MUSCULAR SYSTEM OF THE RED HOWLING MONKEY 



175 



Table 5. — Comparison of the cranial limit of the origin of in. latissim,us dorsi 

 between Alouatta and locomotor groups (data for semibrachiators and quad- 

 rupeds taken from Ashton and Oxnard, 1963; for Alouatta, Sirena, 1871. 

 and this work) . 



Significance of difference in mean cranial limit of origin of m. latissimus dorsi; semibrachiators and quad- 

 rupeds: p<0.001. 



all five are very evident. The most remarkable character of the 

 howler, I think, relates to the function of its gluteus medius in keeping 

 the trunk semi-erect when the animal sits on its haunches (figs. 41 

 and 42). This point is discussed extensively in the section covering 

 that muscle. There is an interesting detail about m. gluteus maximus 

 which should be mentioned. Klaatsch (1900) asserts that it is stronger 

 in the howler than either Ateles or Lagothrix. Bodini (1965) has shown 

 in her studies still in progress that the ectogluteus of the capuchin 

 is larger than in any other monkey with prehensile tail. These obser- 

 vations put Alouatta in a sort of intermediate position between 

 Cebus and the Atelinae, and might, therefore, indicate an adaptation 

 to arboreal quadrupedalism, a kind of locomotion where the lower 

 members are the main propulsors of the body. 



With respect to the hamstring muscles one gets the impression 

 from the current research by Bodini (1965, personal communication) 

 that these muscles are better developed in the howler than in the 

 spider monkey, but less so than in Cebus. This one would expect 

 from Alouatta if it indeed practices quadrupedalism as its main form 

 of locomotion. 



The special interest that attaches to the howler monkey is based 

 on the fact that although it is known to be essentially an arboreal 

 quadruped (Carpenter, 1934, 1960), its musculature is mainly that 

 of a New World brachiator A\ith, however, some quadrupedal charac- 

 ters. The animal has, in addition, several muscular speciaUzations of 

 its own in relation to the production and modification of voice. The 

 foUomng lines of reasoning perhaps may help to explain this peculiar 

 combination of morphological characters and locomotor habits. 



During the middle or early Cenozoic some of the New World 

 primates presumably had invaded the higher branches of the trees, 



