The phenomenon of perforated cattle skulls is well known to archaeozoologists. It is a condition that has not only been reported during routine faunal analyses but, unusually for animal palaeopathological studies, it has also been the subject of dedicated research papers. A revolutionary moment in the understanding of this anomaly came with its detection in the skull of a European bison. As a result of this discovery, a developmental disorder of genetic origin is now accepted as the most probable cause. In this paper, it is suggested that the perforations are caused by a developmental disorder that is expressed during the pneumatisation of the frontal, parietal and occipital bones in animals with a large sinus frontalis. The first published example of this condition in pig supports this hypothesis. It is argued that the anomaly is a hereditary trait caused by a recessive gene, the phenotypic expression of which only appears in homozygous recessive individuals. Higher incidences of this condition in past cattle and pig populations in certain places at times, may therefore testify to the occurrence of population bottlenecks and/or inbreeding practices. Copyright © 2009 John Wiley & Sons, Ltd.