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, who has used magnetic resonance imaging scans to map the brains of dolphin species and compare them with those of primates.
"The neuroanatomy suggests psychological continuity between humans and dolphins and has profound implications for the ethics of human-dolphin interactions," she added.
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.
In one study, Diana Reiss, professor of psychology at Hunter College, City University of New York, showed that bottlenose dolphins could recognise themselves in a mirror and use it to inspect various parts of their bodies, an ability that had been thought limited to humans and great apes.
In another, she found that captive animals also had the ability to learn a rudimentary symbol-based language.
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.
In one recent case, a dolphin rescued from the wild was taught to tail-walk while recuperating for three weeks in a dolphinarium in Australia.
After she was released, scientists were astonished to see the trick spreading among wild dolphins who had learnt it from the former captive.