Neurons are the basic computational units of the brain, but brain size is the predominant surrogate measure of brain functional capacity in comparative and cognitive neuroscience. This approach is based on the assumption that larger brains harbor higher numbers of neurons and their connections, and therefore have a higher information-processing capacity. However, recent studies have shown that brain mass may be less strongly correlated with neuron counts than previously thought. Till now, no experimental test has been conducted to examine the relationship between evolutionary changes in brain size and the number of brain neurons. Here, we provide such a test by comparing neuron number in artificial selection lines of female guppies (Poecilia reticulata) with >15% difference in relative brain mass and numerous previously demonstrated cognitive differences. Using the isotropic fractionator, we demonstrate that large-brained females have a higher overall number of neurons than small-brained females, but similar neuronal densities. Importantly, this difference holds also for the telencephalon, a key region for cognition. Our study provides the first direct experimental evidence that selection for brain mass leads to matching changes in number of neurons and shows that brain size evolution is intimately linked to the evolution of neuron number and cognition.

Behavioural Ecology Group Department of Animal Sciences 6708wd Wageningen Netherlands

Department of Zoology Ethology Stockholm University 10691 Stockholm Sweden

Department of Zoology Faculty of Science Charles University 12844 Prague Czech Republic

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Bahney, J. , and von Bartheld C. S.. 2014. Validation of the isotropic fractionator: Comparison with unbiased stereology and DNA extraction for quantification of glial cells. J. Neurosci. Methods 222:165–174. PubMed PMC

Barton, R. A. 2012. Embodied cognitive evolution and the cerebellum. Philos. Trans. R. Soc. B 367:2097–2107. PubMed PMC

Bates, D. , Maechler M., Bolker B., Walker S., Christensen R. H. B., Singmann H., Dai B., Scheipl F., Grothendieck G., Green P., et al. 2015. Package ‘lme4’. Convergence 12:1.

Baumann, O. , Borra R. J., Bower J. M., Cullen K. E., Habas C., Ivry R. B., Leggio M., Mattingley J. B., Molinari M., Moulton E. A., et al. 2014. Consensus paper: The role of the cerebellum in perceptual processes. Cerebellum 14:197–220. PubMed PMC

Benson‐Amram, S. , Dantzer B., Stricker G., Swanson E. M., and Holekamp K. E.. 2016. Brain size predicts problem‐solving ability in mammalian carnivores. Proc. Natl. Acad. Sci. USA 113:2532–2537. PubMed PMC

Bloch, N. I. , Corral‐López A., Buechel S. D., Kotrschal A., Kolm N., and Mank J. E.. 2018. Early neurogenomic response associated with variation in guppy female mate preference. Nat. Ecol. Evol. 2:1772–1781. PubMed PMC

Braithwaite, V. A. 2006. Cognitive ability in fish Pp. 1–37 in Sloman K., Balshine S., and Wilson R., eds. Behaviour and physiology of fish, vol. 24 Associated Press, San Diego, CA.

Buechel, S. D. , Boussard A., Kotrschal A., van der Bijl W., and Kolm N.. 2018. Brain size affects performance in a reversal‐learning test. Proc. R. Soc. B 285:20172031. PubMed PMC

Burns, J. G. , Saravanan A., Helen Rodd F.. 2009. Rearing environment affects the brain size of guppies: Lab‐reared guppies have smaller brains than wild‐caught guppies. Ethology 115:122–133.

Butler, A.B. , and Hodos W.. 2005. Comparative vertebrate neuroanatomy: Evolution and adaptation. John Wiley & Sons, Hoboken, NJ.

Corral‐López, A. , Bloch N. I., Kotrschal A., van der Bijl W., Buechel S. D., Mank J. E., and Kolm N.. 2017a. Female brain size affects the assessment of male attractiveness during mate choice. Sci. Adv. 3:e1601990. PubMed PMC

Corral‐López, A. , Garate‐Olaizola M., Buechel S. D., Kolm N., and Kotrschal A.. 2017b. On the role of body size, brain size, and eye size in visual acuity. Behav. Ecol. Sociobiol. 71:179. PubMed PMC

Dicke, U. , and Roth G.. 2016. Neuronal factors determining high intelligence. Philos. Trans. R. Soc. B 371:20150180. PubMed PMC

Dos Santos, S. E. , Porfirio J., F. B. da Cunha , Manger P. R., Tavares W., Pessoa L., Raghanti M. A., Sherwood C. C., and Herculano‐Houzel S.. 2017. Cellular scaling rules for the brains of marsupials: Not as ‘primitive’ as expected. Brain Behav. Evol. 89:48–63. PubMed

Fong, S. , Buechel, S. D. , Boussard, A. , Kotrschal, A. , & Kolm, N. 2019. Plastic changes in brain morphology in relation to learning and environmental enrichment in the guppy (Poecilia reticulata). J. Exp. Biol. 222 10.1242/jeb.200402 PubMed DOI

Gonda, A. , Herczeg G., and Merilä J.. 2013. Evolutionary ecology of intraspecific brain size variation: A review. Ecol. Evol. 3:2751–2764. PubMed PMC

Gómez, A. , Durán E., Salas C., and Rodríguez F.. 2010. Cerebellum lesion impairs eyeblink‐like classical conditioning in goldfish. Neuroscience 166:49–60. PubMed

Grant, S. G. 2016. The molecular evolution of the vertebrate behavioural repertoire. Philos. Trans. R Soc. Lond. B Biol. Sci. 371:20150051. PubMed PMC

Guay, P. J. , and Iwaniuk A. N.. 2008. Captive breeding reduces brain volume in waterfowl (Anseriformes). Condor 110:276–284.

Güntürkün, O. , and Bugnyar T.. 2016. Cognition without cortex. Trends Cogn. Sci. 20:291–303. PubMed

Herculano‐Houzel, S. 2007. Encephalization, neuronal excess, and neuronal index in rodents. Anat. Rec. 290:1280–1287. PubMed

Herculano‐Houzel, S. 2009. The human brain in numbers: a linearly scaled‐up primate brain. Front. Hum. Neurosci. 3:31. PubMed PMC

Herculano‐Houzel, S. 2010. Coordinated scaling of cortical and cerebellar numbers of neurons. Front. Neuroanat. 4:12. PubMed PMC

Herculano‐Houzel, S. 2017. Numbers of neurons as biological correlates of cognitive capability. Curr. Opin. Behav. Sci. 16:1–7.

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. PubMed PMC

Herculano‐Houzel, S. , Mota B., and Lent R.. 2006. Cellular scaling rules for rodent brains. Proc. Natl. Acad. Sci. USA 103:12138–12143. PubMed PMC

Herculano‐Houzel, S. , Collins C. E., Wong P., and Kaas J. H.. 2007. Cellular scaling rules for primate brains. Proc. Natl. Acad. Sci. USA 104:3562–3567. PubMed PMC

Herculano‐Houzel, S. , Manger P. R., and Kaas J. H.. 2014. Brain scaling in mammalian evolution as a consequence of concerted and mosaic changes in numbers of neurons and average neuronal cell size. Front. Neuroanat. 8:77. PubMed PMC

Herculano‐Houzel, S. , Catania K., Manger P. R., and Kaas J. H.. 2015a. Mammalian brains are made of these: A dataset of the numbers and densities of neuronal and nonneuronal cells in the brain of Glires, Primates, Scandentia, Eulipotyphlans, Afrotherians and Artiodactyls, and their relationship with body mass. Brain Behav. Evol. 86:145–163. PubMed

Herculano‐Houzel, S. , Messeder D. J., Fonseca‐Azevedo K., and Pantoja N. A.. 2015b. When larger brains do not have more neurons: Increased numbers of cells are compensated by decreased average cell size across mouse individuals. Front. Neuroanat. 9:64. PubMed PMC

Hinsch, K. , and Zupanc G. K. H.. 2007. Generation and long‐term persistence of new neurons in the adult zebrafish brain: A quantitative analysis. Neuroscience 146:679–696. PubMed

Horschler, D. J. , Hare B., Call J., Kaminski J., Miklósi Á., and MacLean E. L.. 2019. Absolute brain size predicts dog breed differences in executive function. Anim. Cogn. 22:187–198. PubMed

Houde, A. 1997. Sex, color, and mate choice in guppies. Princeton Univ. Press, Princeton, NJ.

Hwang, L. D. , Strike L. T., Couvy‐Duchesne B., de Zubicaray G. I., McMahon K., Breslin P. A. S., Reed D. R., Martin N. G., and Wright M. J.. 2019. Associations between brain structure and perceived intensity of sweet and bitter tastes. Behav. Brain Res. 363:103–108. PubMed PMC

Jensen, P. 2017. The ethology of domestic animals: An introductory text. Cabi, Boston, MA.

Jerison, H. J. 1973. Evolution of the brain and intelligence. Academic Press, New York, NY.

Kolm, N. , Gonzalez‐Voyer A., Brelin D., and Winberg S.. 2009. Evidence for small scale variation in the vertebrate brain: Mating strategy and sex affect brain size and structure in wild brown trout (Salmo trutta). J. Evol. Biol. 22:2524–2531. PubMed

Kotrschal, A. , Sundström, L.F. , Brelin, D. , Devlin, R.H. , Kolm, N. 2012. Inside the heads of David and Goliath: Environmental effects on brain morphology among wild and growth‐enhanced coho salmon, Oncorhynchus kisutch . J. Fish Biol. 81:987–1002. PubMed

Kotrschal, K. , Van Staaden M. J., and Huber R.. 1998. Fish brains: Evolution and environmental relationships. Rev. Fish Biol. Fisher. 8:373–408.

Kotrschal, A. , Rogell B., Bundsen A., Svensson B., Zajitschek S., Brännström I., Immler S., Maklakov A. A., and Kolm N.. 2013. Artificial selection on relative brain size in the guppy reveals costs and benefits of evolving a larger brain. Curr. Biol. 23:168–171. PubMed PMC

Kotrschal, A. , Corral‐Lopez A., Amcoff M., and Kolm N.. 2014. A larger brain confers a benefit in a spatial mate search learning task in male guppies. Behav. Ecol. 26:527–532. PubMed PMC

Kotrschal, A. , Buechel S. D., Zala S. M., Corral‐Lopez A., Penn D. J., and Kolm N.. 2015. Brain size affects female but not male survival under predation threat. Ecol. Lett. 18:646–652. PubMed PMC

Kotrschal, A. , Zeng H.‐L., van der Bijl W., Öhman‐Mägi C., Kotrschal K., Pelckmans K., and Kolm N.. 2017a. Evolution of brain region volumes during artificial selection for relative brain size. Evolution 71:2942–2951. PubMed

Kotrschal, A. , Deacon A. E., Magurran A. E., and Kolm N.. 2017b. Predation pressure shapes brain anatomy in the wild. Evol. Ecol. 31:619–633. PubMed PMC

Kruska, D. 2007. The effect of domestication on brain size Pp. 143–153 in Kaas J. H., ed. Evolution of nervous system. A comprehensive reference. Academic Press, New York, NY.

Kuznetsova, A. , Brockhoff P. B., Christensen R. H. B.. 2017. lmerTest package: Tests in linear mixed effects models. J. Stat. Softw. 82 10.18637/jss.v082.i13 DOI

Kverková, K. , Bělíková T., Olkowicz S., Pavelková Z., O'Riain M. J., Šumbera R., Burda H., Bennett N. C., and Němec P.. 2018. Sociality does not drive the evolution of large brains in eusocial African mole‐rats. Sci. Rep. 8:9203. PubMed PMC

LaDage, L. D. , Roth T. C. II, Sinervo B., and Pravosudov V. V.. 2016. Environmental experiences influence cortical volume in territorial and nonterritorial side‐blotched lizards, Uta stansburiana. Anim. Behav. 115:11–18.

MacLean, E. L. , Hare B., Nunn C. L., Addessi E., Amici F., Anderson R. C., Aureli F., Baker J. M., Bania A. E., Barnard A. M., et al. 2014. The evolution of self‐control. Proc. Natl. Acad. Sci. USA 111:E2140–E2148. PubMed PMC

Magurran, A. E. 2005. Evolutionary ecology: The Trinidadian guppy. Oxford Univ. Press, Oxford, U.K.

Markram, H. , Muller E., Ramaswamy S., Reimann M. W., Abdellah M., Sanchez C. A., Ailamaki A., Alonso‐Nanclares L., Antille N., Arsever S., et al. 2015. Reconstruction and simulation of neocortical microcircuitry. Cell 163: 456–492. PubMed

McDaniel, M. 2005. Big‐brained people are smarter: A meta‐analysis of the relationship between in vivo brain volume and intelligence. Intelligence 33:337–346.

Miller, D. J. , Balaram P., Young N. A., and Kaas J. H.. 2014. Three counting methods agree on cell and neuron number in chimpanzee primary visual cortex. Front. Neuroanat. 8:36. PubMed PMC

Montgomery, S. H. , Mundy N. I., and Barton R. A.. 2016. Brain evolution and development: Adaptation, allometry and constraint. Proc. R. Soc. B 283:20160433. PubMed PMC

Mullen, R. J. , Buck C. R., and Smith A. M.. 1992. NeuN, a neuronal specific nuclear protein in vertebrates. Development 116(1):201–211. PubMed

Ngwenya, A. , Nahirney J., Brinkman B., Williams L., and Iwaniuk A. N.. 2017. Comparison of estimates of neuronal number obtained using the isotropic fractionator method and unbiased stereology in day old chicks (Gallus domesticus). J. Neurosci. Methods 287:39–46. PubMed

Olkowicz, S. , Kocourek M., Lučan R. K., Porteš M., Fitch W. T., Herculano‐Houzel S., and Němec P.. 2016. Birds have primate‐like numbers of neurons in the forebrain. Proc. Natl. Acad. Sci. USA 113:7255–7260. PubMed PMC

Price, E. O. 1999. Behavioral development in animals undergoing domestication. Appl. Anim. Behav. Sci. 65:245–271.

Rodríguez, F. , Durán E., Gómez A., Ocaña F. M., Álvarez E., Jiménez‐Moya F., Broglio C., and Salas C.. 2005. Cognitive and emotional functions of the teleost fish cerebellum. Brain Res. Bull. 66:365–370. PubMed

Sarko, D. K. , Catania K. C., Leitch D. B., Kaas J. H., and Herculano‐Houzel S.. 2009. Cellular scaling rules of insectivore brains. Front. Neuroanat. 3:8. PubMed PMC

Sokolov, A. A. , Miall R. C., and Ivry R. B.. 2017. The cerebellum: Adaptive prediction for movement and cognition. Trends Cogn. Sci. 21:313–332. PubMed PMC

R Core Team. 2018. R: A language and environment for statistical computing. R Foundation for Statistical Computing, Vienna.

Schulze, L. , Henninger J., Kadobianskyi M., Chaigne T., Faustino A. I., Hakiy N., Albadri S., Schuelke M., Maler L., Del Bene F., et al. 2018. Transparent Danionella translucida as a genetically tractable vertebrate brain model. Nat. Methods 15:977–983. PubMed

Tasic, B. , Yao Z., Graybuck L. T., Smith K. A., Nguyen T. N., Bertagnolli D., Goldy J., Garren E., Economo M. N., Viswanathan S., et al. 2018. Shared and distinct transcriptomic cell types across neocortical areas. Nature 563(7729):72. PubMed PMC

Tosches, M. A. , Yamawaki T. M., Naumann R. K., Jacobi A. A., Tushev G., and Laurent G.. 2018. Evolution of pallium, hippocampus, and cortical cell types revealed by single‐cell transcriptomics in reptiles. Science 360:881–888. PubMed

Tsuboi, M. , van der Bijl W., Kopperud B. T., Erritzøe J., Voje K. L., Kotrschal A., Yopak K. E., Collin S. P., Iwaniuk A. N., and Kolm N.. 2018. Breakdown of brain–body allometry and the encephalization of birds and mammals. Nat. Ecol. Evol. 2:1492–1500. PubMed

van der Bijl, W. , Thyselius M., Kotrschal A., and Kolm N.. 2015. Brain size affects the behavioural response to predators in female guppies (Poecilia reticulata). Proc. Biol. Sci. B 282:20151132. PubMed PMC

Warren, R. , and Sawtell N. B.. 2016. A comparative approach to cerebellar function: Insights from electrosensory systems. Curr. Opin. Neurobiol. 41:31–37. PubMed PMC

Zeisel, A. , Hochgerner H., Lönnerberg P., Johnsson A., Memic F., der Zwan J. Van, Häring M., Braun E., Borm L. E., La Manno G., et al. 2018. Molecular architecture of the mouse nervous system. Cell 174:999–1014. PubMed PMC

Zhu, F. , Cizeron M., Qiu Z., Benavides‐Piccione R., Kopanitsa M. V., Skene N. G., Koniaris B., DeFelipe J., Fransén E., Komiyama N. H., et al. 2018. Architecture of the mouse brain synaptome. Neuron. 99:781–799. PubMed PMC

The evolution of brain neuron numbers in amniotes

Individual and age-related variation of cellular brain composition in a squamate reptile

The evolution of brain structure captured in stereotyped cell count and cell type distributions