Forest strata use by red colobus monkeys, Piliocolobus badius, in the presence and absence of green vervet monkeys, Chlorocebus sabaeus, in Bijilo Forest Park, The Gambia

Background/Question/Methods

    Tropical forests act as a natural defense system to global climate change by providing various ecosystem services. Unfortunately, deforestation and habitat degradation reduce natural resources and limit ecosystem services.  Ninety percent of primates rely on tropical forests, and almost half are threatened by anthropogenic habitat change. Fragmented landscapes alter the distribution of food trees, increase bushmeat hunting, and alter the frequency and intensity of mixed-species interactions. In a fragmented landscape, mixed-species associations may provide increased resource detection and predator avoidance. We examined mixed-species associations between the arboreal Western red colobus monkey (Piliocolobus badius) and the terrestrial green vervet monkey (Chlorocebus sabaeus) in a forest patch in the Gambia, West Africa.  Red colobus monkeys were observed for three weeks (June 2013) in the presence and absence of green vervets in order to determine any differences in their activity budgets and use of the forest canopy.  We expected a greater proportion of red colobus monkeys to utilize the lower canopy and forest floor when green vervets were present.  If red colobus monkeys are better able to detect food resources when in mixed-species associations, we expected more of them to feed when green vervets were present than when they were absent.     

Results/Conclusions

    The presence of green vervet monkeys affected the canopy use and feeding behavior of the red colobus monkeys. When green vervets were absent (n = 24 observations), the majority of red colobus utilized the middle canopy strata and they were never documented on the forest floor. However, when green vervets were present (n = 26 observations), the distribution of red colobus monkeys in the strata was significantly different (p < .001).  They utilized the understory most frequently and fed, travelled, and socialized on the ground. When in mixed-species associations, a greater proportion of red colobus monkeys fed (p = .01).  The presence of green vervets may allow them to obtain food items otherwise unavailable. Green vervets may act as sentinels and their calls may signal potential terrestrial threats and when it is safe to descend to the ground.  Red colobus monkeys may provide social benefits to green vervets because they were observed playing with and grooming them. Western red colobus monkeys are critically endangered due to habitat loss.  When living in small forest patches, mixed-associations may offer them greater access to resources.  We conclude that conservation efforts should aim to preserve entire primate communities, rather than using a single-species approach.