The cortical arrangement of functional areas has also changed. Cortical layer IV is absent or very poorly developed, and so inputs, outputs, and interneuronal connections are very different than those in other mammals ( Glezer et al., 1988 Morgane and Glezer, 1990). This could indicate a morphological alteration of the telencephalon associated with the return to the marine environment ( Manger et al., 2012).Ĭytoarchitectural organization is very complex ( Hof et al., 2005), and does not resemble that of terrestrial mammals layer I is far more cellular, layer II contains atypical neurons, and layer III contains very large pyramidal neurons ( Glezer, 2002 Hof et al., 2005). Cetaceans have been observed to be the most gyrencephalic mammals studied to date, also more than predicted based on comparison with other mammals. This migration has also changed the shape of the brain and the cranial nerve distribution ( Oelschläger and Oelschläger, 2009). The migration of the blowhole from the front to the top of the head facilitates breathing at the water's surface. It has a very high level of gyrification, and like the rest of the whale body, it possesses several traits that indicate adaptation to water. The cetacean brain differs from the terrestrial mammalian brain in many ways. This is especially true for odontocetes such as the sperm whale ( Physeter macrocephalus) (~10 kg), which has the largest brain in the Animal Kingdom, but it is not the largest animal alive ( Marino, 2004). During secondary adaptation to water, cetaceans underwent major transformations in body form and physiology, resulting in large, highly encephalised, and extremely gyrified brains compared to those of terrestrial mammals ( Oelschläger and Oelschläger, 2002 Marino, 2008). These two suborders appeared and began to diverge in the early Oligocene, about 30 million years ago ( Gingerich et al., 1983). Accordingly, our findings make an important contribution to the ongoing debate over quantitative relationships in the mammalian brain.Ĭetaceans are divided into odontocetes (toothed whales) and mysticetes (baleen whales). 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. 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. 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. These cell numbers are compared across various mammals with different brain sizes, and the function of possessing many neurons is discussed. For the first time, we show that a species of dolphin has more neocortical neurons than any mammal studied to date including humans. 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. Few studies, however, report total number of brain cells in cetaceans, and even fewer have used unbiased counting methods. Their brains, which are the largest in the Animal Kingdom and have enormous gyrification compared with terrestrial mammals, have long been of scientific interest. Possessing large brains and complex behavioral patterns, cetaceans are believed to be highly intelligent.