NO. 5 BEHAVIOR OF THE NIGHT MONKEY — MOYNIHAN 39 



all tended to utter many Gulps during such periods, but relatively few 

 or no other notes (except Sneeze-grunts — see below). Most of the 

 wild Night Monkeys on Barro Colorado Island also uttered many 

 Gulps and relatively few obviously hostile or sexual calls just before 

 dawn. 



960 

 720- 

 480 

 240 



T f 



2.0 4.0 



Fig, 11. — Two Gulps, uttered by an adult. 



Based upon a spectrogram by a "Missilyser." 

 It will be seen that these notes are rather similar to the Sneeze-grunt shown 

 in figure 9. There are, however, a few definite differences. The Sneeze-grunt 

 has a range of frequencies between and approximately 650 cycles per second 

 (c.p.s.), all starting at exactly the same time and having predominant amplitudes 

 and longer signals around 100 c.p.s. and 260 c.p.s. The Gulps begin with high 

 frequencies around 850 c.p.s., and drop rapidly in frequency to fairly equal 

 amplitude signals between approximately 240 and 600 c.p.s. 



The causation and function (s) of this intense (and often apparently 

 "purposeless") activity were difficult to identify. It is possible that 

 Night Monkeys, like some other mammals that normally range over 

 large areas (e.g., many canids — see Lorenz, 1952), have a strong 

 "internal" tendency to perform locomotory movements, a tendency 

 expressed by overt activity even when the other immediate needs of 

 the animals (e.g., food, water, and sex) can be fully satisfied with- 

 out moving about very much. 



If so, many or all Gulps may be expressions of the same "inde- 

 pendent" locomotory tendency. They may be produced when the loco- 



