An international team led by Scotland's University of St Andrews looked at the behaviour of bottlenose dolphins in the Gulf of Trieste in the northern Adriatic sea for more than 16 years.

The dolphin "social network" was split into mixed-sex clusters, including two main groups that enjoyed "stable membership and long-lasting friendships", they found.

Both groups contained "core membership" with extra tiers, while dolphins in one of the groups sometimes formed smaller factions within themselves.

They were connected by several dolphins which acted as "social brokers", preventing "complete cluster isolation", the researchers said.

A third smaller cluster, nicknamed "freelancers", shared much weaker bonds.

The rival groups tended to avoid each other but shared particular areas of water by using them at different times.

Tilen Genov, of St Andrews' Sea Mammal Research Unit (SMRU), said: "We were quite surprised by this.

"It is not uncommon for dolphins to segregate into different parts of the sea, but to have certain times of the day in which they gather is unusual.

"We would sometimes see one social group in the morning and then the group in the same area in the late afternoon."

The researchers also suggested that ecological constraints, such as the availability of prey, could explain the inclusion of older-looking dolphins.

The study, published in Marine Biology, said: "With lack of major prey-aggregating bottom features, spatio-temporal distribution of prey is likely highly variable, which may promote network connectedness.

"Clusters A and B both contained individuals which appeared 'older' based on their external appearance.

"These animals may possess long-term knowledge needed to tackle such constraints and thus play a key role in their community."

Australian Associated Press