Although dolphins seem cute and friendly to us, they can cause serious trouble for their kind. This is quite understandable: animals generally do not tolerate outsiders.
In solitary species such as predatory seals, burrowing rodents, or songbirds, individuals defend their territory from conspecifics (although they may be quite tolerant of neighbors who do not cross boundaries).
In social animals, the situation is similar, but the entire group defends the territory. Of course, if a potential mate 'invades' your territory, there will be a different attitude towards it. Social animals may accept a lone representative from another group who can become a mate or bring benefits to the new family. However, instances where two groups meet and treat each other not only peacefully but also friendly are extremely rare.
Bottlenose dolphins demonstrate such behavior. An article published in Royal Society Open Science describes the behavior of a group of dolphins in the Bimini area of the Bahamas. At one point, other dolphins, separated from a group located 160 km to the north, joined the local group. The meeting was peaceful: the new and local dolphins began to swim together, touch and rub against each other, play, and mate. At least the incoming males clearly showed interest in the local females; however, whether mating occurred will only be known after genetic testing of the offspring. Local males showed no interest in the incoming females, and researchers cannot explain this yet (although it is possible that interest existed but went unnoticed). The outsiders were larger and outnumbered the locals, and in the event of a conflict, they could have displaced them. Nevertheless, no conflict occurred. Communication happened not only between females and males — males from different groups also showed friendliness by touching each other with their pectoral fins, swimming and diving nearby, etc.; thus, it cannot be said that the outsiders were simply tolerated or ignored.
It is known that young male dolphins can form strong friendships regardless of group affiliation, but this concerns juveniles, while in this case, adult individuals are observed. It is possible that there are other dolphin species with similar social openness and tolerance, but at the moment, bottlenose dolphins remain a unique example.
