• coexistence mechanism;
  • contest competition;
  • foraging efficiency;
  • frugivory;
  • giving-up density (GUD);
  • Kibale National Park;
  • patch depletion;
  • Uganda


  • 1
    We examined mechanisms of coexistence between two congeneric species of frugivorous primates, the blue monkey (Cercopithecus mitis) and the red-tailed monkey (C. ascanius).
  • 2
    We used giving-up densities (the amount of food which animals leave in a patch) in fruit trees to measure foraging efficiency and to evaluate possible mechanisms of coexistence. Animals with higher giving-up densities are less likely to persist in the company of those with lower giving-up densities because the former are not able to exploit food patches used previously by the latter. We climbed trees to estimate giving-up densities by counting the fruit which primates left behind.
  • 3
    We tested five possible mechanisms of coexistence. Three mechanisms proposed that each frugivorous species has a lower giving-up density than the other in at least one of the following: (1) different tree species, (2) within-tree foraging zones or (3) seasons. The fourth mechanism predicted that the socially dominant species exploits resources first and that the subordinate species has lower giving-up densities. The final mechanism predicted that one species would find resources more quickly than the other, which would in turn have a lower giving-up density.
  • 4
    Four of the five mechanisms received no support from our data. Only a trade-off between interspecific dominance and giving-up densities was supported.
  • 5
    We discuss the generality of our results and possible interactions with other factors.