![]() The dolphins have larger brains, larger cerebellums, and greater numbers of cortex neurons than would be expected given their body size. The most oxygen is used by the cortex and cerebellar gray matter. The brain consumes an oversized share of oxygen available to the body. Brain metabolism may be one feature limiting dolphin dives. In contrast, the dolphins we studied limit dives to five or 10 minutes. For example, the beaked whale may dive for more than an hour, and the pygmy sperm whale more than a half hour. The beaked whale and pygmy sperm whale we studied dive deeper and for much longer periods than the dolphins. Non-dolphin brains had lower neuron densities compared to all of the dolphins, even the 6215g brain. We compared neuron density of Nissl stained cortex of these two brains with those of the dolphins. To begin a study of non-dolphin toothed whales, we measured a 596g brain of a pygmy sperm whale and a 2004g brain of a Cuvier’s beaked whale. ![]() Despite having a lower neuron density than smaller dolphins, the killer whale has more gray matter and more cortical neurons than any mammal, including humans. As expected, average neuron density in four areas of cortex decreased from the smallest to the largest brain. We studied dolphin brains varying from 783 to 6215g. ![]() Since the work of Tower in the 1950s, we have come to expect lower neuron density in the cerebral cortex of larger brains. ![]()
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