Brain size reduction in dogs was already established at least by the Late Neolithic of Western Europe, 5000 years ago

The timing and causes of brain size reduction in domestic dogs remain uncertain. Using endocast volume as a proxy for brain size, this study provides a first insight into long-term brain size evolution in the wolf-dog lineage. We compared endocranial volumes of 185 modern and 22 prehistoric wolves and dogs ranging from Western Europe to Australia, and spanning the Pleniglacial (35 000 yr BP) to the Late Neolithic (5000 yr BP). Our results reveal that Pleistocene so-called ‘protodogs’ show no brain size reduction compared with coeval Pleistocene wolves. Instead, we observed a slightly larger relative endocranial volume in the 35 000-year-old 'protodog' from Goyet, which could suggest increased behavioural flexibility in the presence of humans. In contrast, Late Neolithic dogs show a drastic brain size reduction (46\%) with endocranial volumes comparable to modern small terrier and toy breeds. We speculate that the anxious and wary temperaments of these Late Neolithic dogs, induced by the brain tissue reorganization associated with such a size reduction, could have served an alerting purpose, among the many other potential roles dogs could have played within these Late Neolithic socio-ecosystems.