Factors impacting the complexity of the leporid intracranial joint

Leporid lagomorphs, the rabbits and hares, exhibit unique cranial traits that distinguish them from their closest relatives, the Ochotonidae (pikas), and all other mammals. Among these features, the intracranial joint stands out as the only example of cranial kinesis in mammals and is hypothesised to dissipate kinetic energy during high-speed locomotion. Despite its potential functional importance, the morphology of the joint remains understudied. High complexity in other cranial sutures has been associated with behavioural traits such as feeding and head butting. In this study, we quantified the complexity of the intracranial joint using two independent metrics and explored its relationships with overall cranial shape, locomotor mode, cranial size and body mass, burrowing habit, and facial tilt angle. We found significant correlations between complexity and locomotor mode as well as with facial tilt angle, indicating that cursorial species have less complex sutures and highlighting a potential link between the complexity of suture interdigitation and facial tilting. However, complexity did not correlate with size or burrowing habit. Our findings shed more light on the functional anatomy of the leporid cranium and emphasise the need for further research on ontogenetic development, biomechanics, and behaviour to fully understand the evolutionary and functional significance of these unique cranial traits.