MUSCULAR SYSTEM OF THE RED HOWLING MONKEY 167 



are amplified in a system of air sacs, but the role of these chambers 

 as resonators is not yet well understood. The enlarged hyoid bone, 

 which is filled by a tube-like membranous expansion of the laryngeal 

 lining, strongly suggests that this bulla is an important resonator. 



The evolution of these massive organs in the neck of Alouatta has 

 very strongly influenced the morphology of the structures around the 

 hyoid and the larynx, and in addition, it seems quite reasonable to 

 conclude that it has also effected changes in other parts of the body. 

 Thus, the acquisition of these anatomical specializations for voice 

 production appear to have led to mechanical problems that had to 

 be solved by the gradual development of adaptations in the locomotor 

 system. I suspect that the peculiar combination of skeletal character- 

 istics, such as one finds in Alouatta, is an expression of these adapta- 

 tions: (a) A relatively high intermembral index (Humerus +Radius/ 

 Femur+Tibia) of 98, a value closer to those of Ateles (105), Brachy- 

 teles (105), and Lagothrix (98) than to that of Cebus (80) (Erikson, 

 1963); (b) the forelimb index value (Humerus + Radius/Trunk Verte- 

 bral Column) in Alouatta of 91 is very close to that of Cebus (90), 

 but, separated by 17 units from the value of 109 for Lagothrix, and 

 even further removed from those of Ateles (150) and Brachyteles 

 (140) (Erikson, 1963); (c) the lumbar segment of the vertebral column 

 in Alouatta seems to be the longest among the brachiator monkeys 

 of the New World (Erikson, 1963); (d) a comparison of the scapulas 

 of Alouatta, Ateles, Brachyteles, and Lagothrix (see Erikson, 1963, 

 plate 3A) reveals that it is the bone of the howler which most closely 

 resembles that of a quadruped (it has a moderately large acromion 

 and its spine has only a slight obliquity in relation to the vertebral 

 border; in addition, the glenoid of the scapula in the howler does not 

 seem to face as much craniad as in the other brachiators) ; (e) the 

 supraspinous fossa of Alovatta appears to be very large (Erikson, 

 1963; Hill, 1962; Schultz, 1930). Moreover, several muscles of the 

 trunk and limbs seem also to have been affected in connection with 

 the modifications that occurred in the locomotor system. 



The howlers are described (Carpenter, 1934, 1960) as arboreal 

 quadrupeds moving in the highest branches of the trees by using 

 their limbs and prehensile tails in a deliberate and carefully aimed 

 manner. Not infrequently they make short jumps between close 

 branches. The animals pro])el themselves \vith the hind Umbs and 

 the extended arms secure a hold on the landing place. This quad- 

 rupedal locomotion is favored by the fact that the displacement 

 backward of the foramen magnum brings the axis of the orbits al- 

 most in hne with the direction in which the animal vdW be advancing. 

 Other prehensile-tailed cebids are jirobably forced to extend their 



