A new study addresses a controversial hypothesis regarding the potential ramming function of the sperm whale's head. The hypothesis, originally proposed by a 19th century whaler, suggests that the forehead of male sperm whales evolved partly to be used as a battering ram weapon when fighting for access to reproductively active females. This hypothesis was instrumental in inspiring Herman Melville to write the novel Moby Dick but its mechanical feasibility had never been addressed.
An interdisciplinary team from Australia, UK, USA and Japan used structural engineering principles to test how the head of the sperm whale might be able to resist strong ramming impacts.
"The sperm whale forehead is one of the strangest structures in the animal kingdom," says Dr. Olga Panagiotopoulou from the University of Queensland, and the lead author of the paper. "Internally the forehead is composed of two large oil-filled sacs, stacked one on top of the other, known as the spermaceti organ and the junk sacs. It is the oil within the upper spermaceti organ that was the main target of the whaling industry in the early 19th century." Panagiotopoulou adds, "This whole complex is highly sexually dimorphic which means that it is much larger in males than in females, a pattern commonly found in species in which males fight to compete for females."
The battering ram hypothesis was originally proposed by a 19th whaler, Owen Chase, after his ship, the Essex, was sunk by a large male that intentionally rammed the ship with his forehead.
Professor David Carrier, a co-author from the University of Utah said "we know that the sperm whale head is important in transmitting sonar clicks and there are many other hypotheses about its role in communication and buoyancy." "But none of these hypotheses could explain how the sperm whale head could function as a weapon capable of sinking ships that are four to five times the mass of the whale."
"The ramming hypothesis was received with reluctance by the scientific community" says Panagiotopoulou. "This was mainly because the front part of the sperm whale head houses sensitive anatomical structures that produce the sounds essential for sonar and would be in harm's way in a ramming event. Also not many people had actually observed sperm whales ramming."
"We were fascinated when we received a report from a pilot and conservation researcher, who documented sperm whales ramming while flying over the Gulf of California" says Carrier. And adds "we then knew that our ramming hypothesis had some merit and looked into the available technology to test it."
"Creating a computer model to simulate ramming in sperm whales was a challenging task" adds Dr. Spyridis, consulting engineer and a co-author of the study. "When analysing bridges, tunnels, or buildings you are given exact measurements and material properties for the simulations but in this case we were restricted to limited published data and we had to perform a series of sensitivity tests to ensure model efficiency," he adds.
"We used probabilistic simulation to study the mechanical effects of impact variation," said Associate Professor Todd Pataky from Shinshu University in Japan and senior author of the paper. "After creating a series of modified versions of the type and direction of impact force on the sperm whale head, we concluded that the connective tissue partitions embedded within the junk absorb impact stresses and protect the skull from fracturing"
"Increased skull stresses at a ramming event can be detrimental for the animal since they can cause fatal fractures," says Panagiotopoulou. "Our findings show that the mechanical advantage of the structure of the junk may be the result of acquired traits related to selection on male to male aggressive behaviour. Although male sperm whales may not fight frequently, we know that aggressive ramming behaviour is a common characteristic in the group of mammals from which whales are derived -- the even-toed ungulates, the artiodactyls. A closer look into the anatomy of the heads of other species of whales that ram may reveal a variety of protective mechanisms."
"Our study has limitations but we hope to stimulate future research to unravel the mechanical function of the head during head-butting events in other species, where aggressive behaviour has been observed, but remains unmodelled" added Panagiotopoulou.
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