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 × 109 neurons, which is almost twice as many as humans, and 127 × 109 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.
Highlights
Cetaceans are divided into odontocetes and mysticetes
Table 2 summarizes the mean cell numbers found in the longfinned pilot whales, and Figure 4 shows the number of neurons and glial cells in the entire neocortex in each individual animal
On average, long-finned pilot whales possess 37.2 × 109 neocortical neurons and 127 × 109 neocortical glial cells, with a glial cell to neuron ratio of 3.4/1
Summary
Cetaceans are divided into odontocetes (toothed whales) and mysticetes (baleen whales). 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). 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). This could indicate a morphological alteration of the telencephalon associated with the return to the marine environment (Manger et al, 2012)
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