The cetaceans are among mammals the most completely adapted to ocean life. They have no dependence on land and the bigger ones can come to grief in shallow water, although there are a few observations of dolphins coming a little way up gently sloping beaches, apparently to take stranded fish. There are at least eighty surviving species of cetaceans; some are known to us only from one or two skeletal remains found on beaches. Like the pinnipeds they are carnivorous, but the baleen whales have specialized in eating huge numbers of small pelagic prey, mainly crustaceans and small fishes. The grey whale is an exception it grubs for benthic animals on the sea-beds. The cetaceans have relatively large heads, primarily to accommodate their specialized jaws, but also especially in the Odontocetes to hold large brains and the organs by which they acoustically sense their environment and communicate with each other. All the baleen whales, and most of the toothed whales and dolphins are marine. But some of the smaller dolphins live far p warm rivers and in estuaries.