title: Quantitative relationships in delphinid neocortex.
abstract: Possessing large brains and complex behavioral patterns, cetaceans are believed to be highly intelligent. Their brains, which are the largest in the Animal Kingdom and have enormous gyrification compared with terrestrial mammals, have long been of scientific interest. Few studies, however, report total number of brain cells in cetaceans, and even fewer have used unbiased counting methods. In this study, using stereological methods, we estimated the total number of cells in the neocortex of the long-finned pilot whale (Globicephala melas) brain. For the first time, we show that a species of dolphin has more neocortical neurons than any mammal studied to date including humans. These cell numbers are compared across various mammals with different brain sizes, and the function of possessing many neurons is discussed. We found that the long-finned pilot whale neocortex has approximately 37.2 × 10(9) neurons, which is almost twice as many as humans, and 127 × 10(9) glial cells. Thus, the absolute number of neurons in the human neocortex is not correlated with the superior cognitive abilities of humans (at least compared to cetaceans) as has previously been hypothesized. However, as neuron density in long-finned pilot whales is lower than that in humans, their higher cell number appears to be due to their larger brain. Accordingly, our findings make an important contribution to the ongoing debate over quantitative relationships in the mammalian brain.
authors.affiliation: Research Laboratory for Stereology and Neuroscience, Bispebjerg and Frederiksberg University Hospitals Copenhagen, Denmark ; Research Department, Environment Agency Torshavn, Faroe Islands.
authors.forename: Heidi S
authors.initials: HS
authors.lastname: Mortensen
authors.affiliation: Research Laboratory for Stereology and Neuroscience, Bispebjerg and Frederiksberg University Hospitals Copenhagen, Denmark.
authors.forename: Bente
authors.initials: B
authors.lastname: Pakkenberg
authors.affiliation: Research Department, Environment Agency Torshavn, Faroe Islands.
authors.forename: Maria
authors.initials: M
authors.lastname: Dam
authors.affiliation: Department of Bioscience, Institute for Bioscience - Arctic Research Centre, Roskilde, University of Aarhus Roskilde, Denmark.
authors.forename: Rune
authors.initials: R
authors.lastname: Dietz
authors.affiliation: Department of Bioscience, Institute for Bioscience - Arctic Research Centre, Roskilde, University of Aarhus Roskilde, Denmark.
authors.forename: Christian
authors.initials: C
authors.lastname: Sonne
authors.affiliation: Museum of Natural History Torshavn, Faroe Islands.
authors.forename: Bjarni
authors.initials: B
authors.lastname: Mikkelsen
authors.affiliation: Research Laboratory for Stereology and Neuroscience, Bispebjerg and Frederiksberg University Hospitals Copenhagen, Denmark.
authors.forename: Nina
authors.initials: N
authors.lastname: Eriksen
keywords: animal cognition; glia/neuron ratio; neocortical cell density; neocortical cell number; stereology