Encephalization Quotient [en-sefa-lie-zay-shun] (EQ) 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 measurement than the raw brain-to-body mass ratio, as it takes into account allometric effects (changes in proportion of various parts of an organism as a consequence of growth).
The relationship, expressed as a formula, has been developed for mammals, and may not yield relevant results when applied outside this group. Brain size usually increases with body size in animals. The relationship is not linear, however. Generally, small mammals have relatively larger brains than big ones. 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 elephants being quite intelligent animals.
Several reasons for this trend are possible, one of which is that neural cells have a relative constant size. As an animal’s brain gets larger, the 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. 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.
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 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 behavior.
Mean EQ for mammals is around 1, with carnivorans, cetaceans, and primates above 1, and insectivores and herbivores below. It also puts humans at the top of the list. The relationship between brain-to-body mass ratio and complexity of behavior is not perfect as other factors also influence intelligence, like the evolution of the recent cerebral cortex and different degrees of brain folding, which increase the surface of the cortex, which is positively correlated to intelligence in humans.
Dolphins have the highest brain-to-body weight ratio of all cetaceans. Manta rays have the highest for a fish, and either octopuses, or jumping spiders have the highest for an invertebrate. 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, shows it had an EQ well above the reptilian range, and just below that of living birds. In the essay ‘Bligh’s Bounty,’ Stephen Jay Gould noted that if one looks at vertebrates with very low encephalization quotients, 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 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 criticized by a very simple argument: the brains of large dinosaurs were frequently tiny. Stegosaurus, 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 behavioral 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.