Parallel Evolution of Skull Form in Three Rodent Genera Inhabiting Steep Elevational Gradients of Ethiopian Highlands

Abstract

Several rodent genera radiated in Ethiopian highlands during Pleistocene, giving rise to species specialized to different elevation zones. The zonation is accompanied by vegetation change from savannahs or forests in lower elevations, through the mid-elevation belt of ericaceous shrublands to Afroalpine moorlands on the mountain tops. Such evolution on elevation gradient is expected to promote phenotypic adaptations and several genera are known to include highly distinctive high-elevation specialists. In this study, we systematically examined shape and size differentiation of skull and mandible in three genera, Lophuromys, Arvicanthis and Stenocephalemys, each including from six to eight species inhabiting different elevation zones on both sides of the Great Rift Valley. Employing computed tomography, three-dimensional virtual models, landmark-based geometric morphometrics and a powerful toolkit of partial least squares analysis, we demonstrated the elevation-related differentiation is a prominent feature of skull and mandible shape variation in all three genera. We identified the parts of skull and mandible which change most between the successive zones. These include, notably, orbits and insertion areas of masticatory muscles, which suggests adaptive shifts in visual perception and diet. We also showed that while the same parts are modified in all three genera, they are modified in different ways and to a different degree. The evidence for size differentiation was mixed despite the universal trend of larger size in higher elevations. Our study confirms the high-elevation specialists represent the most distinctive forms, which is especially true for the species from Bale and Arsi Mountains, East of the Great Rift Valley.