Whaling ships in the 19th century chased sperm whales for the reserves of oil in the spermaceti organ, the spongy bump in their foreheads used for echolocation. That bump, as Ahab and colleagues well knew, can also be used to destroy ships. Now scientists suggest that what caused these bumps to evolve into powerful battering rams was male-male competition.
Moby Dick upends a whaler.
CREDIT: BETTMANN/CORBIS
All toothed whales and dolphins have sonar "melons," oil-filled sacs in their heads that serve as acoustic amplifiers and emitters. But in male sperm whales they are huge, occupying up to a third of their length and a quarter of their body mass. The world's small community of spermaceti researchers has speculated that the size might have evolved to help produce sound pulses to stun prey, control buoyancy, or just serve as a sexual turn-on.
Curious about the biomechanics of a real-life encounter that inspired Moby Dick--in which a 26-meter sperm whale sank the 238-ton whaling ship Essex--comparative physiologist David Carrier and colleagues at the University of Utah, Salt Lake City, ran computer simulations to see how a whale's noggin would perform in a head-on collision. Although the massive organ is springy enough to protect a charging whale, it is still hard enough to deliver lethal blows, they report in the 15 June issue of the Journal of Experimental Biology.
What's more, they found from comparisons with 20 other cetacean species that the larger the melon, the larger the size difference between the sexes and the more likely the males were to be polygynous. Because sperm whales are polygynous, all the evidence points to a key role of melons in males who protect their harems in head-to-head combat, they say.
Experts find the idea intriguing, but whether male sperm whales joust with each other is hard to verify through observation because they rarely stray into shallow water, says bio-sonar specialist Bertel Mohl of Aarhus University in Denmark: "We can only do so much in the field with binoculars, hydrophone arrays, acoustic tags, and midget budgets."
