The sperm whale forehead is one of the largest and strangest structures in the animal kingdom. It contains two large oil-filled sacs, stacked one on top of the other, known as the spermaceti organ and the junk. These sacs extend for one-third of the total length of the whale and can constitute more than one-quarter of the animal’s mass.
A new study looks at a controversial hypothesis regarding one potential function of the sperm whale’s enlarged head: ramming. A team of researchers from Australia, the U.K, the U.S., and Japan used structural engineering principles to test how the head of the sperm whale might be able to resist strong ramming impacts.
A Whale of a Head
The ramming hypothesis was originally proposed by 19th century whalers after large sperm whales attacked and sank two whaling ships, the Essex in 1821 and the Ann Alexander in 1851.
“After the Essex was sunk, Owen Chase, the ship’s first mate, proposed that the whales not only use their heads as battering rams but that ‘the whale’s head is admirably designed for this mode of attack,’” says lead author Olga Panagiotopoulou, of the University of Queensland.
Instances like these inspired the novel Moby Dick and gave rise to the ramming hypothesis: the forehead of sperm whales evolved partly to function as a battering ram in males fighting for access to females.
The sperm whale’s giant head plays an important role in echolocation, but there are other hypotheses about its function in sexual selection, acoustic prey debilitation, and buoyancy control. However, none of these hypotheses explains how the sperm whale head could act as a weapon capable of sinking ships four to five times the mass of the whale.
The ramming hypothesis suggests the unique anatomy of the sperm whale forehead absorbs energy to protect the brain and skull when males use their heads as battering rams to compete over females. One reason this hypothesis is compelling is the great size difference between male and female whales. Sperm whales are the most sexually dimorphic of all cetaceans, with adult males reaching sizes three times larger than females. Among mammals, such size differences are common in species where males fight over females, and are especially pronounced in those characteristics that enhance a male’s ability to dominate other males, such as horns, antlers, and teeth.
“The observations that males are three times larger than females, and the head is the most sexually dimorphic feature, suggest that the ramming hypothesis may be true,” says Panagiotopoulou.
Yet the ramming hypothesis remains controversial. One reason is that the spermaceti organ houses sensitive anatomical structures needed for echolocation, and these would be in harm’s way in a ramming event. Panagiotopoulou and colleagues suggest that it is the junk, and not the spermaceti organ, that is used in ramming. The researchers note that scarring on the heads of adult male sperm whales is largely confined to the end of the junk, suggesting whales avoid contact with the spermaceti organ and use the junk in ramming events.
“If they ram another male or a ship, they should use the junk and not the spermaceti organ,” says Stefan Huggenberger, a biologist at the University of Cologne who was not involved in this research. “It’s like a person head-butting another person – they want to use their foreheads and not their noses, because their foreheads are more stable. The same is true of the junk, which is more stable than the spermaceti organ.”
Another reason the ramming hypothesis is controversial is that not many people have actually observed sperm whales using their heads this way. Panagiotopoulou and colleagues report one documented account of male-male ramming behavior, observed in 1997 outside the Gulf of California. In this instance, a pilot and conservation researcher saw two mature males swim directly at each other and collide forehead-to-forehead, a few miles away from a group of about 50 females.
Panagiotopoulou cautions that not observing a ramming event does not mean that it is unlikely to happen. “When ramming events occur in shallow depths, a human observer has to be directly above the surface of the water to watch it happen,” she says. “The observation from 1997, coupled with the reports of ramming attacks on 19th century whaling ships, suggests that sperm whales do sometimes participate in ramming contests.”
Within the sperm whale’s head, the junk is organized into sections by partitions of connective tissue that contain waxy oil. Panagiotopoulou and colleagues set out to determine if these connective tissue partitions have the potential to reduce stress in the bones of the skull during ramming impact. They used statistical and simulation approaches that engineers use to test the endurance of bridges and tunnels.
“We showed that the connective tissue partitions within the junk have the ability to absorb impact stresses that could otherwise cause bone fractures,” says Panagiotopoulou.
Although the prevalence of ramming in sperm whales is not well documented, ramming is seen in other cetaceans, including humpback whales, narwhales, and killer whales. In addition, ramming is important to male-male competition in many even-toed ungulate species, such as goats and ibex – the group of mammals from which whales evolved.
Although this study demonstrates the ramming hypothesis is plausible, the jury is still out on whether whales regularly engage in this behavior.
“We still don’t know enough about sperm whale behavior to explain the full function of this enormous and fascinating structure,” says Huggenberger.