A 9-year study has uncovered some unusual behavior by common bottlenose dolphins (Tursiops truncatus) living off the coast of Slovenia. Within one population of this species, the animals have divided into two groups that avoid contact by hunting at different times of day—a social strategy not known in marine mammals.
Researchers used photographs of the dolphins’ dorsal fins to individually identify them. They made many observations of 38 of the animals, carefully recording the time, date, and location of each sighting. The marine mammals divided into two major groups of 19 and 13 animals each, with six animals loosely making up a third group, the team reports today in Marine Biology. The 19 members of the larger group tended to hang out—and likely hunt—while following fishing trawlers in the Bay of Trieste, which is located at the eastern top of Italy’s “boot.” The second group’s cadre of 13 never associated with boats when in the Bay of Trieste. Although the dolphins hunted in the same area, they rarely saw each other, the researchers discovered, because the larger group was in that area only between 7 a.m. and 1 p.m. local time, whereas the smaller group showed up between 6 p.m. and 9 p.m.
Other studies have documented groups of dolphins that divide up the waters where they hunt, but this is the first time these marine mammals have been shown to timeshare the sea, the researchers note. Although they don’t know why—or how—the dolphins set these schedules, the fact that the animals are never in the same place likely diminishes unfriendly encounters and reduces direct competition for food.