40 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I46 



motory tendency is too strong to be completely expressed by running 

 and leaping movements alone. 



There are other indications, however, that some or all of the Gulps 

 may be at least partly hostile. Although most overt escape and 

 simple alarm patterns are silent (see page 13), Gulps are often uttered 

 before and/or after such reactions. Very occasionally, Gulps are 

 uttered by individuals that are not running or leaping at the time. One 

 captive individual on Barro Colorado, for instance, uttered a particu- 

 larly loud and rapid series of Gulps while sitting, frozen, in a crouch 

 posture immediately after overt escape. It is also my impression that 

 the wild individuals in the forest tended to utter relatively few Gulps 

 when they were apparently unaware of being observed. The fre- 

 quency of their Gulps sometimes increased greatly at the instant they 

 seemed to become aware of the presence of an observer — even when 

 their other activities continued unchanged. 



It is possible, therefore, that some or all of the Gulps may be pro- 

 duced by a combination of locomotory and escape tendencies, usually 

 when the latter is relatively and/or actually weak. 



The Gulps may exemplify a complex evolutionary change. 



Although they are probably partly hostile in motivation, they do not 

 appear to be hostile in function. (They certainly do not function as 

 warning signals. I have never seen a Night Monkey respond to the 

 sounds of Gulps by performing overt escape.) They may well function 

 as "contact notes." They may help to keep mated individuals and 

 members of the same family group together under natural conditions. 

 The utterance of Gulps by one individual should enable its companions 

 to tell exactly where it is (especially as Gulps are very penetrating, 

 even when not very loud). 



No other species of platyrrhine monkey with which I am familiar 

 utters similar notes with similar frequency in similar situations.^ This 

 would suggest that Gulps, as adult contact notes, are highly specialized. 

 If so, they probably have been derived from some purely hostile 

 Grunt pattern. (They are similar to Gruff Grunts in some aspects of 



2 Adult individuals of some other species, e.g., some tamarins, may utter "lost" 

 notes or calls when completely isolated; but such notes or calls are seldom or 

 never uttered by individuals moving about near other individuals of the same 

 species in the same way as the Gulps of Night Monkeys. 



Adult howler monkeys of the species Aloiiatta palliata may utter some notes 

 which help to maintain contact between the members of a band (Carpenter, 

 1934, and Collias and Southwick, op. cit.) ; but such notes seem to be uttered 

 relatively much less frequently than the Gulps of Night Monkeys, and may be 

 produced by rather different motivation. 



