Most large animals economize by cutting back on neuron density. By contrast, a rodent with a human sized brain, if it followed rodent rules for how neuron numbers increase with brain size, would have only 1/7 as many neurons. When you combine the exceptionally large brain sizes of humans with a standard high primate neuron density, you get an animal with an enormous number of neurons. We have about the same density of neurons as other primates. An x-fold increase in primate brain size corresponds to about an x-fold increase in number of neurons. Increase number of neurons by x and you increase brain mass by x 1.5.) But primates are exceptional the relationship is nearly linear. Increase brain size by x, and you increase number of neurons by about x. Major findings: Among most mammals, the number of neurons increases more slowly than brain size. This allows her to estimate how many neurons there are in different brains. Susan Herculano-Houzel has pioneered a technique that involves chopping up brains (or parts of brains), dissolving their cells to make a kind of brain soup, and counting cell nuclei. They have larger brains than would be expected based on their body sizes.īut we’ve recently learned that primates – especially big ones – are even more special than this graph suggests. The thing to note is that primates defy expectations. 10,000 times the body mass means 1000 times the brain mass. That means that if one mammal has 16 times the body mass of another, it’s expected to have 8 times the brain mass. So Log plotted against Log gives a straight line with a slope of. Equivalently (taking the logarithm of both sides) Log is equal to.
In other words, Brain Mass is proportional to (Body Mass). Instead it multiplies brain mass by about x.
Multiplying body mass by x doesn’t multiply brain mass by x. Larger mammals tend to have larger brains, but the relationship is non-linear. Here’s the conventional graph illustrating that: One reason for being interested in monkeys is that they’re brainy mammals.
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