Male bottlenose dolphins organise gang-like alliances - guarding females against other groups and occasionally "changing sides".
A team studying dolphins in Shark Bay, western Australia, say the animals roam hundreds of square kilometres, often encountering other dolphin groups.
The researchers observed the dolphins there over a five-year period, recording their movements.
Dr Richard Connor, a researcher from the US who took part in this study, first began his studies of the Shark Bay dolphins in the early 1980s.
This latest study reveals that these highly intelligent marine mammals live in an "open society". Rather than males guarding a specific territory, groups have what Dr Connor described as a "mosaic of overlapping ranges".
The fact that the dolphins travel in their troops and frequently encounter strangers reveals a great deal about their intelligence, because when one group meets another, the animals have to decide how to respond.
Shark Bay dolphins deal with this by organising themselves into three different types of alliances.
The first is pairs or trios that work together to capture and herd fertile females. "These consortships can last over a month," Dr Connor explained.
In a "second-order alliance", the animals form "teams" of between four and 14 males which mount attacks on other groups to take their females, or to defend against attacks.
In a third level, the dolphins have "friendly relations" between these larger teams; they join forces to form larger dolphin armies, working together to defend their females against other large, aggressive groups.
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