The following are two lists of animals ordered by the size of their nervous system. The first list shows number of neurons in their entire nervous system. The second list shows the number of neurons in the structure that has been found to be representative of animal intelligence. The human brain contains 86 billion neurons, with 16 billion neurons in the cerebral cortex. Neuron counts constitute an important source of insight on the topic of neuroscience and intelligence: the question of how the evolution of a set of components and parameters (~1011 neurons, ~1014 synapses) of a complex system leads to the phenomenon of intelligence.

Overview

Neurons are the cells that transmit information in an animal's nervous system so that it can sense stimuli from its environment and behave accordingly. Not all animals have neurons; Trichoplax and sponges lack nerve cells altogether. Neurons may be packed to form structures such as the brain of vertebrates or the neural ganglions of insects. The number of neurons and their relative abundance in different parts of the brain is a determinant of neural function and, consequently, of behavior.

Whole nervous system

All numbers for neurons (except Caenorhabditis and Ciona), and all numbers for synapses (except Ciona) are estimations.

Forebrain (cerebrum or pallium) only

Proxies for animal intelligence have varied over the centuries. One early suggestion was brain size (or weight, which provides the same ordering.) A second proposal was brain-to-body-mass ratio, and a third was encephalization quotient, sometimes referred to as EQ. The current best predictor is number of neurons in the forebrain, based on Herculano-Houzel's improved neuron counts. This accounts for variation in the number of neurons in the rest of the brain, for which no link to intelligence has been established. Elephants, for example, have an exceptionally large cerebellum, while birds make do with a much smaller one. Differing methods have been used to count neurons, and these may differ in degree of reliability. The primary methods are the optical fractionator, an application of stereology and the isotropic fractionator, a recent methodological innovation. Most numbers in the list are the result of studies using the newer isotropic fractionator. A variation of the optical fractionator was responsible for the previous total human brain neuron count of 100,000,000,000 neurons, which has been revised down to 86,000,000,000 by the use of the isotropic fractionator. This is in part why it may be considered to be less reliable. Finally, some numbers are the result of estimations based on correlations observed between number of cortical neurons and brain mass within closely related taxa. The following table gives information on the number of neurons estimated to be in the sensory-associative structure: the cerebral cortex (aka pallium) for mammals, the dorsal ventricular ridge ("DVR" or "hypopallium") of the pallium for birds, and the corpora pedunculata ("mushroom bodies") for insects.