Cellular scaling rules for primate brains

Abstract

Primates are usually found to have richer behavioral repertoires and better cognitive abilities than rodents of similar brain size. This finding raises the possibility that primate brains differ from rodent brains in their cellular composition. Here we examine the cellular scaling rules for primate brains and show that brain size increases approximately isometrically as a function of cell numbers, such that an 11؋ larger brain is built with 10؋ more neurons and Ϸ12؋ more nonneuronal cells of relatively constant average size. This isometric function is in contrast to rodent brains, which increase faster in size than in numbers of neurons. As a consequence of the linear cellular scaling rules, primate brains have a larger number of neurons than rodent brains of similar size, presumably endowing them with greater computational power and cognitive abilities.allometry ͉ brain size ͉ evolution ͉ number of neurons ͉ number of glia B rain size varies by as much as 100,000ϫ across mammalian species (1, 2), and a large number of comparative studies have concentrated on finding the shared regularities behind brain morphology and cellular composition across species of different brain sizes (1,3,4). Although any such regularities may reveal general principles underlying the development and evolution of the brain, one must keep in mind that major differences across orders may also exist. Because cellular composition of the brain is one of the major determinants of its computational capacities (5), species belonging to different orders, having different cognitive abilities, and possessing brains of similar sizes would be expected to differ in cellular composition.We recently described the cellular scaling rules that apply to rodent brains from the mouse to the capybara (6). In that work, we showed that the rodent brain scales hypermetrically as a function of its numbers of neurons, and that the average neuronal size is larger in larger brains, whereas the average nonneuronal cell size remains comparatively stable. As in previous reports (1-3, 7-13), neuronal density decreases and the glia/neuron ratio increases with increasing brain size. We also showed that the ratio of total neuronal mass/total nonneuronal mass remains constant across rodent species, and we offered an explanation for how this ratio could be achieved during development (6).These results prompted us to investigate whether the same scaling rules apply to other mammalian orders. Our aim was to establish what rules are shared among mammalian brains, and thus might reflect characteristics inherited from a common ancestor, and what rules differ across orders of mammals, and thus might account for phylogenetic variance across groups. We were particularly interested in cellular scaling differences that might have arisen in primates. If the same rules relating numbers of neurons to brain size in rodents (6) also applied to primates, a brain comparable to ours, with Ϸ100 billion neurons, would weigh Ͼ45 kg and belong to a body of 109 tons, about the mass of the heavies...

The evolution of primate general and cultural intelligence

“…In common with many human scientists and neuroscientists [1,9,11,65], we note that the mechanisms behind the brain volume-cognition relationship are unknown and require study. It is important to establish if, how and why characteristics such as brain cell composition, connectivity, numerosity and diversity are linked to brain volume, why these characteristics vary across brain regions, how this variation is linked to cognitive function and whether common links between structure and function are found across taxa [65].…”

“…It is important to establish if, how and why characteristics such as brain cell composition, connectivity, numerosity and diversity are linked to brain volume, why these characteristics vary across brain regions, how this variation is linked to cognitive function and whether common links between structure and function are found across taxa [65]. Adequate tests of cognitive function will be essential for this exercise.…”

Mice selectively bred for high voluntary wheel running have larger midbrains: support for the mosaic model of brain evolution

“…For example, in rodents, neuronal cell size increases with brain size, such that neuronal density is lower in larger-brained species (albeit with a greater absolute number of neurons) (Herculano-Houzel et al, 2006). Conversely, in primates, neuronal cell size is constant and neuron density is constant, such that total neuron number is much greater with increasing brain size (Herculano-Houzel et al, 2007). Therefore, an increase in the size of a brain region could be the product of a variety of cellular changes (e.g.…”

Decreasing sleep requirement with increasing numbers of neurons as a driver for bigger brains and bodies in mammalian evolution

“…Here I examine this second prediction that total sleep duration decreases together with neuronal density mm 22 across mammalian species and in their development. To this end, I present an analysis of a set of 24 species belonging to six mammalian clades for which cortical numbers of neurons, neuronal density and surface area were available [18][19][20][21][22][23][24][25][26][27][28][29], as well as data for total number of sleep hours per day [7]. Data are provided in table 1.…”