Scaling of forearm muscle weights in primates

Among primates, it has been proposed that the differential development of the forearm flexor compartment is connected to substrate use. Likewise, the projection of the medial epicondyle is thought to reflect this differential development. We examined the scaling properties of forearm muscle wet weights to test the hypothesis that the total masses of various muscle compartments are differentially developed among primates. The forearms of six strepsirrhine (Eulemur fulvus, Lemur catta, Varecia rubra, Hapalemur griseus, Galago senegalensis, Nycticebus coucang) and seven haplorhine (Callithrix jacchus, Macaca sp., Papio sp., Erythrocebus patas, Hylobates lar, Gorilla gorilla, Pan troglodytes) primate specimens were dissected. Distal humerus dimensions and muscle wet weights were measured. Scaling properties of each muscle compartment within the forearm were examined via ordinary least squares regression. The total mass of the flexor and extensor compartments were both found to scale isometrically on total forearm muscle mass, with little unexplained variance (p\0.0001, r250.99). Likewise, the mass of the wrist flexors and digital flexors both scale isometrically to total forearm muscle mass (p\0.0001, r250.98). The mass of the total flexor compartment and the mass of the digital flexors both fail to significantly explain the variance in medial epicondyle projection, when body size is taken into account. It is concluded that the relative masses of the wrist and digital flexors do not predictably vary with substrate use. Locomotor differences in epicondyle development likely serve to reorient the muscles around the joint, rather than reflect any differential development of muscle mass.