Elephants are best known by many people in the world for their teeth. It is because of the human lust for ivory that thousands of people and countless elephants have been slaughtered for their teeth. Ivory is really only dentine and is no different from ordinary teeth. It is the diamond shaped pattern of the elephant's tusk looking from a cross-section which gives elephant ivory its distinctive lustre. The tusks present at birth are only milk teeth which fall out after around one year of age (approximately 5 cm long). The permanent tusks begin to protrude beyond the lips of an elephant at around 2-3 years of age, and will continue to grow throughout its life. Were and elephant's tusks able to grow long enough they would be in the shape of a spiral (similar to the extinct woolly mammoth), because the tusk typically follows a sinusoidal curved growth pattern. The growth rate of tusks is at approximately 15-18 centimeters per year. About one quarter of the tusk is hidden within the socket. Thus, the typical question of why people can not just cut off the elephant's tusks to ward off poachers is answered. There is still a large amount of ivory that is attached inside the head and attached to the skull, which has to be carved out of the head to be removed. The actual base of the tusk is hollow and contains the pulp cavity. This usually extends quite far, and in males may reach beyond the lip line. The tusk grows from its base as fresh dentine is slowly deposited over the surface of the pulp cavity. Interestingly, for females this cavity begins to fill in with age. The pulp is composed of a highly vascular tissue (blood vessels and nerves), which is amongst unspecialized connective tissue. Although both African elephant sexes have tusks, there are large differences in size and weight. Typically, the male tusk has a larger circumference in relation to its length, is stouter, and is much heavier. Interestingly, some elephants are born without tusks. This hereditary condition causes huge differences in the musculature and shape of the neck and the head. Also, the carriage of the head is different and the bones at the back of the skull are less developed. Interestingly, not all male Asian elephant elephants have tusks; approximately 40-50 percent of male Asian elephants are tuskless. These particular males are known as makhnas in India. Some likely reasons for the greater proportion of tuskless Asian elephants compared the African elephants may be due to strong selection in the past by humans killing the tusked male elephants and an gene in Asian elephants which is not as recessive. When fully developed the Asian elephant's tusks does compare to the weight and size of the African elephant's tusks. The tusks of a male African elephant may exceed 200 kg for the pair although such weights are rare today because most of them have been murdered. The heaviest tusks recorded were 209 kg for the pair taken from an old bull shot in 1897 (British Museum of Natural History). The working tusks that an elephant favours tends to be more worn down over the years. The tusks server a variety of functions for an elephant; this is for digging up soil at salt licks, ripping off bark from trees, for resting a heavy trunk, and as weapons.