The case of an insular molarless black rat: Effects on lifestyle and mandible morphology

We report a specimen of an insular black rat (Rattus rattus) from Illa den Colom (Menorca, Western Mediterranean) displaying a singular dental characteristic. It has no molar teeth but displays regular incisors. Its mere occurrence as a regular adult rat is puzzling and we attempted to evaluate what diet and morphological changes in jaw shape were promoted by the total lack of molars, and allowed the successful survival of this specimen. Two approaches were performed: first, bone tissue was analysed to obtain δ15N and δ13C values in order to estimate dietary preferences of the rat. Second, the shape of the jaw was analysed through elliptic Fourier analysis, using outlines as markers of diet. The values for C and N fractionation (−19.89‰ and 10.06‰, respectively) suggest that the molarless rat included animal food in the diet and not exclusively plant material as observed in other mainland rat populations. The morphometric analysis in which the shape of the molarless mandible falls into the range of omnivorous groups leads to a similar conclusion. The adult age of the specimen suggests that it fed efficiently enough with its incisors to allow a normal growth. Although displaying a lack of molar teeth, no deep changes in remodelling jaw morphology can be observed and its shape falls into the variation of regular murines. The molarless rat exemplifies that special ecological features on small islands allow the survival of aberrant morphotypes.