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
A recent study performed in the tropical rain forests of Western Africa has generated a lot of interest and discussion into ancient chimpanzee cultures. The excavation of stone tools dating back 4,300 years has changed a lot of preconceived notions about how chimps acquired this food gathering technique. Modern chimps use large, heavy granite stones to pummel Panda nuts to extract the fruit within the hard exterior shell. The history of chimps performing this task previously dated back only to the 19th century, so many experts believed that this technique was developed from chimps imitating humans. The new stones, excavated by a team led by Julio Mercader of the University of Calgary, predate any known human inhabitants of Noulo at Cote d'Ivoire in the Ivory Coast's Tai National Park. Researchers had previously theorized that chimps learned this technique from ancient humans, and this imitation was passed down through generations. If chimps manufactured and used tools without the presence of humans, it can now be theorized that chimps either developed this idea independently as a result of independent technological convergence, or chimps may share a common ancestor with early humans that first learned this pummelling technique.
The data presented by the authors dealt with variations in the timing and magnitude of the breeding migrations. The size and age structure of breeding adults as well as the production of juveniles were also observed. Mortality rates were recorded from the metamorphic stages to adulthood. The data was collected over a seven-year time period in Carmel Valley located in Monterey County.
By sequencing the mitochondrial DNA hypervariable regions (HVRs) of the control region of modern and fossilized tuataras and radiodating the fossils examined, the group calculated an evolution rate of 1.56 (0.83-2.34) substitutions per nucleotide per million years. This is the highest rate seen among the 10 animal species studied to-date using similar statistical techniques.
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