Elephant society is highly complex and arranged around family units composed of groups of closely related females and their calves. Each family unit contains around ten individuals (6), led by an old female known as the 'matriarch' (2). Family units often join up with other bands of females forming 'kinship groups' or 'bond groups', and larger herds may number well over a hundred individuals (7). Male elephants leave their natal group at puberty and tend to form much more fluid alliances with other males. Elephants are extremely long-lived and although females may reach sexual maturity at ten years old they are at their most fertile between 25 and 45 (2). There is no distinct breeding season, although birth peaks in certain areas may relate to the local rainfall patterns (9). Calves are born after an exceptionally long gestation period of nearly two years, and continue to be dependent on their mother for several years (2). They are also cared for by other females in the group, especially by young females known as 'allomothers' (2). The social bonds between elephants are very strong and if faced with danger they will form a protective circle around the young calves, with the adults facing outwards and the matriarch adopting a threatening pose or even charging the intruder (2). Elephants care for their wounded and also show recognition of, and particular interest in, elephant bones (10). Elephant groups will spend the day wandering their home range in search of food and water (2). An adult elephant requires 160 kilograms of food a day; using their highly mobile trunk they pluck at grasses and leaves or tear at branches and bark with their tusks, which can cause enormous damage (7). Elephants can communicate over large distances and use some vocalisations that are below the range of human hearing (11).