Cetartiodactyls are primary, secondary, and higher-level consumers, filling roles of predator (most cetaceans) and prey (most artiodactyls). Terrestrial cetartiodactyls are plagued by ectoparasites such as fleas, lice, and bot flies. Cetaceans, though aquatic, are not free from external parasites either, and are host to barnacles, copepods, and whale lice. Both terrestrial and aquatic species host internal parasites as well, such as tapeworms, flukes, and nematodes. Interestingly, birds have evolved commensal relationships with both aquatic and terrestrial cetartiodactyls. Seagulls follow schools of dolphins and consume small fish stirred up by the cetaceans, and cowbirds follow herds of cattle and consume insects stirred up by the hooves of the artiodactyls. Also, some cetartiodactyl species are mutualists with animals that feed on their ectoparasites: topsmelt (Atherinops affinis) consume whale lice that live on the skin of gray whales (Eschrichtius robustus), while oxpeckers (Buphagus) remove fleas and other parasites from the skin of various African artiodactyls.