Cooperation and competition in two forest monkeys

Putty-nosed monkeys, <i>Cercopithecus nictitans stampflii</i>, occur at various sites in West Africa, particularly in the transition zone between rainforest and savannah. The species is sometimes seen in primary rainforest, although at a curiously low density compared with that of other monkey species. We conducted a 24-month field study in the tropical rainforest of Taï National Park, Ivory Coast, and found that putty-nosed monkeys require an ecological niche almost identical to that of the Diana monkeys, <i>Cercopithecus diana diana</i>. Moreover, the niche breadth of putty-nosed monkeys was significantly decreased in the presence of Diana monkeys, suggesting that feeding competition with Diana monkeys kept putty-nosed monkeys from successfully colonizing a rainforest habitat. However, contrary to the interspecies competition hypothesis, groups of both species almost completely overlapped in home ranges and formed near-permanent mixed-species associations, rather than avoiding each other. We hypothesized that Diana monkeys tolerated immigrating putty-nosed monkeys and formed mixed-species groups with them, despite high levels of competition, because of their merit in predation defense. Direct observations and a series of field experiments confirmed that male putty-nosed monkeys play a vital role in defense against crowned eagles, suggesting that putty-nosed monkeys obtain access to feeding trees by offering antipredation benefits to Diana monkeys. We discuss these findings in light of biological market theory.