The Nuances of Locomotor Strategies in Suspensory Primates (Apes): Locomotor Costs in Terms of Skeletal Injury

Injury may be a significant locomotor cost, in that a severely injured animal may be unable to secure enough food to maintain normal activities, avoid predation, or find mates. Thus, safety is as likely as energetics to be the impetus of natural selection, and both have potential to impact reproductive fitness (Pontzer and Wrangham, 2004; Thorpe, 2005). The purpose of this study is twofold. First, to assess fracture patterns relative to variation in locomotion, ecology, and body mass among suspensory apes. Second, to quantify change in compact bone geometry associated with bone fracture and bone remodeling in suspensory apes. To obtain the first goal, fracture frequency, severity, and remodeling are examined in limb bones from skeletons of 141 wild-caught primates according to four major predictions. First, the suspensory genera will show a greater percentage of limb bone fractures than quadrupedal baboons. Second, among suspensory apes, the brachiating gibbons will have the highest fracture frequency and the most severe fractures, the quadrumanus orangutans will have frequent and severe fractures, but fewer than the brachiators, and the climbing and knuckle-walking chimpanzees will have the lowest fracture occurrence and severity. As a corollary, terrestrial quadrupedal baboons will have the lowest fracture frequency and severity. Third, fractures will be more prevalent in species with larger bodies. Fourth, fracture rate and severity will be greatest in species that travel on substrates higher in the forest canopy. The results show that body size is a significant predictor of fracture frequency. Increases in body size increase the likelihood of fracture. Travel height and locomotor strategy are not significant predictors of fracture frequency. However, the dual locomotor strategy and low travel heights used by Pan have a negative relationship with deformity and remodeling. Such strategies may provide an evolutionary advantage, as fractures incurred by Pan are less likely to be severely deformed and are more likely to have obtain more complete remodeling.The second part of this study investigates differences in the internal bone, geometry of fractured and unfractured corresponding elements. I test three major hypotheses. First, animals that frequently use terrestrial quadrupedal locomotion, such as baboons and chimpanzees, will show larger differences in cross-sectional area (CSA) when affected by fracture than those that are more suspensory. Second, elements that are consistently weight bearing in the preferred locomotor strategy will show the most difference in CSA. For example, the difference in CSA of the fractured fore-limbs of the highly suspensory gibbons and orangutans will be greater than that of the more terrestrial chimpanzees and baboons. Third, body size will be positively correlated with fracture severity and will have an impact on the difference in CSA between fractured limbs and their corresponding elements. The results of this study demonstrate that chimpanzees have a lower mean CSA for fractured elements than unfractured elements. This difference in bone geometry could be related the chimpanzee’s ability to engage in the dual locomotor behaviors of suspensory locomotion and knuckle-walking, that promote reduction in body mass and impact load on the injured limb.