Fat Heads Sink Ships

Protected by an internal "fender," a sperm whale can deliver a killing blow with its head.

Story by Adam Summers
Art by Pog Summers

In 1851 an enraged sperm whale smashed into the bow of the whaling ship Ann Alexander, causing it to sink in just minutes. The event resulted in a big boost in sales for the just-published Moby-Dick, Herman Melville's fictional account of a white sperm whale that is pursued by, but eventually sinks, the whaleship Pequod. (The 1820 sinking of another whaler, the Essex, by a sperm whale had inspired Melville's tale.)

Sperm whales (Physeter macrocephalus) are the largest of the toothed whales; mature males weigh more than forty tons and stretch fifty feet from nose to fluke. But the whaling ships that sailed out of Nantucket in the mid-nineteenth century were ninety feet long and weighed nearly 250 tons. Why would a whale seek out a collision with such a ship, and-more to the point-what enables it to survive? (To bring the question down to a more comprehensible scale, imagine your 40-pound child dashing headlong into the side of a 250-pound beached rowboat, staving a large hole in its side, then calmly picking herself up and wandering off.) The answer may be intimately connected-anatomically speaking-with the very reason the sperm whale was considered such a desirable catch.

P. macrocephalus was prized by whalers because in addition to the oil that could be rendered from its blubber, a large quantity of higher-quality oil-spermaceti oil-could be ladled out of the enormous, thick-skinned, fiber-reinforced bulb that forms a sort of forehead. This structure, known as the spermaceti organ, has two oil-filled chambers, one of which has room for as much as 500 gallons of spermaceti oil. (The sperm whale's spermaceti organ is so big that it sometimes seems to be the head; on the big males, however, it plainly juts out beyond the jaw.) This organ evolved at least 20 million years ago-clearly not to sink ships. Scientists have speculated that the bulb may focus the whale's vocalizations into a tight beam, capable of sonically stunning prey, or that it may cause the sounds to resonate, thereby increasing the appeal of a whale's song to potential mates. Another theory holds that because the oil is less dense than water, the spermaceti organ is important in buoyancy control. Recently, University of Utah researchers David Carrier and Stephen Deban, together with undergraduate Jason Otterstrom, proposed a pugilistic function: they think the spermaceti organ is a head-mounted boxing glove, used for combat between males.

Many other whale species also have fat-filled forehead fenders, and some have been seen using them as battering rams against their fellows. The Utah biologists observed that forehead size is closely correlated with a common measure of male-to-male aggression: sexual size dimorphism. (From fish to frogs to felines, species in which males are considerably larger than females tend to be those in which males fight for the privilege of mating; the greater the size difference between the sexes, the more competition between males.) In the species that the researchers compared, those with the most striking sexual size dimorphism were also those with the largest spermaceti organ relative to the rest of the body. A big, oil-filled forehead seems to be associated with male aggression, at least in some species. But just how is harder to determine.

Carrier and his group used anatomical information, including the size and shape of the spermaceti organ and of the skeleton that supports it, to build a mathematical model of imaginary collisions between jousting whales. Their goal was to see whether this organ is suited for delivering a useful broadside punch while simultaneously protecting the aggressor's noggin.

In a collision, it is not speed but rather the change in speed over time-the acceleration-that causes injury. The force on an object is the product of its mass multiplied by its acceleration. Thus, the same change in speed will exert more force on a heavier object than on a lighter one: a 3,000-pound car will be hit 100 percent harder than a 1,500-pound one when slowing from sixty miles an hour to zero upon colliding with a wall. The key to surviving a high-speed collision is to make the crash last as long as possible. Automobile designers don't aim to build cars as strong as tanks; they build cars to collapse in a controlled fashion that uses up as much of the collision's energy as possible without compromising the passenger compartment.

In accordance with this principle, an empty, blown eggshell dropped onto my counter from a height of eighteen inches will not break, while a fresh egg, differing from the blown one only in mass (and in not having two tiny holes), will make a small mess. A mouse will survive a drop of several stories and land with a force of about 170 g, or 170 times the acceleration of gravity; a 10-g car crash will break human bones; and a sperm whale will suffer destructive, possibly fatal injuries at just 2 g. (The acceleration due to gravity is 32 feet per second per second.)

So how might all this pertain to a sperm whale set on slamming into a rival? First, it's helpful to take a closer look at the whale's putative battering ram. The spermaceti organ actually consists of two main chambers: a lower section called the junk, which is filled with its own oil plus baffles of connective tissue, and, atop the junk, a chamber often called the case, containing the valuable spermaceti oil. The whole organ sits on the wide upper jaw and the dished-out skull behind it. The posterior six cervical vertebrae are fused, providing a few more feet of solid, bony support.

A head-on collision between whales would be a fender bender, with each animal's spermaceti organ cushioning the blow. But if a male could manage to thump a rival in the brisket or the chops, the outcome would be very different. A conservative estimate is that the spermaceti organ is ten times better at absorbing energy than the (relatively!) thin layer of fatty tissue that covers the rest of the whale. The model created by the Utah researchers predicts that in a broadside collision, the aggressor would experience less than 0.5 g while inflicting a dangerous, or even fatal, blow of more than 2 g. The smart thing for the intended victim to do is either move quickly out of harm's way or turn to face the danger head-on. Compared with the graceful sperm whale, nineteenth-century whaleships were slow, clumsy, and oblivious to threats from below. To the whales that sent them to the bottom of the sea, the Essex, the Ann Alexander, and the fictional Pequod must have seemed punch-drunk opponents just begging to be blindsided-dream targets for an angry sperm whale.

Adam Summers is an assistant professor of ecology and evolutionary biology at the University of California, Irvine (asummers@uci.edu).