MORPHOMETRIC VARIATION IN THE MANDIBLE OF MICROTUS-ARVALIS AND MICROTUS-AGRESTIS (RODENTIA, CRICETIDAE)

Skulls of the sibling species Microtous arvalis and M. agrestis were c ollected from the pellets of Barn owls in France, Germany and the Neth erlands. Initial identification was made according to the shape of the second upper molar, although this is not 100% reliable. Multivariate statistical analysis of 45 distance measurements of the lower mandible clearly revealed two groups which corresponded to the two species, ex cept for 17 specimens from France. In further analyses the number of m easurements was reduced to six and discriminant functions derived whic h yielded 100% correct allocation of the identified material, and a me an a posteriori probability of 98.8% correct allocation of the unident ified material. Within-species geographic variation in mandible shape was found and is parallel in the two species, suggesting either simila r adaptation (genetic or ontogenetic) to a geographically varying envi ronment, or hybridisation with genetic introgression.