain-to-body mass ratio is however simpler to come by, and is still a useful tool for comparing encephalization

Encephalization

Encephalization is defined as the amount of brain mass exceeding that related to an animal's total body mass. Quantifying an animal's encephalization has been argued to be directly related to that animal's level of intelligence. Aristotle wrote in 335 B.C...

within species or between fairly closely related species.

Brain-body size relationship

Brain size usually increases with body size in animals (is positively correlated

Correlation

In statistics, dependence refers to any statistical relationship between two random variables or two sets of data. Correlation refers to any of a broad class of statistical relationships involving dependence....

), i.e. large animals usually have larger brains than smaller animals. The relationship is not linear however. Small mammals like mice

MICE

-Fiction:*Mice , alien species in The Hitchhiker's Guide to the Galaxy*The Mice -Acronyms:* "Meetings, Incentives, Conferencing, Exhibitions", facilities terminology for events...

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

Cognition

In science, cognition refers to mental processes. These processes include attention, remembering, producing and understanding language, solving problems, and making decisions. Cognition is studied in various disciplines such as psychology, philosophy, linguistics, and computer science...

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

Cerebral cortex

The cerebral cortex is a sheet of neural tissue that is outermost to the cerebrum of the mammalian brain. It plays a key role in memory, attention, perceptual awareness, thought, language, and consciousness. It is constituted of up to six horizontal layers, each of which has a different...

and different degrees of brain folding, which increase the surface of the cortex, which is positively correlated

Correlation

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

Dolphin

Dolphins are marine mammals that are closely related to whales and porpoises. There are almost forty species of dolphin in 17 genera. They vary in size from and , up to and . They are found worldwide, mostly in the shallower seas of the continental shelves, and are carnivores, mostly eating...

s have the highest brain-to-body weight ratio of all cetaceans. Either octopuses

Octopus

The octopus is a cephalopod mollusc of the order Octopoda. Octopuses have two eyes and four pairs of arms, and like other cephalopods they are bilaterally symmetric. An octopus has a hard beak, with its mouth at the center point of the arms...

or jumping spider

Jumping spider

The jumping spider family contains more than 500 described genera and about 5,000 described species, making it the largest family of spiders with about 13% of all species. Jumping spiders have some of the best vision among invertebrates and use it in courtship, hunting and navigation...

s have the highest for an invertebrate

Invertebrate

An invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...

. Humans have a higher brain-to-body weight ratio than any of these animals. Shark

Shark

Sharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago....

s have one of the highest for fish

Fish

Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...

(although the electrogenic elephantfish

Mormyridae

The family Mormyridae, sometimes called "elephantfish" , are freshwater fish in the order Osteoglossiformes native to Africa. It is by far the largest family in the order with around 200 species. Members of the family are popular, if challenging, aquarium species...

has a ratio nearly 100 times higher - about 1/34, which is slightly higher than that for humans). The tiny shrew

Shrew

A shrew or shrew mouse is a small molelike mammal classified in the order Soricomorpha. True shrews are also not to be confused with West Indies shrews, treeshrews, otter shrews, or elephant shrews, which belong to different families or orders.Although its external appearance is generally that of...

, 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 whale

Whale

Whale is the common name for various marine mammals of the order Cetacea. The term whale sometimes refers to all cetaceans, but more often it excludes dolphins and porpoises, which belong to suborder Odontoceti . This suborder also includes the sperm whale, killer whale, pilot whale, and beluga...

s have very small brains compared to their weight, and small rodent

Rodent

Rodentia is an order of mammals also known as rodents, characterised by two continuously growing incisors in the upper and lower jaws which must be kept short by gnawing....

s 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

Stephen Jay Gould

Stephen Jay Gould was an American paleontologist, evolutionary biologist, and historian of science. He was also one of the most influential and widely read writers of popular science of his generation....

noted that if one looks at vertebrates with very low encephalization quotient

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 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.

External links

http://www.tufts.edu/as/wright_center/cosmic_evolution/docs/text/text_cult_3.html
http://www.wsu.edu:8080/~taflinge/mindwork/mawint1.html
a graph of body mass vs. brain mass
"Bligh's Bounty" — Stephen Jay Gould
Stephen Jay Gould
Stephen Jay Gould was an American paleontologist, evolutionary biologist, and historian of science. He was also one of the most influential and widely read writers of popular science of his generation....

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