Interdemic variation in mixed-species association patterns: common diurnalprimates of Kibale National Park, Uganda

We used interdemic variation in the tendency to form mixed-species groups t o examine the costs and benefits of association among the primates of Kibal e National Park, Uganda. A year-long survey of six sites revealed that the amount of time that the five common diurnal primates [red colobus (Procolob us rephrosceles), black-and-white colobus (Colobus guereza), redtail monkey s (Cercopithecus ascanius), blue monkeys (Cercopithecus mitis), and grey-ch eeked mangabeys (Lophocebus albigena)] spent in mixed-species groups varied dramatically among sites. In many cases, the proportion of time that speci es associated was positively related to their densities. By using detailed behavioral observations of redtail monkeys and red colobus made over 4 year s (2660 h) at four sites, we were able to reject the null hypothesis that a ssociations occur by chance for only one of four sites. However, a correlat ive approach exploring the costs and benefits of association suggests that ecological variables do influence association patterns. We found that redta il monkeys and red colobus overlapped in diet (19.2% of their foraging effo rt) and traveled further when in mixed-species groups than when alone. Havi ng demonstrated this, we examined the applicability of the ecological const raints model for predicting the proportion of the time spent in mixed-speci es groups based on food availability. For this analysis we concentrated on red colobus from the site with 35 months of observation and demonstrated th at their tendency to be in mixed-species groups was related to food availab ility. We used two methods to examine if mixed-species associations functio n to decrease predation risk. First, chimpanzees are known to prey heavily on red colobus, but rarely kill other primates. The time red colobus spent in mixed-species groups was correlated to chimpanzee density, but it was no t for the other monkey species, suggesting that mixed-species groups serve to decrease predation risk. Second, when red colobus groups contain more in fants and are presumably at the greatest risk of predation, they form mixed -species groups most often. These results demonstrate that the costs and be nefits of mixed-species associations vary dramatically over small spatial a nd temporal scales. If such variation is generally the case, then studies c onducted at different locations or different times could easily highlight t he importance of difference selective agents in favoring mixed-species asso ciations.