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In this issue |
Moby Dick Goes Modelling
kathryn{at}biologists.com
When David Carrier was handed a copy of `In the Heart of the Sea' it just looked like another good sea-faring yarn about a whaling ship called the Essex, and a sperm whale. As Carrier read about the real life encounter that inspired Herman Melville's `Moby Dick', he began wondering whether a male sperm whale really could successfully destroy a huge wooden ship. Would the mammal have `the right attitude', let alone the physical ability? Could male aggression be the main driving force behind the whale's head butting behaviour? Carrier needed to collect some evidence to support this idea. Collaborating with Jason Otterstrom and Stephen Deban, they began analysing the evolutionary development of the melon structure in the male sperm whale's forehead to see how it would perform in a head on collision. In this issue of the J. Exp Biol., they outline their new theory: that the melon structure has evolved specifically as a battering ram for combat (p. 1755).
Most cetaceans have a structure at the front of their heads referred to as the `melon'. In many whales and dolphins, this structure is quite small, but sperm whale melons are particularly well developed, so they must have some function. Dolphins and whales rely on sound for communication, navigation, and even hunting, so it seemed likely that the structure had evolved to focus and amplify sound. But was that the whole story? The first mate of the Essex had described the melon as a battering ram after watching the enormous mammals battling in the ocean. Carrier also knew that beached males were heavily scared on their melons as if they'd been raked by an opponent's teeth in a head-to-head encounter. Could the melon have evolved as a battering ram? Whales would need a strong motive for the melon to have evolved as a weapon, rather then an amplifier.
The team began by correlating melon size with sexual dimorphism for 21 species, including the sperm whale. If the melon had evolved as an instrument of battle, males that needed to protect a harem of females would tend to have more pronounced melons. When the team looked at relative melon size in a phyologenetic context, the correlation was there. But could the melon physically withstand a head-on impact?
The three biologists simulated the forces acting on the melon as it crashed into the side of another whale. To be an effective battering ram, the melon would have to carry enough momentum to cause a serious injury, while protecting the charging whale by absorbing the impact. Deban ran simulations of the assault where he represented the aggressor's melon as an enormous damper. As he changed the stiffness of the damper over a range of physiologically relevant values, the impact on the victim became increasingly violent, while protecting the aggressor. Carrier admits that this isn't conclusive evidence that the melon is a battering ram, but it is supportive.
Returning to the inspiration for the work, it's obvious that sperm whales didn't evolve the melon for mortal combat with a ship. So if Carrier's theory is right, the Essex was probably a victim of mistaken identity.
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