Dolphins have been declared the world's second most intelligent creatures after humans, with scientists suggesting they are so bright that they should be treated as "non-human persons."

Studies into dolphin behaviour have highlighted how similar their communications are to those of humans and that they are brighter than chimpanzees. These have been backed up by anatomical research showing that dolphin brains have many key features associated with high intelligence.

The researchers argue that their work shows it is morally unacceptable to keep such intelligent animals in amusement parks or to kill them for food or by accident when fishing. Some 300,000 whales, dolphins and porpoises die in this way each year. "Many dolphin brains are larger than our own and second in mass only to the human brain when corrected for body size," said Lori Marino, a zoologist at Emory University in Atlanta, Georgia.

Dolphins have long been recognised as among the most intelligent of animals but many researchers had placed them below chimps, which some studies have found can reach the intelligence levels of three-year-old children. Recently, however, a series of behavioural studies has suggested that dolphins, especially species such as the bottlenose, could be the brighter of the two. The studies show how dolphins have distinct personalities, a strong sense of self and can think about the future. It has also become clear that they are "cultural" animals, meaning that new types of behaviour can quickly be picked up by one dolphin from another. Other research has shown dolphins can solve difficult problems, while those living in the wild co-operate in ways that imply complex social structures and a high level of emotional sophistication.

There are many similar examples, such as the way dolphins living off Western Australia learnt to hold sponges over their snouts to protect themselves when searching for spiny fish on the ocean floor. Such observations, along with others showing for example, how dolphins could co-operate with military precision to round up shoals of fish to eat, have prompted questions about the brain structures that must underlie them. Size is only one factor. Researchers have found that brain size varies hugely from around 7oz for smaller cetacean species such as the Ganges River dolphin to more than 19lb for sperm whales, whose brains are the largest on the planet. Human brains, by contrast, range from 2lb-4lb, while a chimp's brain is about 12oz.

Marino and colleagues will present their findings at a conference in San Diego, California next month, concluding that the new evidence about dolphin intelligence makes it morally repugnant to mistreat them. Thomas White, professor of ethics at Loyola Marymount University, Los Angeles, who has written a series of academic studies suggesting dolphins should have rights will speak at the same conference.