Gowans,S., and L. Rendell. In Press. Head-butting in northern bottlenose whales (Hyperoodon ampullatus): a possible function for big heads? Marine Mammal Science.
Northern bottlenose whales (Hyperoodon ampullatus) show marked sexual dimorphism in the melon which is enlarged, flattened and squared off in males. Skulls of males have a mass of dense bone associated with this flattened region, and skin on the front of the melon is often white. During the summer of 1998, we observed two mature male bottlenose whales display a stereotyped circle and head-but behavior, which they repeated several times during two encounters on subsequent days. In between the two head-butting encounters, these same two individuals were observed resting together at the surface. These observations were the first observations of what we considered aggressive interactions in 10 years of study on this species. Our interpretation of these observations is that the head-butting is a form of male-male aggression, similar to the head-to head interactions of many terrestrial ungulates. We believe the behavior reported here may be a rare but important aspect of the social behavior of male bottlenose whales, and it is possible that the dense bone mass in the head of northern bottlenose whales is an adaptation for such interactions. The lack of such structures in females suggests that equivalent interactions are not an important aspect
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