In many solitary mammalian species, females live alone in relatively small home ranges whereas males roam widely and converge around, and compete for matings with fertile females. In primates, orangutans are the only diurnal semi solitary species showing this roving male promiscuity mating system. Here, we develop and test hypotheses about the nature of male-male competition in Bornean orangutans by examining its sexually selected long-distance calls to disentangle the mechanisms through which males compete over mating access. This study uses a novel approach to disentangle the mechanisms of male-male competition. We established an acoustic localization system (ALS) comprising 20 recorders installed in a grid that allowed accurate localization in an area of 450 ha. With this procedure, we triangulated 1615 long calls over 109 days spread over 10 months to examine the males’ ranging and calling decisions. A male’s choice of area was determined mainly by local fruit availability. Once there, however, his calling behavior depended primarily on the number of sexually attractive females and the number of other flanged males present. Both these variables also predicted the proportion of calls to which individual males responded, and the proportion of males present that responded to long calls of other males. We conclude that intra-sexual competition among Bornean flanged males comprises a combination of contest and scramble competition best summarized as confrontational assessment.