Encephalization quotient
Encyclopedia
Species Encephalization quotient (EQ)
Human
Human
Humans are the only living species in the Homo genus...

7.4-7.8
Bottlenose dolphin
Bottlenose Dolphin
Bottlenose dolphins, the genus Tursiops, are the most common and well-known members of the family Delphinidae, the family of oceanic dolphins. Recent molecular studies show the genus contains two species, the common bottlenose dolphin and the Indo-Pacific bottlenose dolphin , instead of one...

4.14
Orca
Orca
The killer whale , commonly referred to as the orca, and less commonly as the blackfish, is a toothed whale belonging to the oceanic dolphin family. Killer whales are found in all oceans, from the frigid Arctic and Antarctic regions to tropical seas...

2.57-3.3
Chimpanzee
Chimpanzee
Chimpanzee, sometimes colloquially chimp, is the common name for the two extant species of ape in the genus Pan. The Congo River forms the boundary between the native habitat of the two species:...

2.2-2.5
Rhesus monkey 2.1
Elephant
Elephant
Elephants are large land mammals in two extant genera of the family Elephantidae: Elephas and Loxodonta, with the third genus Mammuthus extinct...

1.13-2.36
Dog
Dog
The domestic dog is a domesticated form of the gray wolf, a member of the Canidae family of the order Carnivora. The term is used for both feral and pet varieties. The dog may have been the first animal to be domesticated, and has been the most widely kept working, hunting, and companion animal in...

1.2
Cat
Cat
The cat , also known as the domestic cat or housecat to distinguish it from other felids and felines, is a small, usually furry, domesticated, carnivorous mammal that is valued by humans for its companionship and for its ability to hunt vermin and household pests...

1.00
Horse
Horse
The horse is one of two extant subspecies of Equus ferus, or the wild horse. It is a single-hooved mammal belonging to the taxonomic family Equidae. The horse has evolved over the past 45 to 55 million years from a small multi-toed creature into the large, single-toed animal of today...

0.9
Sheep 0.8
Mouse
Mouse
A mouse is a small mammal belonging to the order of rodents. The best known mouse species is the common house mouse . It is also a popular pet. In some places, certain kinds of field mice are also common. This rodent is eaten by large birds such as hawks and eagles...

0.5
Rat
Rat
Rats are various medium-sized, long-tailed rodents of the superfamily Muroidea. "True rats" are members of the genus Rattus, the most important of which to humans are the black rat, Rattus rattus, and the brown rat, Rattus norvegicus...

0.4
Rabbit
Rabbit
Rabbits are small mammals in the family Leporidae of the order Lagomorpha, found in several parts of the world...

0.4
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...

0.18


Encephalization Quotient (EQ), 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
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...

 of a given size, which is hypothesized to be a rough estimate of the intelligence of the animal.

This is a more refined measurement than the raw brain-to-body mass ratio, as it takes into account allometric effects. The relationship, expressed as a formula, has been developed for mammals, and may not yield relevant results when applied outside this group.

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. Generally, small mammals have relatively larger brains than big ones. 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 ratio similar to humans (1/40), while elephants have comparatively small brain/body size (1/560), despite elephants being obviously quite intelligent animals.

There are possibly several reasons for this trend, but one of them is that neural cells
Neuron
A neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form networks. Neurons are the core components of the nervous...

 have a relative constant size. As an animal's brain gets larger, addition of 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. Thus 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. To compensate for this factor, a formula has been devised by plotting the brain/body weight of various mammals against each other and a curve fitted so as to give best fit to the data.

The formula for the curve varies, but is usually given as Ew(brain) = 0.12w(body)2/3. As this formula is based on data from mammals, it should be applied to other animals with caution. For some of the other vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...

 classes the power of 3/4 rather than 2/3 is sometimes used, and for many groups of invertebrates the formula may give no meaningful results at all.

Intelligence in animals is hard to establish, but the larger the brain is relative to the body, 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. The EQ formula, as opposed to the method of simply measuring raw brain weight or brain weight to body weight, makes for a ranking of animals that coincide better with observed complexity of behaviour. Mean EQ for mammals is around 1, with carnivora
Carnivora
The diverse order Carnivora |Latin]] carō "flesh", + vorāre "to devour") includes over 260 species of placental mammals. Its members are formally referred to as carnivorans, while the word "carnivore" can refer to any meat-eating animal...

ns, cetacea
Cetacea
The order Cetacea includes the marine mammals commonly known as whales, dolphins, and porpoises. Cetus is Latin and is used in biological names to mean "whale"; its original meaning, "large sea animal", was more general. It comes from Ancient Greek , meaning "whale" or "any huge fish or sea...

ns and primates above 1, and insectivores
Insectivora
The order Insectivora is a now-abandoned biological grouping within the class of mammals...

 and herbivore
Herbivore
Herbivores are organisms that are anatomically and physiologically adapted to eat plant-based foods. Herbivory is a form of consumption in which an organism principally eats autotrophs such as plants, algae and photosynthesizing bacteria. More generally, organisms that feed on autotrophs in...

s below. It also puts humans at the top of the list. 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 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....

 to intelligence in humans.

Comparisons between groups

Dolphin
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. Manta ray
Manta ray
The manta ray is the largest species of the rays. The largest known specimen was more than across, with a weight of about . It ranges throughout waters of the world, typically around coral reefs...

s have the highest for a 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...

, and 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...

. Despite the jumping spider having a huge brain for its size, it is minuscule in absolute terms, and humans have a much higher EQ, despite having a lower raw brain-to-body weight ratio. 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, possibly due to lower thermoregulation and/or motor control demands. Estimation of brain size in the oldest known bird, Archaeopteryx
Archaeopteryx
Archaeopteryx , sometimes referred to by its German name Urvogel , is a genus of theropod dinosaur that is closely related to birds. The name derives from the Ancient Greek meaning "ancient", and , meaning "feather" or "wing"...

, shows it had an EQ in the upper reptilian range, but below that of living birds.
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


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
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...

 quotient
Quotient
In mathematics, a quotient is the result of division. For example, when dividing 6 by 3, the quotient is 2, while 6 is called the dividend, and 3 the divisor. The quotient further is expressed as the number of times the divisor divides into the dividend e.g. The quotient of 6 and 2 is also 3.A...

s, 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 whole brain size is a better measure of cognitive abilities than EQ for primates at least.

The concept of EQ as a measure of intelligence can be strongly criticised in a very simple argument. The brains of large dinosaurs were frequently tiny. Stegosaurus
Stegosaurus
Stegosaurus is a genus of armored stegosaurid dinosaur. They lived during the Late Jurassic period , some 155 to 150 million years ago in what is now western North America. In 2006, a specimen of Stegosaurus was announced from Portugal, showing that they were present in Europe as well...

, weighing about the same as an average elephant, had a comparatively small brain—160 g compared to about 5 kg for an elephant. While Stegosaurus undoubtedly was an animal of very limited behavioural complexity, this fact undermines the idea on which EQ is based - that a larger animal requires a larger brain to look after a large body.

If Stegosaurus could survive with this tiny brain, it would seem that any animal with anything bigger must be using it for non-essential abilities. However, mammalian evolution has repeatedly improved the effectiveness of a bodily function by innervating it more; the digestive and immune systems are examples. Thus, while an elephant has a much larger brain than a Stegosaurus, a substantial part of the excess brain is bound up in bodily functions rather than cognitive functions. This can account for some differences between classes of animals, but not species within a class. More associative brain tissue, cortex, still indicates a level of mental activity above the reptilian form. Some of these abilities may be sensory and/or physical, and some may be intellectual. The actual intelligence of an animal therefore depends on the size of the brain and the proportion of the brain that is used for intellectual abilities, rather than advanced sensory or physical skills. Critics point out that EQ gives only a very rough estimate of these proportions.

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....

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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