Male dolphins have a very open but complex social structure, similar to that of human males, say researchers.
Professor Bill Sherwin, of the University of New South Wales, and colleagues, report their findings today in the journal Proceedings of the Royal Society B.
"It's the first study to show this level of complexity in male dolphin behaviour," says Sherwin
Researchers have always known that the social structure of male dolphins is very complex.
Small groups of two to four male dolphin 'cousins' are known to gang up to make sure at least one of them mates with a particular female.
This ensures their genes are passed on to someone they are related to, if they are not the successful mate themselves, says Sherwin.
But in addition to these "first order alliances", which are permanent, male dolphins are also known to form other less permanent groups, ganging up with other gangs at particular times, and competing with them at other times.
Past attempts to find an appropriate model from the animal kingdom to describe the social structure of male dolphins have failed, says Sherwin.
In the most recent attempt, he and colleagues checked two different previously untested models to see if they applied to dolphins.
One model, taken from the social structure of chimps, involves a small number of male gangs defending the boundaries of the community. These gangs cover the territory of all other individuals.
The other model is one where gangs defend small groups of females during the mating season, by covering a smaller territory.
In one of the most extensive studies of its kind, the researchers tracked the movements of dolphin gangs in Western Australia's Shark Bay.
They followed the movements of hundred of dolphins, all of which had been tagged and given names, such as 'Captain Hook' and 'Flat Fin' based on the shape of their fins.
But much to their disappointment, Sherwin and colleagues found no evidence to support the "community defence" or the "mating season defence" models.
It appears that dolphins have an open social network with fluid boundaries as well as a complex hierarchy of groups, says Sherwin.
"You can't just draw a bound a boundary around the edge and say this is where everyone interacts and outside that they don't interact very much," he says.
Sherwin says humans are the only other creatures that have the same kind of social structure.
They interact most often within families and less often within communities and nations but there is free flow between different groupings.
Such an open social network requires a lot of brain power and the researchers suggest that dolphins' big brains may be related to their social structure.
Similarly, their streamline body might also support an open social work, because it enables dolphins to easily travel between different groups.