Cetaceans — whales and dolphins — are among the brainiest of beings. In terms of sheer brain size, the sperm whale is tops on Earth, with a brain six times larger than that of a person.

And now, scientists have identified key differences among cetaceans linked to brain size. A study of 90 cetacean species published on Monday found that those with larger brains exhibit greater complexity in social structures and behaviors, with species like the killer whale and sperm whale leading the way.

"Dolphin and whale societies are at least as complex as what we have observed in primates," said evolutionary biologist Susanne Shultz of the University of Manchester in Britain.