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Relative Growth of the Titanothere Horn: A New Approach to an Old Problem

Steven M. Stanley
Evolution
Vol. 28, No. 3 (Sep., 1974), pp. 447-457
DOI: 10.2307/2407166
Stable URL: http://www.jstor.org/stable/2407166
Page Count: 11
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Relative Growth of the Titanothere Horn: A New Approach to an Old Problem
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Abstract

The titanothere horn, which underwent extraordinary evolutionary growth relative to other skeletal dimensions, served as a device to protect the head and neck region against injury during butting, which was probably chiefly intraspecific in nature. In terms of morphological and behavioral traits related to butting, titanotheres are to rhinos within the Perissodactyla as sheep are to goats within the Caprini. Rhinos and goats have sharp horns and do not commonly engage in severe intraspecific combat. Advanced titanotheres display adaptations for head-on ramming analogous to those of sheep. Growth of the titanothere horn apparently served a compound function. First, it provided a broad, solid surface of impact distal to vulnerable portions of the facial region. Second, it brought the force vector of impact toward horizontal alignment with the occipital condyles, reducing the likelihood of neck damage; a slight offset probably remained in titanotheres, as in sheep, allowing the nuchal ligament to absorb some of the impulse and reduce the maximum force of impact. And third, by producing two divergent flanges, it formed a self-righting device that reduced the possibility of lateral twisting of the neck and may also have prolonged the duration and lessened the maximum force of impact. Large-horned titanothere species, like large sheep, possibly used their horns in part for display, to reduce the frequency of physical combat, but this function is questionable because titanotheres had small and presumably weak eyes. By analogy with sheep, sexual selection probably produced the butting adaptations of titanotheres, and it probably also produced the marked increase in body size. Frequency and severity of injury may or may not have decreased in titanothere evolution, depending on how behavior changed and how forces of impact and strength of resisting structures scaled with size, but any argument that relative growth of the horn was nonfunctional or deleterious is totally unjustified.

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