An electromyographic study of the pectoralis major in Atelines and Hylobates, with special reference to the Evolution of a pars clavicularis

Abstract

Among primates there is striking variation in the extent of the origin of pectoralis major from the clavicle. A significant clavicular attachment (pars clavicularis) occurs only in Alouatta, Lagothrix, Hylobates, Pan (troglodytes, paniscus and gorilla), and Homo. Interpreting this trait in nonhuman primates as an adaptation to frequent use of a mobile forelimb in climbing and suspension is contraindicated by the absence of a clavicular origin in Ateles and Pongo. We have undertaken a telemetered electromyographic study to determine any special role of the most cranial part of the pectoralis major in comparison to its caudal part, and to the deltoid, during vertical climbing, pronograde quadrupedalism, and armswinging in Ateles, Lagothrix, Alouatta, and Hylobates. The results show that the cranial pectoralis major possesses a role not shared by the caudal fibers: initiation of the recovery phase of the locomotor cycle. When ability to execute rapid or powerful recovery of the adducted forelimb is required in an animal with a shoulder joint lying on a plane cranial to that of the manubrium, the movement will be facilitated if the origin of the pectoralis major is extended onto the clavicle. Such is the case in nonhuman primates possessing this trait. The absence of a clavicular origin in Ateles and Pongo may be related to diminished selective pressures to perfect locomotor modes such as pronograde quadrupedalism, armswinging, or climbing thick vertical trunks, that demand rapid or powerful recovery of the adducted forelimb. If the arboreal ancestor of humans had evolved a clavicular origin of pectoralis major, this animal would be preadapted for certain uses of the forelimb in its bipedal descendant.

Clavicle, a neglected bone: Morphology and relation to arm movements and shoulder architecture in primates

“…Their clavicle has two pronounced curvatures in dorsal view, and thus they cannot have an acromioclavicular joint as in gibbons. The different types of brachiation of spider monkeys and gibbons do not reflect muscle differences (Stern et al, 1980), but depend on clavicle morphology.…”

“…This insertion on the clavicle appears to be a unique feature among primates (Stern et al, 1980). The pectoralis major is very important for arm flexion, especially at the start of the movement, at least in humans (Gagey, 1985) and gibbons (Stern et al, 1980;Jungers and Stern, 1981).…”

“…Ateles do not have a pectoralis major pars clavicularis, but do exhibit an extended clavicular insertion of the deltoideus (Campbell, 1937;Asthon and Oxnard, 1963; Stern et al, 1980), which takes the place and function of the pectoralis major (Stern et al, 1976). The insertion of the pectoralis major or deltoideus on the clavicle may not reflect locomotor behavior differences (Stern et al, 1980), but may rather reflect different adaptations to similar ecological forces.…”

“…The insertion of the pectoralis major or deltoideus on the clavicle may not reflect locomotor behavior differences (Stern et al, 1980), but may rather reflect different adaptations to similar ecological forces. In other words, Ateles and Hylobates have both developed brachiation, but in different ways.…”

“…First, young orangutan clavicles are S-shaped (Schultz, 1930), and this species has a very short pectoralis major insertion on the clavicle when it is present (Sullivan and Osgood, 1927;Jouffroy, 1962). Two, gorillas possess a long and large insertion of the pectoralis major pars clavicularis (Raven, 1950;Asthon and Oxnard, 1963;Stern et al, 1980), and the internal curvature of their clavicle is only slight or absent. Jenkins et al (1978) have proposed that the internal curvature allows the clavicle to join the manubrium and the acromion without crossing the hole (i.e., the thoracic outlet) of the rib cage in those primates, such as hominoids, that have a dorsal scapula.…”