958 ANIMAL BEHAVIOUR, 29, 3 group on E1 Guayacan, the male was most visible to the rest of the group at feeding times. His descents from the surrounding trees to the feeder areas were responded to with vocalizations from the immature monkeys, often leading to a more complete harassment episode. The adult male in a corral-living group, on the other hand, is continuously visible to the other group members. Habituation to the male's presence is likely to occur. In the corral environment, therefore, witnessing a copulation may be necessary to excite immature monkeys, whereas in the free-ranging state, the male's presence alone is sufficient to excite the young monkeys, regardless of the male's behaviour. Another issue concerns the male's tolerance of harass- ment. This might be best understood in terms of his genetic relationship to the young patas of that group. In stable, one-male primate groups, age cohorts would be half siblings, all sharing a common paternity ('paternal sibships'; Altmann 1979). By being tolerant of immature monkeys possessing tittle potential to inflict serious in- jury, the adult male's genetic investment, as likely father of all the offspring, is protected. Finally, several explanations and interpretations have been offered to account for harassment behaviour. Among these are the protection Of the female from a male's post-copulatory aggressiofi (Gouzoules 1974), male--male competition (DeVore 1965), inter-female com- petition (Hall 1965), an excitatory response (de Benedictis 1973), and dominance testing (Loy & Loy 1977). Only the latter two can be applied to the harassment described in this report, since neither dominance testing nor general excitation are context-specific, but could occur any time that the male is in view. We speculate that dominance testing is a more viable explanation in a species that is organized into one-male units, such as Erythrocebus, and that at some point, the male's tolerance of harassment would wane. Consideration of the environmental influ- ences on the behaviour of any species, in conjunction with that species' social structure and social organization and the social context, is crucial in both the interpretation of behavioural data and the evaluation of functional explanations for those observed patterns of behaviour. This research was supported by a faculty grant from the University of Alabama in Birmingham to Jay R. Kaplan. We thank the administration and staff of the Caribbean Primate Research Center facility in La Parguera for logistic support; and the reviewers of the original version of this report for their comments. Department of Psychology, Emory University, Atlanta, GA 30322, U.S.A. EVAN L. ZUCKER* JAY R. KAPLAN* Department of Anthropology, University o f Alabama in Birmingham, Birmingham, AL 35294, U.S.A. *Present address: Department of Comparative Medicine, Bowman Gray School of Medicine, Wake Forest University, Winston-Salem, NC 27103, U.S.A. References Altmann, J. 1974. Observational study of behavior. sampling methods. Behaviour, 49, 227-267. Altmann, J. 1979. Age cohorts as paternal sibships. Behav. EcoL SociobioL, 6, 161-164. de Benedictis, T. 1973. The behavior of young primates during adult copulation: Observations of a Macaca irus colony. Am. Anthrop., 75, 1469-1484. De Vore, I. 1965. Male dominance and mating behavior in baboons. In: Sex and Behavior (Ed. by F. A. Beach), pp. 266-289. New York: Wiley. Gouzoules, H. 1974. Harassment of sexual behavior in the stumptail macaque, Macaca arctoides. Folia PrimatoL, 22, 208-217. Hall, K. R. L. 1965. Behaviour and ecology of the wild paras monkey, Erythrocebus patas, in Uganda. J. ZooL, 148, 15-87. Hall, K. R. L., Boelkins, R. C. & Goswell, M. J. 1965. Behaviour of patas monkeys, Erythrocebus patas, in captivity with notes on the natural habitat. Folia Primatol., 3, 22-49. Kaplan, J. R. & Zucker, E. 1980. Social organization in a group of free-ranging paras monkeys. Folia Primatol., 34, 196-213. Loy, J. & Loy, K. 1977. Sexual harassment among captive paras monkeys (Erythrocebus patas). Primates, 18, 691-699. Struhsaker, T. T. & Gartlan, J. S. 1970. Observations on the behavior and ecology of the patas monkey (Erythrocebus patas) in the Waza Reserve, Cameroon. J. ZooL, 161, 49-63. (Received 24 March 1980; revised 20 March 1981; MS. number: As-94) Sexual Selection and Aggressiveness in Male Red-Winged Blackbirds During the breeding season, male red-winged b/ackbirds (Agelaius phoeniceas) appear to be more aggressive than females. One explanation of this sexual difference in behaviour is that aggressiveness is favoured by sexual selection; since sexual selection acts more powerfully on male than on female redwings (Payne 1979), males would be expected to be more aggressive than females under this hypothesis. There is a simple mechanism by which sexual selection could favour aggressiveness in red- winged blackbirds. Female redwings are known to be strongly influenced by territory quality in their choice of mates (Searcy 1979; Lexington 1980). If male aggressiveness influences male success in competition for territories, then the most aggressive males would obtain the best territories, which would attract the most females, and thus aggressive males would have high mating success. Here I describe a test of the hypothesis that aggressive- ness aids male red-winged blackbirds in competition for territories. The method used was to manipulate aggres- siveness in male redwings using implants of an androgen (testosterone) or an antiandrogen (flutamide), and then to compare the success of the experimental males in competition for territories to that of control males im- planted with cholesterol. An earlier experiment showed that captive male red-winged blackbirds treated with testosterone were more aggressive than control males, while males treated with flutamide were less aggressive (Searcy & Wingfield 1980). The study was conducted March through June in 1978 and 1979. Two study sites were used, both in Dutchess County, New York. One site, the Millbrook School Marsh, was dominated by clumps of annual grasses in wet areas and by perennial bushes in drier areas. The second site, the Roskow Marsh, was dominated by purple loosestrife (Lythrum salicaria). In both years, males were