The simplex and other forms of the upper third molar (M3) in the common vole (Microtus arvalis) in Zeeland, the Netherlands

The frequencies of the various forms of M3 (simplex, transient and normal) in common (Microtus arvalis) voles were determined in different regions (all former islands) in the province of Zeeland, the Netherlands. A possible higher incidence of left-sidedness of simplex and transient M3 forms was also investigated. The frequency of simplex forms of M3 has not apparently been studied in any other part of the Netherlands, except for the north. Common vole specimens (1254 skulls) from owl pellets that were collected between 1995 and 2010 were studied. The M3 on both sides was assessed and the length of the left molar row was measured. This article presents an analysis of the prevalence of the simplex, transient and normal forms. Based on the measured molar lengths, an agedependent bias in the classification of the morphotypes is unlikely. In the sample the simplex form of M3 occurred 4.9% on the left hand side and 6.7% on the right. Breaking the sample down into regions almost all the statistical limits of confidence for the left side remain within these limits of that region at the right side and vice versa. The only exceptions were for the regions of Schouwen-Duiveland and Zeeuws-Vlaanderen where the confidence limits on both sides exceeded each other.