Size and shape variability in relation to species differences and climatic gradients in the African rodent Oenomys

Aim The modern African murine rodent Oenomys is a semiarboreal genus characteristic of the rain forest zone. The size and shape geographical differentiation is studied in order to discuss the possible evolutionary patterns and constraints leading to this distribution. Location Two species of Oenomys have been previously recognized, corresponding to the West African forest block on the one hand and to the main forest block on the other hand. Both species have been sampled, and forty localities permit the study of: the whole geographical range of Oenomys, from Guinea to Kenya. Methods A Fourier analysis applied to the outlines of the first upper and lower molars allows a quantification of the size and shape variations across the geographical range of Oenomys. Results This morphometrical analysis defines three morphological groups of Oenomys, corresponding to the West, Central, and East African forest zone. This result suggests the existence of an additional cryptic species. The Western group is characterized by broad asymmetrical molars and a small size. The Central African group has more elongated and larger molars while the Eastern African group displays extremely constricted molars and a small size. Main conclusion The shape can be considered as charactheristic of each group and its variations appear to be mostly related to phylogeny while size exhibits variations within each group related to climatic gradients. This discrepancy could be related to different genetic determinants for both characters, the shape being strongly genetically constrained while size can vary according to a wide range of environmental factors.