Morphology of the Humerus and Femur in African Mangabeys and Guenons: Functional Adaptation and Implications for the Evolution of Positional Behavior

Abstract

The morphology of the humerus and femur was examined in three mangabey species (Cercocebus albigena, Cercocebus torquatus, Cercocebus galeritus) and three guenon species (Cercopithecus mitis, Cercopithecus mona, Cercopithecus aethiops). Cercocebus albigena, Cercopithecus mitis, and Cercopithecus mona are strictly arboreal whereas Cercocebus torquatus, Cercocebus galeritus and Cercopithecus aethiops are more frequently utilize terrestrial substrates. Morphological differences, which presumably reflect different positional behaviors, were found within both Cercocebus and Cercopithecus genera. The arboreal Cercocebus albigena differs from the more terrestrial Cercocebus torquatus and Cercocebus galeritus in having more mobile joints and more grecile bones. In Cercocebus torquatus and Cercocebus galeritus, joint movements tends to be restricted to the parasagittal plane emphasizing the economy of parasagittal excursion of the limbs. Similar tendencies were observed between the arboreal Cercopithecus mitis and Cercopithecus mona and the semi-terrestrial Cercopithecus aethiops. However, the morphological distinctions, associated to arboreality vs. terrestriality, are not identical between Cercopithecus and Cercopithecus. Semi-terrestrial mangabeys exhibit stronger adaptations for terrestrialiy by comparison with the semi-terrestrial guenon. While the postcrania of the arboreal mangabey seem to be adaptive for deliberate movements in trees, those of the arboreal guenons suggest more rapid and agile activities. In addition, the arboreal Cercopithecus mona exhibits features related to leaping, which are not found in the more deliberately moving Cercopithecus mitis. Despite these differences, the common morphotype of the humerus and femur in the Cercopithecinae is exhibited in both Cercocebus and Cercopithecus, contrasting with the morphology shown by other primates (Colobinae, Hominoidea and Cebidae). The cercopithecine morphotype is characterized by the restriction of joint movements for economical fore-aft excursions of the limbs. Both the large-bodied cebids and apes exhibit a morphotype that allows greater mobility of the joints involving axial rotations and abduction. The humerus of apes is further characterized by specializations for suspensory positional behaviors. The morphotype of the limb bones of the Colobinae differs from that of the Cercopithecinae in having less restricted joint movements. However, the morphotypes of these two subfamilies show a greater similarity to each other than to those of large-bodied cebids and apes. It is presumed that positional behaviors of ancestral cercopithecids involved parasagittal excursions of the limbs.