Friday, April 11, 2008

On Cellular Scaling Rules for Primate Brains

ResearchBlogging.orgStudent 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.Marmoset.jpg

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?


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


Anonymous April 13, 2008 at 10:00 PM  

What do you want to know about reptiles or amphibians? What their volume/neuron ratio is? I do not see why they wouldn't be able to do that in any species containing a central plexus of neurons. They used several species in the study. I would say, it is obviously applicable to humans. The study was conducted on a primate brain and we have a primate brain too.

Heidi Rivera April 14, 2008 at 12:24 PM  

What I want to know about reptiles or amphibians is whether this cellular scaling model can be applied to other organisms such as reptiles or amphibians; specifically organisms other than mammals? You said, "I do not see why they wouldn't be able to do that in any species containing a central plexus of neurons." This is exactly what I'm wondering. Is it really applicable to ANY species? With the different anatomical and physiological differences among a vast variety of species, I have to question whether or not it's that widely applicable.

Anonymous April 15, 2008 at 12:52 AM  

Can the technique be used? The technique, yes. All they did was measure the number of neurons and weight of the brains for the species. The study did not take into account the anatomical and physiological differences of rodent vs. primate brain. It was purely a cytological study to determine size of neurons and number of neurons, as well as the weight of the respective brains. Any species can have the number of the neurons in the brain counted, and their brain weighed. The question to ask is, would the results mean anything? If for instance, a reptile brain has the same scaling ratio as a human brain, then that would suggest our computational differences cannot be accounted for by this model (which it already doesn't because a reptile brain is functionally equivalent to our brainstem). But, the technique can of course be applied to any brain. It sounded like asked if the technique of the experiment could be used on other organisms, which it could. But the conclusions may not be significant.


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