l8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I46 



SILENT ARCH POSTUEES 



All the captive animals on Barro Colorado Island assumed dis- 

 tinctive "Arch Postures" quite frequently. 



In these postures, the back was raised, usually to a considerable 

 extent, and strongly arched or curved. The raising movement itself 

 was always rapid, but the posture generally was maintained for an 

 appreciable length of time (up to a minute in some cases) . Sometimes 

 an individual stood up on its hind limbs as it assumed an Arch Pos- 

 ture. In this case, the front limbs were allowed to hang downward, 

 usually with the elbows more or less akimbo. Generally the hands were 

 held with the backs facing inward and the palms outward and the 

 hind limbs were straight or only slightly bent at the knees. A typical 

 Arch Posture of this type is shown in figure 3c (such postures were 

 quite similar to some semierect preleaping postures in many respects, 

 but could always be distinguished by the more extreme curvature of 

 the back). In other Arch Postures, the performing individual did 

 not stand up, but remained clasping the perch with both hands and 

 feet. In such cases, all four Hmbs were usually nearly straight. Typi- 

 cal Arch Postures of this type are shown in figures 3a and 3b. (It 

 will be noticed that the relative positions of the head and back are 

 nearly the same in both Arch Postures of this type and some head- 

 down postures ; but the effect is achieved by different methods in the 

 two patterns, lowering the head in one case and raising the back in 

 the other, and the general impression or "gestalt" of the two patterns 

 is quite different.) 



Irrespective of these minor variations in physical form, the Arch 

 patterns of the captive animals could be divided into two main cate- 

 gories — one silent; the other accompanied by Resonant Grunts (see 

 below). 



Silent Arch Postures were assumed during all sorts of purely and 

 partly hostile intraspecific encounters ; but they were most common, 

 and most long-sustained on the average, during the longest and most 

 violent disputes between previously unacquainted individuals of the 

 same sex and between "territorial" rivals in adjoining cages. They 

 were often preceded and followed by a great variety of purely or 

 partly hostile vocal patterns, produced by both attack and escape tend- 

 encies (see below), and/or by ambivalent unritualized hostile move- 

 ments. When they were closely associated with an unambiguous un- 

 ritualized hostile reaction, however, it was always overt attack, not 

 escape. Individuals assumed silent Arch Postures with appreciable 

 frequency both before and after delivering overt attacks. 



