The body center of mass in primates: Is it more caudal than in other quadrupedal mammals?

OBJECTIVES: Whole body center of mass (BCoM) position values are lacking for a comparative sample of primates. Therefore, it still remains unknown whether the BCoM in primates is more posteriorly located than in other mammals. The aim of the present report is to provide data for a large sample of primate species and to compare the position of the BCoM in primates to non-primate mammals. MATERIALS AND METHODS: We collected morphometrics on eight primate species belonging to various families: Hylobatidae (Nomascus grabriellae, Nomascus Siki), Cercopithecidae (Cercopithecus roloway, Cercopithecus lhoesti, Colobus guereza, Trachypithecus francoisi), Cebidae (Sapajus xanthosternos), and Atelidae (Ateles fusciceps). Using a geometric model, we assessed the position of the BCoM in a natural quadrupedal posture and in a control posture. To complete our comparative sample with a wider range of morphotypes, we added the data available in the literature for hominoids (Pan paniscus, Pan troglodytes, Gorilla gorilla, Pongo pygmaeus, Hylobates lar) and another cercopithecoid species (Papio anubis). We also evaluated the phylogenetic signal of the position of the BCoM in primates. RESULTS: The variation in the position of the BCoM in primates is very large, ranging from 40% of the distance between the hip and the shoulder in Ateles fusciceps to 63% in Hylobates lar. We observed a strong phylogenetic signal for this trait: hominoid species, as well as the baboon, have a cranial BCoM relative to the midline between the hip and the shoulder, arboreal cercopithecoids and the spider monkey have a caudal BCoM, and the capuchin monkey has a BCoM positioned at mid-trunk. The variation observed in non-primate quadrupedal mammals lies inside the variation range of primates, from 51% in Felis catus to 63% in Canis familiaris. DISCUSSION: The BCoM of primates is not more posteriorly located than in other quadrupedal mammals; however, there is a substantial range of variation in primates, from caudal (in arboreal quadrupeds) to cranial (in hominoids and terrestrial quadrupeds) positions. This variation is related to a phylogenetic model that suggests stabilizing selection for this trait. It seems that the BCoM position mostly depends of the size of the appendicular system (i.e., limbs) and the tail. Therefore, it may also reflect a general trend in quadrupedal mammals with arboreal species exhibiting a caudal BCoM and terrestrial species exhibiting a cranial BCoM. These results are discussed in the context of the locomotor evolution of primates including locomotor habits and gait mechanics. We also propose a new "passive" mechanism for the explanation of the particular weight support pattern observed in primates with tails.