August 26, 2002 — Male sperm whales that rammed and sank ships – the inspiration for the fictional “Moby Dick” – may have evolved such aggressive behavior when they butted heads in fights over females, according to a study by University of Utah biologists.
“Herman Melville’s ‘Moby Dick’ was based on the sinking of a real ship, the Essex, by a big sperm whale in 1820,” says David Carrier, an associate professor of biology. “We believe this ability to sink ships is the result of whales evolving so that males could butt heads to compete for females.”
Biology student Jason Otterstrom, who co-authored the study for his undergraduate research project, says: “We were trying to find out how sperm whales are able to hit ships so much bigger than them and swim away. Humans can’t run into things that much bigger than us, and come away unfazed. … We make a fairly strong argument that the body of a sperm whale is designed to withstand severe impacts unharmed. It is evolutionarily advantageous to their survival.”
The study was published in The Journal of Experimental Biology on June 15. Carrier and Otterstrom conducted it with postdoctoral fellow Stephen Deban.
Carrier says that while there have been other accounts of sperm whales sinking ships, only two are well-documented: the 1820 Essex incident, and the 1851 sinking of the Ann Alexander, also in the Pacific. The Ann Alexander’s crew was rescued.
Carrier was inspired to conduct the study after reading Nathaniel Philbrick’s 2000 book, “In the Heart of the Sea: The Tragedy of the Whaleship Essex,” about how an 85-foot-long, 80-ton sperm whale rammed the Essex in the Pacific Ocean in 1820, punching a hole in the ship and sinking it in 10 minutes. The 20 crew members survived the sinking and fled in three rowboats, but 12 of them died of starvation or dehydration as they attempted to reach South America.
“What caught my attention was the audacity that whale had to attack a ship three or four times it’s size and built out of solid oak,” Carrier says. “The whale thought he could take on the ship. And he was right.”
Melville learned details of the Essex in 1840 when he worked aboard a whaling ship and met the son of Owen Chase, who was first mate of the Essex and who wrote an 1821 book about the incident. Carrier says Chase’s book argued the forehead of sperm whales is a battering ram.
The sinking of the Essex inspired Melville’s 1851 novel “Moby Dick,” which tells the story of Ahab, an old whaler who had lost a leg to Moby Dick in an earlier encounter and sets out on the ship Pequod to seek vengeance. After the giant whale escapes during two bouts, Ahab declares: “Forehead to forehead I meet thee, this third time, Moby Dick!” – a line Carrier quotes at the beginning of his study.
A major part of the whale study involved studying the relationship between the sexual dimorphism – the male-female size difference – of various species of cetaceans (whales, dolphins and porpoises) and the size of each species’ forehead or “melon.”
In polygynous species – those in which individual males reproduce with a harem of many females – males compete for access to the females. And the bigger the male-female size difference, the larger the harem, the more intense the competition among males and the larger the weapon used if the competition results in fighting. That is because the biggest males with the biggest weapons successfully reproduce and pass on their genes.
“Among groups of mammals, you find the species with the largest harems also have the largest weapons,” Carrier says. Among antelope, for example, “the larger the harem, the larger the size of the antlers.”
Carrier and colleagues examined data on 21 species of cetaceans and found “those species in which males are much larger than females tend to have the largest foreheads. If the forehead was the battering ram used for male-male aggression, then you would expect it would be relatively larger in those species in which males are larger than females” and compete more vigorously for females.
“The findings are consistent with the weapon hypothesis” and head-butting among male sperm whales, Carrier says.
In the second part of the study, Carrier and colleagues used computers to simulate head-butting collisions between average-size sperm whales weighing about 86,000 pounds (39,000 kilograms), with the attacker swimming at almost 7 mph.
A sperm whale’s forehead “is a third of the length of the whale from front to back,” Carrier says. “It’s a big forehead, and is anywhere from 25 to 30 percent of the weight of the whale.”
It includes the spermaceti sac, which is the upper chamber of the forehead filled with the oil prized by whalers, and the “junk,” which is a lower chamber that contains thicker oil and is divided by vertical sheets of connective tissue.
“For our hypothesis to work, the forehead has to function both as a battering ram and as a shock absorber to protect the attacking whale,” says Carrier. “We envision the forehead acting basically like an airbag in your car. We used a wide variety of shock-absorption values in the head-butting simulation because we don’t know the true values.”
He says the computer simulations aimed to determine “is there enough energy in the momentum of that forehead to damage another whale of equal size? We found there was. Also, when you give the attacking whale enough shock absorption to protect itself, the energy delivered to the target whale still appears to be large enough to do serious injury.”
The Utah researchers’ study has generated skepticism among some marine mammal experts. They contend sperm whales use their forehead or melon to aim sounds so they can communicate with other whales and use sound waves to locate objects in their environment.
“There is no doubt it is involved in focusing sound for sonar and communication,” Carrier says. “But we are suggesting it also may be involved in male-male aggression and used as battering ram.”
Critics, however, wonder why whales would risk damage to a sound-focusing organ by using it as a battering ram. Carrier says photos of sperm whales show tooth-mark scars on the junk, indicating the lower part of the forehead is used as the impact surface, while the overlying spermaceti – the main area where sounds are focused – adds to the momentum of impact.
Carrier acknowledges that a weakness of the study is that he lacked the time and money to directly measure the forehead sizes of the 21 species of whales, dolphins and porpoises. Instead, the sizes used in the study were based on those shown in a well-known field guide. But he says any errors in forehead sizes in those drawings are unlikely to affect the conclusion that cetacean species with larger foreheads also had a larger male-female size difference.
Carrier says he and his colleagues are not whale experts, but “I have a lot of confidence in the hypothesis. All the bits and pieces fit together,” including sperm whales sinking ships, scars on their foreheads and the prevalence of head butting among related species.
Male hippopotamuses – which may be the closest living relative of whales and other cetaceans – engage in “open-mouthed charges where they ram their open mouths together and get in a pushing contest,” he says.
Carrier says some other whale species have been observed butting heads, so “it shouldn’t come as a surprise that sperm whales may also.”
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