What does "arboreal locomotion" mean exactly and what are the relationships between "climbing", environment and morphology?

Abstract

The characteristics of "climbing" in the sense of locomotion or posture on three-dimensional substrates are discussed from a biomechanical viewpoint. For this purpose, the mechanical conditions of the most widely spread modes of locomotion or gaits used in arboreal surroundings are reviewed. This allows precise identification of morphological characteristics of traits that are advantageous, and therefore have a positive selective value. Further, at least some of the environmental and substrate characteristics that need to be present for using a specific gait, are noted. It turns out that the extremity which is placed lower on the substrate, has to carry a higher load. If this extremity is consistently the hindlimb--which actually is the case in primates, because of understandable, though complex reasons--a division of labor is likely to occur between the limbs: the hindlimb becoming stronger and the forelimb weaker, but more versatile. A very specific, and advantageous feature of the primates is their possession of prehensile hands and feet. That means the autopodia are able (1) to produce by themselves, without the aid of body weight, very high frictional resistance, and (2) to transmit tensile forces as well as torsional moments on the substrate. The above-mentioned division of labor between fore- and hindlimbs implies that the former make the first contacts with and explore the properties of parts of the environment. As a next step, prehensile hands on long arms may easily replace length and mobility of the neck in getting hold of food items. So very characteristic traits of human body shape can be derived to a large extent from the necessities of arboreal locomotion: Prehensile hands, long arms, concentration of body weight on the hindlimbs, shortness of the trunk in comparison to limb length.