Even though the sperm whale brain should be one of the most interesting to study, and their carcasses wash up upon our shores regularly, little work has been done on them since the mid nineteenth century. A few papers were published either side of the second world war, but they were on unusual topics of peripheral interest, such as on their chloride content after brain tissue had been frozen then thawed. This dearth of information makes any investigation hard. Even the terminology has changed. What does it mean when a report on dissections published in the 1860s describes their cerebrum as the ‘most fibrous’ he had ever seen? Are they talking of neuron density, or something else entirely? Had it not been for a solitary paper published in 2014, I would have been able to make only qualitative statements below!
Here is what we know. Sperm whales possess the largest brains ever known to have existed. If theirs was just a scaled up version of a typical cetacean brain, we would still expect them to be the smartest mammal in the seas (see my section on EQ), yet its structure is variant from the that standard. The cerebrum makes up 91% of its volume*. The next nearest animals by proportion are all around 75-80%. You may be pleased to hear that the next in line by this measure might still prove to be the human brain, since it has sometimes been thought that the brain with the highest proportion as cerebrum would be cognitively superior.
Recently, the most comprehensive work of comparative brain anatomy across the whole mammal class, seems to have been on cerebellum volume to whole brain mass ratio. As we move across mammalian brains in order of size, there is a slight trend for this ratio to decrease. These are called allometric trends and, because of this one, we might have expected the relative size of the cerebellum to be smaller in sperm whales than the cetacean average. The allometrically adjusted ratio is referred to as CQ. Cetaceans have a significantly higher CQ than other mammals, including the few individual killer whale brains that are comparable in size to the sperm whale, and the dwarf sperm whale (the sperm whale’s closest living relative). The CQs of more than 350 mammal species have been measured, and the sperm whale has a CQ 15% lower than any other animal. If we used an unadjusted ratio this would give it the record by an even wider margin. Note, this is not 15% below the mammalian average, nor 15% below the next lowest cetacean, but 15% less than the most extreme brain previously recorded. It demands the question: what selective pressures have driven it to have the smallest proportion of its brain devoted to body control and sensory input of any animal? Why is its brain almost entirely cerebrum where abstract ideas are processed?
Another factor of interest is a leading hypothesis that attempts to explain why cetacean brains are so large in general. Since cetaceans sleep one cerebral hemisphere at a time, their brains are not free to specialise by hemisphere in the fashion that every terrestrial mammalian brain can. They have to ‘double up’ all functions for redundancy. Whatever the mechanism, the example of seals gives powerful evidence that unihemispheric sleep reduces brain efficiency. Seals sleep just like cetaceans when in the ocean but, incredibly, they switch to bihemispheric sleep when on land for any extended time. In 2009 it was discovered that sperm whales are the sole exception among cetaceans, as they sleep and dream just as we do. Evolution has driven this change even though they have the biggest brain, and therefore the least need among cetaceans to do so, and even though this change makes them vulnerable to attack from killer whales. This implies that the marginal evolutionary profit from squeezing extra brain power (already above and beyond that of any other cetacean), has higher potential for them than increased lethality from predation.
Even from gross brain anatomy alone, a picture is starting to emerge of an animal with a complex and cerebral way of living that is without peer among mammals.
FIND why brain size is a better predictor of intelligence in mammals when no adjustments are made for body size
or CONTINUE to find why the conduct of whales during the whaling era matches behaviour know in humans
*Weight/weight figures differ markedly from volume/volume ones when it comes to brain structure. If you are working by weight then, by analogy with other mammals, the proportion of cerebrum in sperm whale would equate to circa 95%.