Encephalization Quotient - Brain-body Size Relationship

Brain-body Size Relationship

Brain size usually increases with body size in animals (is positively correlated); 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 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: 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 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.