Researchers graphed these two characteristics to find a general mathematical relationship to study and examine the correlation between brain size and body weight. This graph shows the relationship between these two characteristics for mammals. 

University of California, Davis

Brain size varies among species and is mostly relative to the general size of the animal. It is not an automatic indicator of behavioral capability. 

Brain Size 

Breedlove, S. & Watson, N. (2017). Behavioral neuroscience, eighth edition. Oxford University Press.

Behavioral Neuroscience 8th Edition

The mathematical measure of brain size relative to body size; the greater this value, the "more brain" a species has for their size. 

Encephalization Factor  

There is a link between the increased size of specific brain structures and the necessity of the behavior/capability it is responsible for in some species (i.e., those that rely heavily on their sense of smell possess a relatively large olfactory bulb). Likely occur through the process of natural selection favoring individuals that adapted to improve that capability.

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