Swans are birds of the family Anatidae within the genus Cygnus. The swans' close relatives include the geese and ducks. Swans are grouped with the closely related geese in the subfamily Anserinae where they form the tribe Cygnini. Swans feed in the water and on land. They are almost entirely herbivorous, although they may eat small amounts of aquatic animals. In the water food is obtained by up-ending or dabbling, and their diet is composed of the roots, tubers, stems and leaves of aquatic and submerged plants. Although swans only reach sexual maturity between 4 and 7 years of age, they can form socially monogamous pair bonds from as early as 20 months that last for many years, and in some cases these can last for life. The lifespan of the mute swan is often over 10 years.