20 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I46 



These facts would suggest that all or most silent Arch Postures are 

 essentially purely hostile per se, and usually are produced when hostile 

 motivation is rather strong. The attack tendency must be stronger 

 than the escape tendency in all or most silent Arch patterns, but 

 probably relatively weaker than in actual overt attack. 



Both morphological types of Arch Postures, standing up and with 

 both hands and feet grasping the perch, were common during silent 

 Arch performances. 



(A relatively very few Arch Postures were accompanied by a few 

 Gruff Grunts, Gulps, and/or "Squeaks" [see below]. These perform- 

 ances appeared to be closely related to, perhaps nothing more than 

 abnormal variants of, typical silent Arch performances. They may 

 have been produced by motivation intermediate between that usually 

 producing typical silent Arches and that producing the accompanying 

 notes without Arches.) 



Silent Arch Postures seem to function as threat. The assumption 

 of an Arch Posture by one individual during an intraspecific dispute 

 usually induces retreat by its opponent. The sight of an Arch Posture 

 apparently activates or stimulates the escape tendency (and probably 

 suppresses or weakens the attack tendency) of the perceiving indi- 

 vidual. This effect is probably partly due to the fact than an animal 

 in an Arch Posture looks larger than usual. 



The Arch Posture is particularly interesting from a comparative 

 point of view. The only other New World primate known to have 

 a similar pattern is the howler Alouatta palliata. 



Part of the Arch Posture may have been derived from an intention 

 movement of leaping. Even when it does not assume a semierect pos- 

 ture, a Night Monkey usually "shifts" its body backward, to put more 

 of its weight on its hind limbs, just before leaping for any reason in 

 any circumstances; and this shifting tends to increase the normal 

 curvature of the back. The straightening of the limbs in the Arch may 

 have been derived from another source. Many other platyrrhines {e.g., 

 Cehus spp.) tend to hold the limbs very straight and stiff when jump- 

 ing up and down in rage. It is conceivable that the Night Monkey 

 has retained the "stiff-leggedness" of such a pattern, even though it 

 has lost the jumping component. At least, I have never seen similar 

 straightening of the limbs by any other species (except Alouatta 

 palliata) in other circumstances. 



OTHER VISUAL PATTERNS 



An important negative feature of the hostile repertory of Night 

 Monkeys is the comparative rarity of "displacement" activities, i.e., 



