The threat of killer whales could be why societies of sperm whales in the Atlantic and Pacific differ so much, researchers say.

The sperm whale, the largest toothed whale, possesses the largest brain of any organism on Earth. These leviathans mostly live on giant squid and other creatures of the deep, pursuing them with the most powerful natural sonar known.

The sexes lead very different lives in sperm whales. Males leave their mothers to form ephemeral bachelor groups or to live alone, while females can develop complex societies with multiple levels of organization. At the most basic level of these societies are nearly permanent units of about 10 females that care for and suckle each other's progeny and defend their companions from attack.

Mysteriously, although the female sperm whale societies of the north Atlantic and the east Pacific are genetically similar, their social structures are substantially different. In the Pacific, units of females often temporarily gather with other units of the same clan — groups composed of thousands of females that share distinct patterns of vocal clicks known as codas.

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