title: Cellular scaling rules for the brain of afrotherians.
abstract: Quantitative analysis of the cellular composition of rodent, primate and eulipotyphlan brains has shown that non-neuronal scaling rules are similar across these mammalian orders that diverged about 95 million years ago, and therefore appear to be conserved in evolution, while neuronal scaling rules appear to be free to vary in evolution in a clade-specific manner. Here we analyze the cellular scaling rules that apply to the brain of afrotherians, believed to be the first clade to radiate from the common eutherian ancestor. We find that afrotherians share non-neuronal scaling rules with rodents, primates and eulipotyphlans, as well as the coordinated scaling of numbers of neurons in the cerebral cortex and cerebellum. Afrotherians share with rodents and eulipotyphlans, but not with primates, the scaling of number of neurons in the cortex and in the cerebellum as a function of the number of neurons in the rest of the brain. Afrotheria also share with rodents and eulipotyphlans the neuronal scaling rules that apply to the cerebral cortex. Afrotherians share with rodents, but not with eulipotyphlans nor primates, the neuronal scaling rules that apply to the cerebellum. Importantly, the scaling of the folding index of the cerebral cortex with the number of neurons in the cerebral cortex is not shared by either afrotherians, rodents, or primates. The sharing of some neuronal scaling rules between afrotherians and rodents, and of some additional features with eulipotyphlans and primates, raise the interesting possibility that these shared characteristics applied to the common eutherian ancestor. In turn, the clade-specific characteristics that relate to the distribution of neurons along the surface of the cerebral cortex and to its degree of gyrification suggest that these characteristics compose an evolutionarily plastic suite of features that may have defined and distinguished mammalian groups in evolution.
authors.affiliation: Instituto de Ciências Biomédicas, Universidade Federal do Rio de Janeiro Rio de Janeiro, Brasil ; Instituto Nacional de Neurociência Translacional, CNPq/MCT São Paulo, Brasil.
authors.forename: Kleber
authors.initials: K
authors.lastname: Neves
authors.affiliation: D'Or Institute for Research and Education (IDOR) Rio de Janeiro, Brasil ; Bioimaging National Center, Federal University of Rio de Janeiro Rio de Janeiro, Brazil.
authors.forename: Fernanda M
authors.initials: FM
authors.lastname: Ferreira
authors.affiliation: Instituto de Ciências Biomédicas, Universidade Federal do Rio de Janeiro Rio de Janeiro, Brasil ; Instituto Nacional de Neurociência Translacional, CNPq/MCT São Paulo, Brasil ; D'Or Institute for Research and Education (IDOR) Rio de Janeiro, Brasil ; Bioimaging National Center, Federal University of Rio de Janeiro Rio de Janeiro, Brazil.
authors.forename: Fernanda
authors.initials: F
authors.lastname: Tovar-Moll
authors.affiliation: Department of Zoology and Entomology, University of Pretoria Pretoria, South Africa.
authors.forename: Nadine
authors.initials: N
authors.lastname: Gravett
authors.affiliation: Faculté des Sciences, University of Kisangani Kisangani, Democratic Republic of Congo.
authors.forename: Nigel C
authors.initials: NC
authors.lastname: Bennett
authors.affiliation: Department of African Zoology, Royal Museum for Central Africa Tervuren, Belgium.
authors.forename: Consolate
authors.initials: C
authors.lastname: Kaswera
authors.affiliation: Laboratory of Histology and Neuropathology, Université Libre de Bruxelles Brussels, Belgium ; Department of Anthropology, University of Arkansas Fayetteville, AR, USA.
authors.forename: Emmanuel
authors.initials: E
authors.lastname: Gilissen
authors.affiliation: School of Anatomical Sciences, University of the Witwatersrand Johannesburg, South Africa.
authors.forename: Paul R
authors.initials: PR
authors.lastname: Manger
authors.affiliation: Instituto de Ciências Biomédicas, Universidade Federal do Rio de Janeiro Rio de Janeiro, Brasil ; Instituto Nacional de Neurociência Translacional, CNPq/MCT São Paulo, Brasil.
authors.forename: Suzana
authors.initials: S
authors.lastname: Herculano-Houzel
keywords: cortical expansion; evolution; glia-neuron ratio; gyrification; numbers of neurons