They look friendly and gentle enough. But dolphins also have a dark side, a study has revealed.
When they want to get their way, they resort to 'rape'.
An international team of researchers spent six years studying the behaviour of 120 bluenose dolphins in Shark Bay, Western Australia.
They observed males asserting their authority by forcefully mounting other males. These were short-term shows of strength, used to dominate males from other groups.
The study also showed a complex 'open society' where dolphins exhibited periods of homosexuality and bisexuality.
So complicated are the bonds between dolphins that one of the researchers has described them as 'mentally and physically exhausting' - and declared he was glad he wasn't a dolphin.
When faced with the challenges of day-to-day life, males formed three different types of friendship.
In 'first-order alliances', two or three males co-operate to herd a female for mating.
Most males are also members of 'second-order alliances' - long-lasting agreements in which they fend off other groups trying to take their females. These gangs also mount attacks on other groups to take their females.
Sometimes, the second-level alliance can be strengthened by forming 'third-order alliances' with males from other groups.
This can lead to bloody battles in which more than 20 dolphins bite and bash each other over the right to one female.
The females, in contrast, don't form any strong friendships.
The journal Proceedings of the Royal Society B reports there was only one observation of females forming a temporary coalition against young males. Most animals form alliances to defend their territory. But the study did not find any evidence of this.
Instead, the dolphins appeared to live in a society without boundaries, in which animals paired and teamed up for a time, before splitting apart or even switching sides.
Researcher Richard Connor, of the University of Massachusetts - Dartmouth, said dolphins would have to be incredibly smart to compute the 'soap operatics' of such a life.
Only human society has such a complex social structure.
Professor Connor told Discovery News: 'I work on the male dolphins and their social lives are very intense. It seems there is constant drama.'
The study is just the latest example of dolphin intelligence.
The creatures also communicate through body language and can be taught to understand the basic elements of human language including vocabulary, sentences, questions and demands.
Daily Mail, London