Filtering whales, unlike the gregarious porpoises and dolphins, seldom school. Even when they appear to be abundant in a limited area, they have congregated for feeding and not for social interaction. Toothed cetaceans, on the other hand, are generally sociable. The sperm whale travels in large groups of females dominated by a single bull whale. The other males have been driven away and the victorious male exercises control of the harem only as long as he wins these contests. Once the dominant male is defeated, he becomes a solitary individual.