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Adaptive and phylogenetic influences on musculoskeletal design in cercopithecine primates

J. D. Polk

Department of Anthropology, Harvard University, 11 Divinity Avenue, Cambridge, MA 02138, USA

View larger version (23K): [in a new window]   Fig. 1. Rationale for the effects of body proportion on limb posture. Animals with longer limbs will have longer moment arms (R) and will experience increased joint moments compared with shorter-limbed animals [assuming that force magnitude, ground reaction force (GRF), and body posture are constant]. As a consequence, animals with longer limb segments below a joint are expected to adopt more extended joint postures to moderate the joint moments that they experience; that is, the product of GRF and R is smaller for the limb on the left (with R1) than for the limb on the right (with R2).

View larger version (25K): [in a new window]   Fig. 2. Body proportions for (A) all adult monkeys and (B) 15 kg monkeys. Relative lengths for adult monkeys were calculated as the length of each segment/(mass). Proximal segments (arm and thigh) are unshaded, intermediate segments (leg and forearm) are hatched and distal segments (hand and foot) are in black. In the adult monkeys (A), similar limb proportions are found within each species (except for the vervet hindlimb), and the interspecific differences in limb proportions are not well correlated with body mass. The greatest discrepancy in segment lengths between the 15 kg animals (B) is found below the knee, where the patas has longer distal hindlimbs than either baboon (Papio). The patas has longer hindlimbs and forelimbs than either baboon, and longer feet and distal forelimbs (forearm + hand) than the female baboon. The patas also has shorter hands than the male baboon. F, female; M, male; SA, subadult.

View larger version (15K): [in a new window]   Fig. 3. Range of walking speeds. The range of relative speeds, v/(gh)0.5, is illustrated. v is velocity, g is the gravitational constant and h is hip height during quiet standing, adopted by each of the individuals in this study. Walking gaits were used at all speeds in this study. The male patas tended to use faster walking speeds than the other monkeys, while the male baboon (Papio) tended to move more slowly. The median speed for each individual is indicated in bold, the shaded box contains the interquartile range (IQR), and whiskers may extend to 1.5xIQR. Values beyond the whiskers (open circles) are outlying data points. Values of N are given below the box-and-whisker plot. F, female; M, male; A, adult; SA, subadult.

View larger version (25K): [in a new window]   Fig. 4. Differences in joint angles between pairs of 15 kg monkeys. Each stick figure represents a comparison of two monkeys. Markers at each joint (A), limb segment (B) and angular excursion line (C) are shaped to indicate whether a difference in joint angle (or excursion) was predicted to exist between these monkeys (based on the differences in body proportions; see Table 2). Markers are shaded to indicate whether that prediction was observed. Solid lines represent individual limb segments, while broken lines represent either forelimbs or hindlimbs. Monkeys with longer limb segments were predicted to have more extended joint postures and lower protraction and retraction angles than monkeys with shorter limb segments. Markers in B illustrate whether predicted differences were observed in shoulder, forelimb, hip and hindlimb protraction and retraction angles. Markers in C illustrate comparisons between joint angular excursions. Where significant differences exist, the animal with longer limb segments had more extended joint postures at mid-stance, as well as lower protraction and retraction angles at touch-down (TD) and lift-off (LO), respectively. M, male; F, female.

View larger version (18K): [in a new window]   Fig. 5. Joint moments. This figure illustrates the difference in joint moments between each pair of 15 kg monkeys. Joint moments in animals with longer limb segments were predicted to be lower or equivalent to those of shorter-limbed animals. While there are some exceptions, this pattern is generally observed. Most importantly, where difference in limb proportions are greatest (i.e. below the knee), lower joint moments are consistently observed. M, male; F, female.

View larger version (19K): [in a new window]   Fig. 6. Effective mechanical advantages (EMA; ratio of anatomical to GRF moment arms, where GRF is ground reaction force) at the elbow (A), knee (B) and ankle (C) for the phylogenetically constrained sample of primates and the diverse sample of mammals from Biewener (1989). EMA for the elbow and knee increase with body mass (in kg) for both primate and non-primate samples. This indicates that body mass has a similar influence on EMA (and consequently on the muscle force required to resist gravity) in the phylogenetically constrained and diverse samples. In contrast, ankle EMA does not increase with body mass in the primate sample, but does increase significantly in the diverse mammalian sample.

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