Just like some humans, dolphins are picky about their friendss and shun rival groups.
However, an international team of researchers, led by St Andrews University, discovered different groups found a way to cooperate with each other.
The study, published in Marine Biology, investigated the social network of dolphins in the northern Adriatic Sea.
It showed dolphins formed distinct social groups, and some do not like each other.
It is known that dolphins usually live in groups, and sometimes members join or leave.
But the research shows groups are not random.
Instead dolphins prefer to spend time with others who could be described as their “best friends”.
The researchers, from the Morigenos Slovenian Marine Mammals Society and the Sea Mammal Research Unit at St Andrews, studied the dolphins for more than 16 years.
They found the dolphin society was made up of three distinct groups – two large, with stable membership and long-lasting friendships, and a smaller third, nicknamed freelancers, with much weaker bonds.
Although the two large groups tended to avoid each other, they did manage to share areas of the sea, using it at different times of the day.
St Andrews’ Tilen Genov 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.”