Planet Earth: An Illustrated History

Sea Life
A pod of narwhals makes its way through an ice passage in the Arctic. The unicorns of the sea, Monodon monoceros are members of the order Cetacea, along with whales, dolphins and porpoises. Unlike baleen whales, which filter small prey from the water, narwhals are toothed whales that feed on small fish. Male narwhals grow a distinctive single tusk, though some female narwhals have one. The tusks grow in a clockwise spiral and can reach almost 10 ft. in length; narwhals themselves are 23 to 26 ft. in length.

• The Book, "TIME Planet Earth: An Illustrated History"