Introduction
Many animals strike with their heads at conspecifics, in ritualized flank-butting, head-to-head shoving matches and head-butting combat. Correlates cited for head-butting in modern ungulates include cranial sinuses [1] that form strut-perfused osseous domes above the brain, and secondary correlates include neurovascular canals supplying a protective keratin covering on the skull surface [2]. In addition to colliding with their horns (which spreads the impact in dual horn-horn contacts [3]), bighorn sheep (Ovis canadensis) vigorously impact each other on the apices of their heads between the horn cores [4]. Duikers (Cephalophinae) are small bovids with thick, rounded frontals, which they use in intraspecific head-to-head impacts [5], [6]. Similarly dome-shaped crania of pachycephalosaurian dinosaurs have been hypothesized as appropriate for head- or flank-butting, but internal histology appears to contradict such capability in young and old individuals [7] unless a thick keratinous covering protected the osseous dome [8].
Dome function in pachycephalosaurs has been controversial, with trabeculae within the dome interpreted as developmental traces inconsistent with head-butting [7], or as structures that would halt or absorb strain during collisions [9]. A highly vascular cancellous zone [7] undoubtedly sped the development and growth of pachycephalosaur domes. However, quantitative tests have supported a complementary energy-absorbing role for trabeculae. Farke [1] determined that trabeculae within cranial sinuses of goats would better dissipate strain than sinuses alone, similar to what Snively and Cox [8] found for cancellous regions of some pachycephalosaur domes. Maity and Tekalur [10] corroborated this phenomenon in bighorn sheep, despite a different loading pattern.
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We use CT (computed tomographic) scanning and finite element analysis (FEA) to compare structural capabilities of crania in head-striking artiodactyls (the duiker Cephalophus leucogaster, musk ox Ovibos moschatus and giraffe Giraffa camelopardalis [6], [10], [11], [12], [13]) and a possibly analogous combatant, the pachycephalosaur Stegoceras validum. As controls we examine specimens of other artiodactyls that engage in a spectrum of combative behaviors. Bighorn sheep [Ovis canadensis) butt heads, and regress in horn-horn contact as these ornaments take a greater display function in older rams [4]. Male pronghorn Antilocapra americana collide with cranial ornaments but do not butt heads [14], whereas neither behavior occurs in female elk (wapiti) Cervus canadensis, peccary Tayassu tajacu and llama (Lama glama). Expanding on previous studies of pachycephalosaur crania, we perform 3D finite element analysis (FEA) of simulated head impacts with models based on CT scans.
These methods test three main hypotheses and predictions. First, we test the prediction that like some pachycephalosaurs [8], head-butting artiodactyls will have a deep layer of cancellous bone beneath dense compact bone. Second, we extend Farke’s hypothesis for goats ([1], [5], and references therein), that frontal sinuses with trabeculae would dissipate strain, to other head-butting bovids. The morphology of soft tissue covering pachycephalosaur domes is unknown, and we test the effects of different-shaped keratin pads by applying concentrated and broad forces to the dome. Finally, we examine whether Stegoceras, musk ox and duiker cephalic structures would experience similar stress levels under similar impact loads.
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