Male animals already provided with efficient cutting or tearing teeth for the ordinary purposes of life, as in the carnivora, insectivora, and rodents, are seldom furnished with weapons especially adapted for fighting with their rivals. The case is very different with the males of many other animals. We see this in the horns of stags and of certain kinds of antelopes in which the females are hornless. With many animals the canine teeth in the upper or lower jaw, or in both, are much larger in the males than in the females; or are absent in the latter, with the exception sometimes of a hidden rudiment. Certain antelopes, the musk-deer, camel, horse, boar, various apes, seals, and the walrus, offer instances of these several cases. In the females of the walrus the tusks are sometimes quite absent.[291] In the male elephant of India and in the male dugong[292] the upper incisors form offensive weapons. In the male narwhal one alone of the upper teeth is developed into the well-known, spirally-twisted, so called horn, which is sometimes from nine to ten feet in length. It is believed that the males use these horns for fighting together; for “an unbroken one can rarely be got, and occasionally one may be found with the point of another jammed into the broken place.”[293] The tooth on the opposite side of the head in the male consists of a rudiment about ten inches in length, which is embedded in the jaw. It is not, however, very uncommon to find double-horned male narwhals in which both teeth are well developed. In the females both teeth are rudimentary. The male cachalot has a larger head than that of the female, and it no doubt aids these animals in their aquatic battles. Lastly, the adult male ornithorhynchus is provided with a remarkable apparatus, namely a spur on the fore-leg, closely resembling the poison-fang of a venomous snake; its use is not known, but we may suspect that it serves as a weapon of offence.[294] It is represented by a mere rudiment in the female. When the males are provided with weapons which the females do not possess, there can hardly be a doubt that they are used for fighting with other males, and that they have been acquired through sexual selection.