| abstract
| - Browsers are the largest animals living in the coniferous forest regions. They feed mainly on young twigs and needles in the summer and subsist on bark, mosses and lichens during the rest of the year. Across much of the Northern Continent the most prolific species are those that are derived from the gigantelopes that originated in Africa. These northern antelope, although much heavier than their distant ancestors, are still not nearly as huge as the African gigantelopes. Only the shaggy tundra-dwelling forms of the far north can compare in size with these. File:Ancestral hornhead.gif This difference in size between the two different northern forms is due to two separate periods of migration. The first took place about 40 million years ago, before the great mountain barriers between Africa and Europe were thrown up and at about the time that the rabbuck was ousting the antelope from its traditional home on the African plains. The gigantelopes, then at an early stage of evolution, were forced to spread northwards into the coniferous forest, where they later flourished and developed into the hornheads. The second migration took place more recently, about 10 million years ago, when the African gigantelopes had reached their present elephantine proportions. The erosion of the Himalayan Mountain chain that once separated the Indian Subcontinent from the rest of Asia opened up new paths to the north and led to their gradual colonization of the tundra and the evolution of the woolly gigantelopes. Soon after their arrival in the coniferous forest the ancestral hornheads' jaws and horns began to evolve in response to their new environment. In common with all the now almost extinct ruminants, most of these creatures possessed no upper incisor teeth. They cropped grass by working their lower incisors against a bony pad on the roof of the mouth. However, this system is not particularly effective for browsing from forest trees. The first change that took place was that the horny head plate became extended forward to form a sort of beak. The lower lip became muscular and grew forward to meet it, thus extending the mouth some distance beyond the front teeth. This fairly primitive arrangement is still found in several species, for example the helmeted hornhead. In more advanced forms, however, like the common hornhead, the lower jaw is also extended so that the lower front teeth meet the horny beak instead. These adaptations are the result of evolutionary pressure that enabled only those forms that could feed successfully on the twigs, bark and lichens of the coniferous trees to survive. The elaborate horn formation above the eyes is also used for defense. The horn structure has been carried one stage further in the water hornhead. The hornhead's horn formation grows gradually throughout adolescence and early adulthood and in the case of helmeted hornhead takes its final form around three years of age.
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