Big-brained cetaceans, such as the sperm whale, show sophisticated behaviors, including babysitting. (Reuters File Photo/Reuters)

Whales and dolphins — both members of the cetacean family — are among the brainiest beings. Scientists have now identified differences among them that are tied to relative brain size.

A study of 90 cetacean species published Monday found that those with larger brains exhibit more complex social structures and behaviors, with the killer whale and the sperm whale leading the way.

Dolphins and whales "are extremely playful; they learn from each other, have complex communication," said biologist Susanne Shultz of the University of Manchester in Britain. "One problem for understanding just how smart they are is how difficult it is to observe them and to understand their marine world. Therefore, we have only a glimpse of what they are capable of."

Researchers created a database of brain size, social structures and cultural behaviors across cetacean species. The group of species with the largest brain relative to body size were large dolphins, such as the killer whale and the pilot whale, Shultz said. Killer whales' food preferences are one of their complex behaviors.

Other big-brained cetaceans also show sophisticated behaviors. Mother sperm whales, for example, organize babysitting duties to protect their young while they hunt for food. Bottlenose dolphins use sea sponges to protect their beaks while foraging for food, and they live in structured communities.

Some of the largest cetaceans — including the blue whale — were on the low end of relative brain size.

— Reuters