DEBAN, S.M.; OTTERSTROM, J.; CARRIER, D.R.: The face that sunk the Essex

In the 19th century, sperm whales (Physeter macrocephalus) were prized commercially for the high quality oil contained in their derived and greatly enlarged melon, known as the spermaceti organ, and also feared by whalers because of their aggressive behavior. Herman Melville’s (1851) fictional portrayal, in Moby Dick, of the sinking of the Pequod was inspired by historical instances in which large sperm whales sank whaling ships by ramming the ships with their heads. We hypothesize that the ability of sperm whales to sink ships is a product of specialization for male-male aggression in which the spermaceti organ functions as a weapon in head-butting behavior. Head-butting is widespread in whales and in the nearest outgroup, the artiodactyls, and is thus likely to be ancestral for whales. An independent contrasts analysis using a published cetacean phylogeny (Messenger & McGuire, 1998) reveals that the evolution of enlarged melons is significantly and positively correlated with the evolution of sexual size dimorphism (toward larger males). This result supports the hypothesis that enlarged melons have evolved in lineages with high levels of male-male aggression. A mechanical model in which the spermaceti is modeled as a mass + damper system (25% body mass) suggests that it can function as a battering ram that damages the target whale and a shock absorber that protects the attacking whale. When accelerations of the target whale exceed an estimated injury threshold of 19.6 m/s/s for a whale of 34,000 kg (Farlow, et al, 2000; Alexander, 1996), accelerations of the attacking whale remain below the injury threshold. Results are in accord with the hypotheses that sperm whales use the spermaceti organ in head-butting contests, and can injure opponents (e.g., whaling ships) without fatally injuring themselves.