On Barro Colorado Island, Panama, a mature male howler monkey (Alouattapalliata) ... based on examples in the literature (e.g., CARPENTER, 1965).
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PRIMATES,22(3): 424-426, July 1981
Chasing Behavior Between Males Within a Howler Monkey Troop* ORREY P. YOUNG
University of Maryland ABSTRACT. On Barro Colorado Island, Panama, a mature male howler monkey (Alouattapalliata) was observed chasing a young adult male howler, both members of the same troop. Detailed observations on births, deaths and solitary males before and after the chase suggest a strong correlation between an unstable social structure within the troop and overt aggression between different-aged males.
OBSERVATIONS Overt aggression between members of a howler monkey troop appears to be a rare event, based on examples in the literature (e.g., CARPENTER, 1965). The general consensus is that howlers are "extremely tolerant of each other in proximity or contact" (BERNSTEIN, 1964). During the period January-June 1974 1 had the opportunity to conduct intensive studies of the howler monkey on Barro Colorado Island (BCI), Panama, and observed several instances o f aggression between males within a troop. I have chosen one series of observations for presentation and discussion, both because of its completeness and because it represents behaviors typically seen during the same period. The quality of the observations has also been enhanced by R. W. THORINGTON and associates, who have laboriously marked by collars and/or tail bands many of the howlers of BCI. On February 26, 1974, Troop III spent the entire day in the Mosquito Ridge area o f Lutz Watershed, feeding on emergent Platypodiurn sp. leaves and ripe Ficus fruit when they were not resting. At 16:30 the entire troop was in one large fig tree, quietly consuming its fruit, when a sudden commotion started in one cluster of monkeys. After 30 sec of "yipping" and grunting, one animal began chasing another in a northerly direction through the trees. The individual in the lead was a young adult male (No. 35) with testicles only partially descended. The monkey in the rear (No. 25) was one of the three older adult males o f the troop. The chase lasted approximately 3 min, during which the young male continually "yipped" and barked and looked over his shoulder as he moved very rapidly (for a howler) through the sub-canopy and mid-story. The older male seemed to keep himself positioned above the younger male, so that when they were very close together the young male would go down to escape. The net result after -4-200 m was that the younger male was 15 ft above the ground in a sapling tree and jumped or fell to the ground to avoid the final lunge of his chaser. Animal No. 35 then proceeded to run along the ground, still toward the north, as No. 25 slowly ascended toward the south into the sub-canopy. No. 25 soon sat down beside an adult female (No. 32) who had followed the pair during the chase, the remainder of the *Support for this field work was received from the Environmental Sciences Program, Smithsonian Institution; and the National Zoological Park, Washington, D.C.
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troop having stayed in the original feeding tree. When No. 25 reached the sub-canopy, the young male began climbing a sapling tree about 50 ft from where he had first touched ground, finally stopping his climb about 20 ft above ground and 4-100 m from No. 25. The two males faced each other at this distance for 5 min before adults No. 25 and No. 32 turned and headed back to the feeding tree. Young male No. 35 then slowly climbed into the sub-canopy and followed the pair, maintaining the 100 m distance. Male No. 35 was first captured and marked on January 7, 1974. At this time he was classified as a Juvenile-3 and was a member of Troop III. He was observed in Troop III during January and February 1974 before the aforementioned chase, and was subsequently seen in March, April and May 1974 in the same troop. The composition o f the troop in January 1974 was 3 adult males (plus young adult male No. 35), 6 adult females, 3 infants and 3 juveniles (total = 16). During February one of the adult males (No. 16) disappeared and two infants were born, so in March the troop composition was 3 adult males (including No. 35), 6 adult females, 4 infants and 4 juveniles (total ----- 17). During April a large adult solitary male was incorporated into the troop and another birth occurred, making the May troop composition 4 adult males (including No. 35), 7 adult females, 4 infants and 4 juveniles (total = 19). DISCUSSION Although specific details are lacking, CARPENTER(1965) suggests that agonistic behavior o f adult howler males toward younger males may be a "vector o f force" resulting in some males leaving troops. This process of ejection of a male, probably occurring over many weeks or months in a troop, apparently has never been documented. I hypothesize that the "chase" reported herein would be one of the behaviors characteristic of the male ejection process and, if repeated frequently enough, would eventually be successful in conferring "solitary male" status on the ejected individual. Unfortunately for this hypothesis, subsequent events in Troop III did not provide further support. It would appear that when one of the adult males disappeared from the troop in February-March, during the ejection process for the young male (No. 35), the socionomic sex ratio (mature males to mature females) increased to a value above the norm for the population (CARPENTER, 1965). The internal social structure of the troop may then have been unstable and required the addition of another male, and the ejection process for male No. 35 was halted. Observations in April indicated that No. 35 was functioning as an adult male and accepted by the troop as such, with no avoidance of the other males and free access to females. Additional births and a successful first pregnancy by one of the juvenile females again increased both the socionomic sex ratio and the size of the troop, creating the instability that led to addition o f a fourth adult mate. This male did not move into the troop at the bottom of the hierarchy (and potentially compete with No. 35 to avoid that distinction.) but appeared to assume the role o f principal male, perhaps due to his large size and prime condition. A complete census o f the troop in late May indicated that with four adult males and seven adult females, the socionomic sex ratio of 1:1.8 was identical with the mean value of six troops censused by MITTERMEIER(1973) in the same area in 1970. REFERENCES
BERNSTEIN,I. S., 1964. A field study of the activities of howler monkeys. Anita. Behav., 12: 92-97.
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CARPENTER,C. R., 1965. The howlers of Barro Colorado Island. In: Primate Behavior, I. DEVORE (ed.), Holt, Rinehart & Winston, New York, pp. 250--291. MITTERMEIER,R. A., 1973. Group activity and population dynamics of the howler monkey on Barro Colorado Island. Primates, 14. 1-19. --Received September 24, 1980; Accepted November 28, 1980 Author's Name and Present Address: ORREYP. YOUNg,Southern Grain lnsects Research Laboratory, Tifion, Georgia 31793, U.S.A.
