SERRATUS VENTRALIS FUNCTION IN VERVET MONKEYS (CERCOPITHECUS-AETHIOPS) - ARE PRIMATE QUADRUPEDS UNIQUE

The serratus ventralis in mammals is a fan-shaped scapulo-thoracic mus cle that is believed by most morphologists both to support body weight and to rotate the scapula during quadrupedal locomotion. Electromyogr aphic studies of this muscle in cats, dogs and opossums confirm the du al supportive and rotatory roles of the serratus ventralis. Although t his muscle has been studied in several primate species, the concentrat ion on arboreal locomotion has resulted in an inadequate data set to p ermit direct comparisons to non-primate terrestrial quadrupeds. In ord er to provide a more comparable data set, we examined cranial, mid- an d caudal thoracic regions of the serratus ventralis during terrestrial quadrupedalism in the vervet monkey, Cercopithecus aethiops. Our resu lts indicate that the serratus ventralis does support the body during the stance phase of quadrupedalism in this primate. However, unlike se veral non-primate mammals, it plays a relatively insignificant rotator y role during swing phase. Additional published and unpublished data o n Lemur, Alouatta, Ateles and Pan indicate a pattern of serratus ventr alis recruitment similar to that of the vervet during terrestrial quad rupedalism. However, the mid-thoracic digitations of serratus ventrali s are recruited during swing phase of quadrupedalism on a branch. We s uggest that serratus ventralis activity during swing phase will cause ventral displacement of the scapula that has the potential for interfe ring with the ability of the forelimb to clear the substratum. The occ urrence of swing phase activity in serratus ventralis during branch qu adrupedalism reflects reduced constraints on forelimb clearance since the hand can be allowed to pass laterally along the bough. The potenti al negative effect of ventral girdle displacement may also explain the contrast in serratus ventralis recruitment between primates and non-p rimates during swing phase of terrestrial quadrupedalism. Primates are known to have uniquely long stride lengths relative to non-primate ma mmals owing in part to their highly protracted arm position at touchdo wn. Ventral displacement of the scapula caused by swing phase recruitm ent of serratus ventralis could cause premature touchdown that would h inder proper hand placement in primates. Such displacement is more tol erable in non-primate mammals owing to their highly retracted arm posi tion throughout swing phase. The unique de-emphasis on the serratus ve ntralis during swing phase of terrestrial quadrupedalism in primates d oes not indicate a lack of involvement of this muscle in scapular rota tion. Data on muscle activity during reaching behaviours demonstrates a major contribution by the serratus ventralis to scapular rotation wh en it is combined with scapular protraction, thus providing further su pport for the importance of this muscle in arm-raising activities in p rimates.