Waser's gas model applied to associations between red colobus and Diana monkeys in the Tai National Park, Ivory Coast

We performed a study on the association between a group of red colobus monkeys (Colobus badius) and a group of Diana monkeys (Cercopithecus diana) in the Tai National Park, Ivory Coast. The groups spent more than 62% of observation time together. We were able to show that the association rate was higher than would be expected on the basis of chance encounters alone by comparing the observed data with a null model known as Waser's gas model. The two study groups had almost identical home ranges and hardly ever associated with conspecific groups, The red colobus group was primarily responsible for the initiation and maintenance of associations. At another site in West Africa (Tiwai Island, Sierra Leone), these species did not associate more often than expected by chance. The predation pressure on Tiwai Island is considerably lower than in Tai. This suggests that reduction of predation risk could be the main ultimate cause for association between red colobus and Diana groups in Tai.