Size Dependence in Prosimian Locomotion and Its Implications for the Distribution of Body Mass

Abstract

The mechanical requirements for arboreal life are reviewed and the constraints which these requirements impose on the body of a prosimian are defined. The mechanical necessities can be fulfilled only by animals which possess the appropriate morphological characters. It is incorrect to refer to these morphological traits directly as ‘adaptations’. Instead their a priori existence must be considered as the precondition for the acquisition of a certain life-style. Once such a life-style has been acquired, a strong selective pressure acts towards a further refinement of such ‘adaptations’ or ‘pre-adaptations’. Postcranial morphology must be seen in a context of following natural laws and is strictly related to the mechanics of posture and locomotion. The traits emphasised and explained here are body proportions – specifically the relative lengths of body segments and the distribution of (muscle) mass on these segments.

Spatio-temporal gait characteristics of the hind-limb cycles during voluntary bipedal and quadrupedal walking in bonobos (Pan paniscus)

“…Common chimpanzees, bonobos, and modern humans differ considerably in body proportions (relative leg masses, muscularization, distribution of mass over the leg segments; Zihlman, 1984;Kano, 1992;Preuschoft et al, 1998) and in posture (erect vs. bent-hip/bent-knee walking; e.g., Crompton et al, 1998;Li et al, 1996). However, our results suggest that these differences do not greatly affect the overall dynamics of the oscillating legs (reflected in the spatio-temporal gait characteristics) during bipedal walking.…”

The biomechanics of leaping in gibbons

“…Biomechanical studies on leaping in primates have mostly focused on specialized leapers from the primate sub-order Strepsirrhini (nontarsier prosimians), particularly the Lemuridae, Indriidae, and Galagidae. Such studies (including: Demes and Gü nther, 1989;Preuschoft et al, 1996;Aerts, 1998;Preuschoft et al, 1998;Crompton and Sellers, 2007) have revealed functional relationships between leaping performance, body mass and segment length in primates.When leaping, larger animals are limited by muscle force, since force is proportional to length squared while mass is proportional to length cubed (see Alexander et al, 1981;Demes and Gü nther, 1989;Gü nther, 1989), and tend to minimize bone stresses by adopting shorter segments and smaller load arms (Biewener, 1989;Preuschoft et al, 1998;Scholz et al, 2006). Indeed, Demes et al (1999) demonstrated that while take-off forces are higher in larger animals, relative forces decrease with increasing body size.…”

“…Indeed, Demes et al (1999) demonstrated that while take-off forces are higher in larger animals, relative forces decrease with increasing body size. Smaller animals, however, require maximization of impulse (force 3 time) to reach the required take-off velocity and so possess long distal segments to facilitate longer contact times at take-off (Preuschoft et al, 1998). Remarkably, despite a particularly forelimb-dominated locomotor repertoire, gibbons actually have long hind limbs in comparison to other nonhuman ape species (Schultz, 1936; Jungers, 1985), which could be particularly adaptive to maintaining contact with the compliant, deflecting branches, from which 67%-85% of gibbon leaps are performed (Fleagle, 1976;Gittins, 1983;Sati and Alfred, 2002).…”

“…use of the tail to manipulate CoM rotation etc. ; Demes et al, 1991Demes et al, , 1995Demes et al, , 1999Preuschoft et al, 1998) and have a body build which places the center of gravity along the take-off trajectory (Crompton and Sellers, 2007; Crompton et al, in press). However, field and laboratory research into optimal leaping strategies for a number of specialized and less specialized leapers suggests that theoretical optima (based on expectations from ballistic science) may be utilized only during near maximal leaps even in the most specialized leapers (Crompton et al, 1993;Linthorne et al, 2005; Crompton et al, in press).…”

Limb growth in captive Galago senegalensis: getting in shape to be an adult

“…Adult shape is therefore reached through differential growth patterns of individual skeletal elements. These skeletal growth patterns must meet functional demands [Humphrey, 1998;Wells et al, 2002] that may change with increases in size [Preuschoft et al, 1998]. Studies of hominoids indicate that function may be more important than phylogeny in determining growth patterns, since suspensory apes exhibit ontogenetic similarities, as do knucklewalkers, despite their phylogenetic affinities [Jungers & Cole, 1992].…”