WHY BE VIGILANT - THE CASE OF THE ALPHA-ANIMAL

We compared patterns of vigilance behavior in a male- and a female-dom inant species-white-faced capuchins and ring-tailed lemurs-and used th e results to test four hypotheses to explain vigilance behavior in pri mates. Adult male white-faced capuchins spent significantly more time vigilant than females did, and much male vigilance appeared to be dire cted toward males from other social groups. This finding supports the protection of paternity hypothesis. No sex difference existed in vigil ance behavior among the ring-railed lemurs, and subjects of both sexes exhibited more vigilance toward predators/potential predators than to ward extragroup conspecifics, which supports the predator detection hy pothesis. A trade-off argument, suggesting that females tolerate males in a group in return for greater male vigilance, does not apply to ri ng-tailed lemurs in our study. In both the male-dominant capuchins and the female-dominant ring-tailed lemurs, the alpha subject in the majo rity of the study groups was significantly more vigilant than other gr oup members were. In white-faced capuchins, the alpha male mates more often than subordinate males do; therefore, the greater degree of vigi lance exhibited by the alpha male may correspond to the protection of his reproductive investment. In ring-tailed lemurs, there can be more than one matriline in a group. Thus, the greater amount of vigilance b ehavior exhibited by the alpha female may be related to protection of her matriline, which could ultimately lead to greater inclusive fitnes s. Alpha subjects in our study groups exhibited certain behaviors more frequently or exclusively. Accordingly, there may be a constellation of behaviors characteristic of alpha animals.