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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