The elephant brain in numbers.

Suzana Herculano-Houzel,Kleber Neves,Josã© Maldonado,Paul R Manger,Jairo Porfã­Rio,Kamilla Avelino-De-Souza,Larissa Mattos Feijã³,Dã©Bora Messeder

doi:10.3389/fnana.2014.00046

Abstract

What explains the superior cognitive abilities of the human brain compared to other, larger brains? Here we investigate the possibility that the human brain has a larger number of neurons than even larger brains by determining the cellular composition of the brain of the African elephant. We find that the African elephant brain, which is about three times larger than the human brain, contains 257 billion (109) neurons, three times more than the average human brain; however, 97.5% of the neurons in the elephant brain (251 billion) are found in the cerebellum. This makes the elephant an outlier in regard to the number of cerebellar neurons compared to other mammals, which might be related to sensorimotor specializations. In contrast, the elephant cerebral cortex, which has twice the mass of the human cerebral cortex, holds only 5.6 billion neurons, about one third of the number of neurons found in the human cerebral cortex. This finding supports the hypothesis that the larger absolute number of neurons in the human cerebral cortex (but not in the whole brain) is correlated with the superior cognitive abilities of humans compared to elephants and other large-brained mammals.

Highlights

What explains the superior cognitive abilities of the human brain, with richly complex and flexible behaviors, compared to other brains (Premack, 2007), some even larger than ours (Roth and Dicke, 2005)? Neuroanatomical correlates have been sought in total brain mass, cerebral cortical mass and cortical folding index, but these values are smaller in the human brain than in several other species
We find that the brain of the African elephant, weighing 4618.6 g, about 3 times heavier than the human brain, holds a total of 257.0 billion neurons, 3 times more than the average of 86 billion neurons found in the human brain (Azevedo et al, 2009)
The rest of the brain structures, excluding the cerebral cortex and cerebellum, holds as many neurons in the elephant brain as in the human brain: 0.7 billion neurons, despite being 4 times larger in the elephant brain (564.7 g compared to 117.7 g in the human brain)

Summary

Introduction

What explains the superior cognitive abilities of the human brain, with richly complex and flexible behaviors, compared to other brains (Premack, 2007), some even larger than ours (Roth and Dicke, 2005)? Neuroanatomical correlates have been sought in total brain mass, cerebral cortical mass and cortical folding index, but these values are smaller in the human brain than in several other species (reviewed in Herculano-Houzel, 2011a). To smaller-brained species such as rodents, the human brain has a much larger number of neurons, both in the cerebral cortex and in the cerebellum (Herculano-Houzel, 2012); testing the hypothesis that absolute numbers of neurons correlate with cognitive abilities across species, including humans, requires determining the cellular composition of brains larger than the human brain.

Results

Conclusion