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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.anbehav.2005.02.012 Byline: John C. Mitani, David P. Watts Abstract: Territorial boundary patrols are a distinctive and unique aspect of wild chimpanzee, Pan troglodytes, behaviour. Although patrolling has been frequently observed in nature and several proposed functional explanations for it exist, scant information is available regarding the proximate factors that affect this activity. We found that there is considerable temporal variation in the frequency of patrolling by chimpanzees at Ngogo, Kibale National Park, Uganda. We examined several ecological and social factors that potentially influence this variation. Results of logistic regression analyses revealed that male party size was the single best predictor of the tendency to patrol. Lethal intergroup aggression occurs in chimpanzees, and patrols are likely to be dangerous and costly. Our findings are consistent with the hypothesis that chimpanzees at Ngogo reduce these costs by patrolling together in large parties. Author Affiliation: (a) Department of Anthropology, University of Michigan (a ) Department of Anthropology, Yale University Article History: Received 9 September 2004; Revised 27 November 2004; Accepted 3 February 2005 Article Note: (miscellaneous) MS. number: A9992