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. Name Neurons in the brain & whole nervous system Synapses Details Image Source Sponge 0 Sherwood L, Klandorf H and Yancey P (2012) Animal Physiology: From Genes to Organisms Cengage Learning, p. 150. . Trichoplax 0 Despite no nervous system, it exhibits coordinated feeding and response behaviors. Asplanchna brightwellii (rotifer) Brain only Tardigrade Brain only Ciona intestinalis larva (sea squirt)|| || (central nervous system only)
| Caenorhabditis elegans (roundworm) First organism with its whole connectome (neuronal "wiring diagram") completed. Starfish ring of neurons surrounding the mouth Hydra vulgaris (fresh-water polyp) Megaphragma mymaripenne Box jellyfish – Adult Tripedalia cystophora (8 mm diameter)Does not include 1000 neurons in each of the four rhopalia Medicinal leech Pond snail California sea hare Amphioxus central nervous system only Leptopilina Larval zebrafish Fruit fly Another organism with the connectome mapped, 139,255 neurons in the female brain and ~20,000 neurons in the ventral nerve cord. Wandering spider Calliopsis (bee) Ant Varies per species Perdita (bee)MelissodesBombus impatiensWestern honey bee Honey bee Cockroach Coconut crabc. 1,000,000 interneurons are dedicated to analysing olfactory input alone.California carpenter beeSteudner's dwarf geckoBrown anoleMochlus sundevalliiPeloponnese slowwormCommon house geckoTakydromus sexlineatusAnolis cristatellusPapua snake lizard Guppy Natal Midlands dwarf chameleonAcontias percivaliSand lizardOcelot geckoDarevskia raddeiAnolis evermanniEchis carinatusCerastes cerastesTenerife geckoDraco sumatranusBlue-throated keeled lizardCrested geckoLacerta strigataSan Francisco garter snakeRed-eyed crocodile skinkEmoia cyanuraEast African spiny-tailed lizardChalcides ocellatusCylindrophis ruffusCat geckoAspidoscelis deppiiBrown water snakeAspidelaps lubricusScincus scincusMontivipera xanthinaHispaniolan curlytail lizardSceloporus malachiticusCrotalus durissusAgama agamaAgama aculeataGekko kuhliLeopard geckoLatastia longicaudataRazor-backed musk turtleGreek tortoisePhelsuma grandisAcanthocercus atricollisTokay geckoRussian tortoiseMarginated tortoisePsammophis elegansXenopeltis unicolorZonosaurus KarsteniOplurus quadrimaculatusMalagasy giant chameleonAhaetulla prasinaDasia olivacea Adult zebrafish cells (neurons + other) Timon tangitanusCorn snakeAcanthosaura capraGallotia gallotiEutropis multifasciataRainbow boaEast African black mud turtleIndochinese spitting cobraCyrtodactylus irianjayaensisUromastyx ornataYellow-throated plated lizardMeller's chameleonTwist-necked turtleWestern whiptailGehyra voraxWhite-lipped mud turtlePainted wood turtleSheltopusikStanding's day geckoCommon snapping turtlePainted turtleOcellated lizardOplurus cuvieriAfrican spurred tortoiseBoa imperatorRed-eared sliderPlumed basiliskRed-bellied short-necked turtle Frog Corallus hortulanaBlue-tongued skinkGran Canaria giant lizardChinese water dragonChlamydosaurusNorthern snake-necked turtleSpiny-tailed monitorBoa constrictorAmeiva ameivaChinese softshell turtleAmboina box turtleAlligator snapping turtleYellow anacondaMexican beaded lizardMajor skinkSudan plated lizardMata mataNaked mole-ratWeber's sailfin lizardGreen iguanaArgentine black and white teguCyclura nubilaGold teguVaranus macraeiLittle free-tailed batSmoky shrew Sand goannaShort-tailed shrew Pig-nosed turtleCommon bent-wing batAfrican sheath-tailed batHottentot golden moleWoermann's batZenker's fruit batCommerson's roundleaf batRufous trident bat House mouse Asian water monitorKing quailNile crocodile Heart-nosed batDiamond dove Golden hamster Short-palated fruit batAnsell's mole-ratMashona mole-ratCommon quailHairy-tailed moleEastern rock elephant shrewStar-nosed mole Zebra finch Brain only African collared doveSilvery mole-ratNorthern bobwhiteFour-toed elephant shrewEurasian blackcapWahlberg's epauletted fruit batGoldcrestCape mole-ratGrey partridgeEgyptian fruit batMechow's mole-ratDamaraland mole-ratFranquet's epauletted fruit bat Brown rat Guyenne spiny ratEastern mole Elegant crested tinamouRed junglefowlGreat titGreen-rumped parrotletQuebracho crested tinamouGuinea pig Chilean tinamouGray mouse lemurCommon wood pigeonCommon treeshrew Hammer-headed bat Wood duckPigeon Brain only BudgerigarPygmy falconCape dune mole-ratMallardEurasian tealCommon blackbirdMarbled duckReeves's pheasantEurasian sparrowhawkFerretCockatielGray squirrelBanded mongoosePrairie dogRed-breasted gooseCommon starlingWild turkeyEuropean rabbit Octopus Bigfin reef squid Western tree hyraxEurasian pygmy owlIndian peafowlVictoria crowned pigeonLittle owlCommon kestrelCommon marmoset Eastern rosellaYellow-knobbed curassowBarn owlMonk parakeetGreylag gooseAzure-winged magpieRock hyrax Cat Black-rumped agouti MagpieCommon hill mynaNorthern greater galago Western jackdawLong-eared owlMute swanCommon buzzardBlack vultureGreater rheaEurasian jayAlexandrine parakeetRaccoon dogTanimbar corellaSaker falconEmuThree-striped night monkey Rook Grey parrotTawny owlCapybara Common ostrichWhite-tailed eagleJackalSnowy owlEurasian eagle-owlFoxSulphur-crested cockatooRaccoonKeaRaven Brain only Domestic pigDogDog (Chihuahua)Dog (Chow Chow)Dog (Yorkshire Terrier)SpringbokDog (West Highland White Terrier)BlesbokBlue-and-yellow macaw Brain only Common squirrel monkey Crab-eating macaqueDog (German Shepherd)Tufted capuchin Bonnet macaqueStriped hyenaDog (Komondor)Dog (Transylvanian Hound)LionGreater kuduRhesus macaque Brown bearGiraffeYellow baboon Chimpanzee Orangutan Gorilla Common minke whale Human Neurons for average adult Short-finned pilot whale African elephant
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.