Written by Lila Cart; Illustration by Daniela Gamba

Not all brains in the animal kingdom are built with the same framework. Rodent and primates are a great example of this. Rodent neurons increase in mass as you compare across the evolutionary tree from small to large rodents. Primates neuron do not change in mass like rodents do. This means that primate have an equal relationship between the number of neurons and brain mass. Primates are more efficient at packing neurons into their brains. A primate brain will have close to double the amount of neurons as a same size rodent brain. Due to the difference in how brains are built if a rodent were to have 86 billion neurons like a human its brain would weigh 99 pounds and its body would be over 100 tons. Check out the Zoo Brains posters for a full illustration.

Imagine taking a cookie recipe, the brain, with chocolate, the neurons. A primate adds in more chocolate chips which creates a larger cookie to stay balanced. A rodent replaces the chocolate chips with chocolate chunks. These chunks take up more space within the same sized cookie compared to the chocolate chips.


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  • Herculano-Houzel, S. (2018, July 18). Zoo Brains [Online interview].
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