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Abstract

Previous studies of vigilance behaviour have focused mainly on the influence of predation threat, whereas the influences of conspecific factors, such as within-group threats, are relatively unstudied. To elucidate the influences of conspecific factors, this study examined vigilance behaviour in wild chimpanzees (Pan troglodytes schweinfurthii) in Mahale Mountains National Park, Tanzania. Vigilance level was lower during foraging than during resting, which indicated a conflict between vigilance and foraging activity. In addition, vigilance level was higher when chimpanzees were on the ground where an encounter with leopards (Panthera pardus) is likely than when the chimpanzees were in trees. Males, but not females, increased their level of vigilance as the number of individuals within 3 m increased. In both males and females, daily party size – an index of group cohesion – did not affect the vigilance level. The level of maternal vigilance was higher when a dependent infant was separated from its mother than when the offspring was in contact with its mother. Both males and females increased their vigilance when a less-associated group member was nearby, when compared with when there was no less-associated group member nearby. This finding suggests that variation in relationship quality influences the vigilance level and that individuals need to increase their level of vigilance when the level of within-group threats is high. This study indicated that variation in vigilance cannot be understood unless conspecific factors, such as variation in the relationship quality with associates, are considered.