The males gang up together to corner their desired females. They are so promiscuous that the females do not know who their babies' fathers are.
Some days, the mothers and their young ones stay together. On other days, the youngsters go off and hang out among themselves.
They are also altruistic, and the
adults often club together to protect their young from predators.
No, it's not some frontier human society. It's bottlenose dolphins of the kind that Whangarei lifeguards say protected them from a shark attack this month.
Auckland University graduate student Fabiana Mourao, who is studying the dolphins' social behaviour, has observed that the same individuals often hang out together even though the animals' promiscuity means that there is no such thing as family groups of mum, dad and the kids in dolphin society.
She is working out whether the animals that hang out together are of the same or different genders - a difficult thing to determine in dolphins because their genitalia are not usually visible.
"If females hang out with females it may be to raise their calves," Ms Mourao said.
"If males hang out with males, two or three males may have long-term alliances to fight for resources - which can be either food or mates.
"Imagine that you're in a bar and try to approach one girl. There's more probability with three guys that they can succeed - it's going to be hard for her to escape."
Male gangs of this kind have been observed among dolphins in Florida and Western Australia, but not in Moray Firth in Scotland.
Ms Mourao has not yet got far enough in her study to find out if it is common in the population of 446 adult bottlenose dolphins that live between the Bay of Islands and the Hauraki Gulf.
Overseas studies have also observed larger male groupings, both in dolphins and in apes, that coalesce occasionally to fight rivals for food or mates.
"It compares to a war," Ms Mourao said.
She said dolphin society was very fluid. "They don't hang out with the same group every day. Last week the mothers and calves were staying together, but in some weeks the juveniles hang out together.
"It's not like families. It's not stable like chimpanzees, who always travel together in the same group."
Ms Mourao, a masters student from Brazil, and Argentine doctoral student Gabriela de Tezanos Pinto, are also studying interactions between the Bay of Islands-Hauraki Gulf dolphins and New Zealand's other three groups of bottlenose in the Marlborough Sounds, Milford Sound and Doubtful Sound.
The populations are tiny - only 65 in Doubtful Sound, a small number in Milford Sound and perhaps several hundred in the Marlborough Sounds. Associate Professor Scott Baker, who is supervising the dolphin research, says there are probably fewer than 1000 bottlenose dolphins around New Zealand.
They are not necessarily endangered, because bottlenose dolphins are found in temperate and tropical waters all over the world. The New Zealand study is also looking at genetic links with dolphins in Kiribati and elsewhere in the Pacific.
But studies of maternally-inherited DNA show that the four groups of bottlenose around New Zealand have been largely isolated from each other, let alone the rest of the world, for hundreds and possibly thousands of years.
"They may be connected back through some other, larger, pelagic [ocean-going] population that we don't know anything about."
The males gang up together to corner their desired females. They are so promiscuous that the females do not know who their babies' fathers are.
Some days, the mothers and their young ones stay together. On other days, the youngsters go off and hang out among themselves.
They are also altruistic, and the
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