Only skull-deep? Headbutting adaptations may not extend to the brain cavity

High-impact headbutting behavior makes the muskox (Ovibos moschatus) a charismatic species. While many theorize how these headbutting bovids might protect their brain during such encounters, few have investigated their claims anatomically. We investigated the anatomical function of digitiform impressions in the bovid brain cavity and their relationship to headbutting. This work is divided into two parts. First, we provide an anatomical description of the muskox brain cavity for the first time in literature. We used computed tomography (CT) scans of adult and juvenile muskoxen skulls and scans of a goat (Capra hircus) skull for comparison. As the texture of the muskox brain cavity appeared pronounced, we sought to quantify the digitiform impressions that create this rough appearance, with the hypothesis that they support the brain and reduce injury during headbutting. We developed a simple measurement to quantify digitiform impressions called the endocranial roughness index (ERI). In the second part of this work, the ERI was applied to a sample of 59 headbutting and non-headbutting bovid species. Contrary to our expectations, the quantitative assessment of digitiform impressions across various taxa revealed no correlation between endocranial roughness and headbutting. We conclude that endocranial roughness either only affects brain protection at a smaller scale, or endocranial roughness at the measured scale does not have an effect on brain protection significant enough to cause selective evolutionary pressure.