NO. 5 BEHAVIOR OF THE NIGHT MONKEY — MOYNIHAN 5 



trees are usually accomplished by great bounds. When leaping, most 

 of the propulsive power appears to come from the hind limbs, the 

 forelimbs being used chiefly to absorb the shock of landing. The 

 long and rather heavy tail seemingly is used as a counterweight, never 

 as a prop or support. (This is another resemblance to Callicebus 

 and Pithecia.) Night Monkeys run along branches less frequently 

 than monkeys of the genera Cehus and Saimiri or tamarins such as 

 Saguiniis geoffroyi. (The generic names used in this paper follow 

 Hershkovitz, 1958.) 



As noted by Hill (1957), Night Monkeys are among the most 

 nearly completely quadrupedal monkeys. Not only do they usually 

 use both fore and hind limbs in locomotion, but they generally grasp 

 a supporting branch with one or both hands even when sitting quietly. 

 Just before leaping, however, they may take their hands off the 

 supporting branch and sit or stand in a semierect posture, balancing 

 on the hind limbs alone (see figure 1). The hind limbs generally 

 are flexed in this posture, the back may be nearly straight or curved 

 to an appreciable extent, and the front limbs may hang downward or 

 the hands may be slightly raised (presumably in preparation for 

 grasping a branch after the leap) . An individual may remain poised 

 in this way for several seconds or more. 



Night Monkeys seem to eat a great variety of fruits and insects 

 under natural conditions, and learn to eat artificial foods without 

 difficulty in captivity. Their method of approaching and "handling" 

 nonmoving food in captivity is interesting. They generally begin by 

 sniffing at the food. (For other indications that they may have a good 

 sense of smell, see below.) After sniffing for several seconds, an 

 animal usually will seize the food with its teeth, and then sit up. Only 

 then, in most cases, will it raise one or both hands to hold and possibly 

 manipulate the food as it begins to chew. 



The various ways in which different species of platyrrhine monkeys 

 use their hands and fingers have not been well described. Some in- 

 correct statements and misinterpretations are current in some of the 

 published literature. Sanderson (1957), for instance, states that 

 Night Monkeys have opposable thumbs and use their hands in much 

 the same way as human beings. In a later passage, he also implies 

 that they usually oppose the first two fingers of the hand against the 

 last three (in much the same way as monkeys of the genera Pithecia 

 and Aloiiatta). Both statements are somewhat misleading. 



Night Monkeys often spread their fingers more or less radially 

 when grasping an object. At such times, all the fingers tend to diverge 



