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flickr | Amila Tennakoon

Have you ever wondered why the sperm whale, Physeter macrocephalus, has such a big head? Nearly one third of the cetacean’s entire body mass is made up by its gigantic melon, and at first glance it looks very impractical. Plowing its way through the water like a school bus, it's hard to imagine how much energy must be burnt in the process. When you compare it to the very streamlined bottlenose dolphin, the sperm whale’s head appears to be a clear impediment. So why is it so huge?

It turns out that beneath its seemingly inefficient exterior, the sperm whale is actually a highly complex hunter. In the top half of the whale’s skull, an enormous cavity as big as one quarter of its body holds 2,000 litres of spermaceti. Early whalers believed that this white waxy oil was semen, hence the name. It was harvested for oil lamps, cosmetics, candles, pharmaceuticals, lubricants, and textiles. As the oil never went rancid it was favoured over the oil whalers produced from blubber, and the industry led to the animal’s near extinction.

Once scientists proved that spermaceti was not in fact sperm and had no connection at all to the whale’s reproductive system, they hypothesized that it was used as a buoyancy device. By restricting blood flow to the spermaceti organ the temperature of the oil was thought to cool, causing it to become denser and allowing the whale to sink faster. The opposite was believed to happen if the blood flow was increased - the oil would heat and the whale would be aided in its ascent.

There has been some debate on the accuracy of this theory, and recently more biologist have swayed to a different idea, that the spermaceti is a tool for echolocation. Like bats, toothed whales produce pulses of sound which bounce off of objects, echoing back to them and creating an “image” of their surroundings. Perhaps the spermaceti focuses the sound waves like an acoustic lens.

The sperm whale has been recorded at depths over 2,000 meters, ranking it as one of the deepest diving mammals. Here there is virtually no sunlight and the whale must use echolocation alone to chase down its favourite meal - the giant squid. Still, catching prey as agile and speedy as cephalopods is an incredible feat for a school bus and there have been other theories that the spermaceti may be capable of focusing sound like a stun gun.

Whatever the case may be, its some pretty interesting stuff.

By Carolyn Skelton - Tenerife Assistant Research Officer

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