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dolphin (or other small toothed whales, incl. porpoises, belugas, etc.)
水族館で海豚のショーを見た。
I watched a dolphin show at the aquarium.
海豚は知能が高く社会性がある。
Dolphins are intelligent and social.
船の周りを海豚が泳いでいた。
Dolphins were swimming around the boat.