Dolphins are picky about who they are friends with and shun rival groups, new research has found.
However, an international team of researchers, led by the University of St Andrews, found that the groups still managed to cooperate by sharing the sea – taking turns to inhabit particular areas.
The study, published in Marine Biology, investigated the social network of dolphins in the northern Adriatic Sea. It showed that dolphins living in the Gulf of Trieste form distinct social groups, and some don't like each other.
It is widely known that dolphins usually live in groups, and in the case of the common bottlenose dolphin the composition of these groups changes often, with members often joining or leaving.
However, these groups are not random. Rather, it is individual dolphins preferring to spend time with other individuals who could be described as their "best friends".
The researchers, from the Morigenos Slovenian Marine Mammals Society and the Sea Mammal Research Unit (SMRU) at the University of St Andrews, studied the dolphins in the region for more than 16 years.
They found that the dolphin society in the Gulf of Trieste comprised three distinct groups: two large social groups with stable membership and long-lasting friendships, and a smaller third social group, nicknamed "freelancers", with much weaker bonds and no long-lasting friendships.
However, although the two large groups tended to avoid each other, they did manage to share particular areas of the sea, with each group using them at different times of the day. Such temporal partitioning based on time of day has not previously been documented in whales and dolphins, nor in other mammals.
Tilen Genov, of SMRU at St Andrews, 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 study demonstrates how different segments of the same animal population may behave very differently. In turn they may react differently to human behaviour.
Explore further: Banned toxins passed from mother to young in European dolphins
Tilen Genov et al. Behavioural and temporal partitioning of dolphin social groups in the northern Adriatic Sea, Marine Biology (2018). DOI: 10.1007/s00227-018-3450-8