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MOVEMENTS OF A WILD NIGHT MONKEY (AOTUS TRIVIRGATUS) Richard W. Thorington, Jr., Nancy A. Muckenhirn, and Gene G. Montgomery INTRODUCTION The natural history of the night monkey (Aotus trivir- gatus) is known in rough outline and is described in Enders (1935), Cabrera and Yepes (1940), and Moyni- han (1964). The last presents detailed descriptions of the behavior of night monkeys. These and other pa- pers, however, lack data on the home ranges and activity patterns of Aotus that are important for estimating and censusing wild populations. With the increased use of Aotus in medical research in the United States (averaging 4.500 animals per year in 1968 and 1969), it is becoming increasingly important to have data that can be applied to proper population management. Although this study was conducted on a near-minimal sample of one animal, we present our data to document (1) the behavior and strategy of an animal released in a strange environment (possibly simulating the dispersal of a young Aotus from its parental home range), (2) the movements of a night monkey in a tropical forest, and (3) the feasibility of studying Aotus by radio-tracking. MATERIALS AND METHODS An 800-g young male Aotus, purchased in the local Panamanian market, was fitted with a 40-g dummy transmitter (packaged as a neck collar) and was main- tained in a cage approximately 2 x 2 x 2 m for 4 weeks on Barro Colorado Island, C.Z. The animal seemed undisturbed except for occasionally chewing on the whip antenna of the dummy transmitter and regularly holding the antenna with one hand. On 14 September 1971, when we released the ani- mal, the dummy transmitter was replaced with a 37 g radio transmitter (AVM Inst. Co., Champaign, 111., Model ST-1) with a predicted battery life of 85 days. The collar, made of rubber-coated, test-lead wire, 4 mm in diameter, acted as a transmitting antenna. We used two harmonics of the basic transmitter frequency (Montgomery et al., 1973) at about 150 MHz and 450 MHz for locating the animal. A 10.2-cm-long whip antenna of the rubber-coated wire increased transmit- ter output at the higher frequency. To locate the Aotus we used a hand-held Model LA-11-5 receiver with an auxiliary 450-MHz converter (AVM Inst. Co., Champaign, 111.) with two different antennas. A two-element yagi was used to receive the stronger 150-MHz signal from up to 1.2 km away from the transmitter. A five-element yagi antenna was used to locate the transmitter precisely from close range with the weaker 450-MHz signal (Montgomery et al., 1973). The activity pattern of the animal was determined by recording changes in intensity of signal from the transmitter with a Rustrack model 291 recorder (Inst. Control Co.. Minneapolis. Minn.). Fluctuations in intensity attributable to activity of the animal (Sun- quist and Montgomery, 1973) and direct observations of the animal's activity were summed in 5-min incre- ments for each hour of the night to produce estimates of the percentage of time during which the Aotus was active or inactive. Visual observations were occasionally possible with the aid of headlamps and binoculars 7 x 35 mm. We also used a Starlight scope, but this was not as useful 32
MOVEMENTS OF A WILD NIGHT MONKEY, AOTUS TRIVIRGATUS 33 because of the lower magnification (4 x) of the Star- light scope, the backlighting of the monkey by star- light, and the shallow depth of field of the scope. RESULTS The night monkey was released at the site of its cage on 14 September 1971. We located the animal irregu- larly during the first three nights (a total of 15 radio- locations) and then followed it almost constantly (ap- proximately 80 hours) for the following nine nights. During the first night after release, the animal moved approximately 120 m northwest and found a vine- covered tree, which served as its home tree for the next 6 days. After the seventh night, it used another tree 45 m west of the first and two other trees within 15 m of the second (Figure 1). These home trees served as daytime refuges for the next 6 days. All four trees were covered with masses of vines in which the animal concealed itself. The first home tree was large, approx- imately 25-30 m tall, and contained a den. The other three were small, 10-15 m high, and did not contain dens. Travels of the animal from its home trees alternated between feeding sessions in the vicinity of the original home tree and exploratory excursions from the vicin- ity. On the third, fourth, eighth, and tenth nights, it was not found outside a 30- x 20-m area centered west of the first home tree. On the second night the animal moved 200 m north of the home trees, while on the fifth and sixth nights it returned to an area near its holding cage. On the ninth night the animal moved 150 m south; on the eleventh night, 150 m south and 250 m west. In returning to home trees from these explora- 100 FIGURE 1 Study area on Barro Colorado Island. Percentages indicate relative amounts of time the Aotus spent at night within and without the encircled areas. Broken lines indicate routes described in the text. U U oc u Q- 80- 60 4O 20 6PM HOURS I2AM 6AM FIGURE 2 Nocturnal activity of Aotus. The average percent- ages of activity (A) and inactivity (A) per hour are given for the period 14-26 September 1971. Percentages do not total 100 percent for each hour because activity levels could not always be determined. tory excursions, the animal approximately retraced its original path through the forest canopy. These routes and the areas most used by the Aotus are shown in Figure I. The animal spent daylight hours 10-15 m above the ground. During the night, some of its pathways brought it closer to the ground (as low as 3 m), but much of its activity took place in the forest canopy. Arboreal pathways used by the animal included very large to very small branches. It moved through the trees quietly, seldom making long jumps. When cross- ing between trees over small branches, it used its tail extensively for balance. When feeding on small terminal fruits, such as those of Brosimum bernadette, the Aotus stretched out to the ends of branches, collected a fruit, sat on a more stable perch to eat it, then repeated the procedure. When feeding on other undetermined foods, perhaps buds and insects, it moved about almost constantly. The nocturnal activity patterns of the Aotus are summarized in Figure 2. The animal became active at dusk between 1700 and 1815 hours and remained active, feeding and moving for several hours. Usually it became inactive after midnight for an hour or two, then increasingly active toward dawn. On at least four of the mornings, it fed intensively for one-half to a full hour before returning to its home tree. It usually retired to a daytime refuge between 0550 and 0600 hours. However, based on Rustrack recordings, it was
34 THORINGTON, MUCKENHIRN, and MONTGOMERY evident that the animal was active in or near its home tree. CONCLUSION When released, the strategy of the Aotus appeared to be locating a good home tree with a good feeding area nearby. The animal explored outward from this place and subsequently changed home trees. However, it did not change its principal feeding area during the course of the study. It is unfortunate that there are no similar data on dispersal of other primates for comparison of strategies. As shown in Figure 1, the Aotus spent 72 percent of its time in an area of 800 m2 and 85 percent of its time in an area of one-half hectare. The latter area is slightly smaller than the territories of two to four Callicebus, as described in Mason's (1966) study. Further studies will be required to determine if our animal occupied a home range typical for Aotus in this habitat and if the size of the home range varies in different habitats. As we radio-tracked the animal, there were long periods during which we could not see or hear it. It moved quietly through the trees, did not vocalize, and usually did not drop rinds or other debris as it fed. With a headlamp we occasionally saw its eyeshine. The animal could not have been followed without use of the radio equipment. Based on our experience with this animal, we feel that it would be feasible to conduct a detailed ecological study of wild Aotus by using radio- telemetry and probably impossible to conduct such a study without this field technique. ACKNOWLEDGMENTS This study was supported by Air Force Contract No. F44620-67-C- 0063 to the Smithsonian Institution (RWT and NAM) and by Smith- sonian Research Foundation Grant No. SG 3540001 (GGM). W. W. Cochran designed the two-frequency radio-location system. The transmitter was purchased by Animal Resources Branch, NIH, USPHS contract PH-43-64-44 (Task Order 12) and Headquarters U.S. Army Medical Research and Development Command Contract No. DADA 17-71-C-1117 to the National Academy of Sciences. REFERENCES Cabrera, A., and J. Yepes. 1940. Historia natural Ediar; mamiferos sub-americanos. Buenos Aires. 1 ink-is R. K 1935. Mammalian life histories from Barro Colorado Island, Panama. Bull. Mus. Comp. Zool. 78:385-502. Mason, W. A. 1966. Social organization of the South American monkey, Callicebus moloch: a preliminary report. Tulane Stud. Zool. 13:23-28. Montgomery, G. G., A. S. Rand, and M. E. Sunquist. 1973. Postnesting movements of iguanas from a nesting aggregation. Copeia 1973:620-622. Moynihan, M. 1964. Some behavior patterns of platyrrhine mon- keys. I. The night monkey (Aotus trivirgatus). Smithson. Misc. Coll. 146(5):iv. 1-84. Sunquist, M. E., and G. G. Montgomery. 1973. Activity patterns of two- and three-toed sloths. J. Mammal. 54:946-954.
