Brain mass and total number of neurons for the mammalian species examined so far with the isotropic fractionator.
Brains are arranged from left to right, top to bottom, in order of increasing number of neurons according to average species values from Herculano-Houzel et al., 2006 (rodents),Herculano-Houzel et al., 2007 (non-human primates), Sarko et al., 2009 (insectivores) and Azevedo et al., 2009 (human brain). Rodent brains face right, primate brains face left, insectivore brains can be identified in the figure by their bluish hue (due to illumination conditions). All images shown to the same scale. Primate images, except for the capuchin monkey and human brain, from the University of Wisconsin and Michigan State Comparative Mammalian Brain Collections (www.brainmuseum.org ). Insectivore images kindly provided by Diana Sarko, and human brain image by Roberto Lent. Rodent images from the author. Notice that some rodent brains, such as the agouti and the capybara, contain fewer neurons than primate brains that are smaller than them.
The image above is part of a review article written by one of the scientists that developed isotropic fractionation, a quantitative method that determines the cellular composition in the brain in humans, nonhuman primates and rodents. This method produces cell counts derived from tissue homogenates from anatomically defined brain regions (Herculano-Houzel and Lent, 2005 ).
Herculano-Houzel, S. (2009). The human brain in numbers: a linearly scaled up primate brain. Frontiers in Human Neuroscience 3 (31).
Herculano-Houzel, S., and Lent, R. (2005). Isotropic fractionator: a simple, rapid method for the quantification of total cell and neuron numbers in the brain. J. Neurosci. 25, 2518–2521.