There is a large variation in brain size among vertebrates and the relationship between brain size and brain complexity has always been a fascinating topic of study. What is meant by the term encephalization quotient (EQ)?
What information does EQ provide, what are two problems with using EQs, and is there a better way to address the questions of brain complexity?
What will be an ideal response?
Answer: EQ is the ratio of the observed brain mass to the expected brain mass. When we compare EQ quotients, humans have the largest EQs, followed by some cetaceans such as dolphins, and then various species of primates. Social animals and carnivores have higher EQs than solitary and herbivorous animals. These findings suggest that EQs provide some information about cognitive capacity, and in general, behavioral complexity correlates well with EQ. Using EQs becomes problematic because the values obtained depends on the taxa used to obtain the regression line. The data used to generate the regression is also problematic because each point in the regression represents one entire species. As we know, there is much variation in body mass of different individuals in a species. To further complicate matters, even the mass of individuals can change dramatically over time, thus changing the EQ. Recent research by Dr. Herculano-Houzel uses a technique, which directly measures the number of neurons in the brain. The nuclei of brain cells are stained, which allows one to distinguish nuclei of neurons from non-neural glial cells. Dr. Herculano-Houzel's results comparing rodents and primates with similar-sized brains reveals that primate brains have more neurons per unit area, which may explain the higher cognitive capacity of primates. In addition, more neurons per unit area may also facilitate the formation of more synapses. Thus, counting neuron numbers holds some promise to potentially provide more knowledge about brain cognition and complexity.
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