In the calm, shallow waters of Slovenia's Gulf of Trieste, a resident population of bottlenose dolphins has solved an unusual social problem in a way no other marine mammal is known to: they have split into two groups that use the same sea at different hours of the day, almost never meeting each other.
Over nine years of field work, marine biologists followed a single population of common bottlenose dolphins (Tursiops truncatus) in the northern Adriatic by photographing the distinctive notches on each animal's dorsal fin — the dolphin equivalent of a fingerprint. The long record revealed a clear and persistent pattern: the animals divide into two distinct social groups. One group hunts and socializes during daylight; the other takes the water at dusk and into the night. The two groups occupy the same coastline, but they go to lengths to avoid contact with one another.
In most dolphin societies, group membership is fluid: individuals join and leave pods, and social ties are woven by frequent, casual contact. The Adriatic dolphins behave more like cliques, with strong bonds within each group and an almost visible line between them. Researchers suggest the time-partitioning is a way of keeping the two cliques apart — a "timesharing" arrangement that reduces friction between groups that would otherwise compete for the same fish and the same stretch of sea.
The finding challenges the old assumption that bottlenose dolphin social structure is uniform and fluid worldwide. Here, in a small, semi-enclosed bay, dolphins have settled into unusually stable factions that partition the day to live side by side without colliding. It is a reminder that even a well-studied, charismatic species still holds social surprises.