Brain, Primate

Relative Brain Size

Primates tend to have relatively larger brains than most other terrestrial mammals. This would lead one to assume that mammals with larger brains are more intelligent. However, this is not always the case. For instance, if one believed that absolute brain size, used alone, is a good predictor of intelligence, then one would find themselves less intelligent than the elephant, who has a brain 4times larger than humans. Another way to investigate the size of the brain is by applying a simple ratio: brain weight over body weight. As body weight increases, so does the weight of other organs in the body, such as the heart, liver, intestines, and, of course, the brain. This happens because larger bodies require larger organs to meet their everyday energy demands. Although this simple ratio seems like a good way to measure potential cognitive abilities, it also has its pitfalls. Take, for instance, the squirrel monkey, which has a brain/body weight ratio of 0.032, and compare it with the human brain/body weight ratio of 0.020. If this ratio were used as a measure of potential intelligence, then squirrel monkeys would defeat modern humans in intellectual capacity. The main problem with this ratio is that the size of the brain increases at a slower rate than one's body weight. Therefore, the larger the mammal, the smaller the brain/body weight ratio.

The way around this impasse is to use a regression line graph. It was in 1885 that Charles Darwin's cousin Francis Galton invented this idea of a regression line to illustrate the average tendency within a population. This technique enables researchers to overcome the problem of comparing the human brain with that of a smaller primate. On this graph, you would find the weight of the body on the X-axis and the weight of the brain on the Y-axis. Once these points have been illustrated on the graph, a “regression” line can be drawn through these points to represent the trend among the primate species. This line allows researchers to compare the brain of a human with that of a theoretical primate of the same weight. This method along with comparisons of anatomical structures has been very helpful in comparing the brain of a human with that of other primates. These methods of analysis have enabled researchers to determine that the brain of a human is 3 times larger than the brain of a chimpanzee, whose body weight is approximately the same.

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