On Cellular Scaling Rules for Primate Brains

Student post submitted by Heidi K. Rivera.

Why are primates smarter than rodents with similar-sized brains? To get the full explanation read this paper. Or, in short, read on. This paper analyzed the cellular scaling rules of primate brain sizes and cellular composition across six species of primates (marmoset (pictured here), galagos, owl monkey, squirrel monkey, rhesus monkey and the tree shrew). In a previous paper, the authors described the cellular scaling rules that apply to rodent brains. That research showed that rodent brain scales hypermetrically as a function of its numbers of neurons and that the average of neuronal size is bigger in larger brains, while the average nonneuronal cell size remains relatively stable.

The results of that study encouraged the scientists to extend their research to other mammalian species. Would this reflect the characteristics or traits inherited from a common ancestor? This begged the question, “what rules differ across orders of mammals, and thus might account for phylogenetic variance across groups?” After doing some calculations, they found that if the same cellular scaling rules for rodents applied to primate brains, “a brain comparable to ours, with approximately 100 billion neurons, would weigh >45 kg and belong to a body of 109 tons, about the mass of the heaviest living mammal, the blue whale!” Realizing this obviously indicated that the cellular scaling rules differ between rodents and primates. This is also supported by the fact that it is know that rodents and primates have very different cognitive abilities even when they have a similar brain size. The main difference in the cellular scaling rules for building rodent and primate brains is that increased numbers of neurons in primates are not accompanied by decreased neuronal densities, indicating that the average neuronal cell size remains stable across primate species. After completing their research, they found that primate brain sizes increase isometrically with body size across primate species. Primate brains increase in size as a linear function while rodent brains hyperscale as they gain neurons. This suggests that, “there has been a selective pressure against increase in average neuronal size with brain size.” This type of increase allowed primate brains to accumulate large numbers of neurons without becoming prohibitively large. If the rodent cellular scaling model applied to say, the macaque brain, which has approximately 6.4 billion neurons, would weigh about 575g instead of it’s actual weight of 87g! “These findings suggest that the divergence of primate evolution away from the common ancestor with rodents involved mechanisms that favored the concentration of larger numbers of neurons per unit volume of brain tissue.” The larger number of neurons per unit volume apparently provides primates with a larger computational capacity than rodent brains of the same size. This answers the question of why primates are smarter than rodents with similar-sized brains. They concluded with possibly applying the research found from both the rodent paper and this paper to apes and humans. My question is whether this “model” of cellular scaling and cellular composition can be applied to other species. What about reptiles or amphibians? What do you think?

Reference:

Herculano-Houzel, S., Collins, C.E., Wong, P., Kaas, J.H. (2007). Cellular scaling rules for primate brains. Proceedings of the National Academy of Sciences, 104(9), 3562-3567. DOI: 10.1073/pnas.0611396104