A spatio-temporal decrease in molar size in the western European house mouse

We investigate the existence and persistence of a temporal and a spatial gradient in molar size of the house mouse, Mus musculus domesticus, from France, Europe and northern Africa. Specimens include both modern samples extending from the beginning of last century to extant populations as well as archeological material. Several patterns in size variation were evidenced: i) there is a size gradient in molar teeth decreasing from North to South in modern mice, ii) a similar North-South gradient is present in archeological material indicating that it has existed at least since Gallo-Roman times, and iii) there is a decrease in size between the Gallo-Roman samples and modern ones. The geographical gradient was not gradual, but was characterized by a step cline on both sides of the 45th parallel, that is between the Mediterranean zone and the western European Continent. Within each of these compartments, no latitudinal variation from North to South was observed. The decrease in size of the house mouse since the Gallo-Roman period, or likely since the end of the Middle-Ages, could be due to landscape clearance and expansion of cereal-like cultivations. The commensal way of life, associated with an increase in population density, may have also had an effect.