PRIMATES, 38(3): 281-291, July 1997 281 Dyadic Associations of Red Colobus and Diana Monkey Groups in the Ta'i National Park, Ivory Coast OLIVER P, HONER Institut fiir Verhaltenswissenschaften, ETH Ziirich LORENZ LEUMANN Zoologisches Institut, Universittit Zfirich and RONALD NOi~ Max-Planck-Institut fiir Verhaltensphysiologie, Seewiesen ABSTRACT. Members of the genus Colobus have been observed to associate frequently with Cercopithecus monkeys in several African sites. In the Tai National Park, Ivory Coast, one group of western red colobus was found to be in association with one particular group of diana monkeys more than could be expected by chance (HOLENWEG et al., 1996). We show that dyadic association is not an idiosyncrasy of these two groups, but rather a pattern that is general for our study site. All five red colobus groups we studied were closely associated with diana monkeys during more than 60% of the time. Four groups had one particular diana partner group, the fifth two different partners. Apart from the red colobus, three more primate species, the olive colobus, Campbell's monkey, and the lesser spot-nosed monkey, were also strongly attracted to diana monkeys. Key Words: Colobus; Cercopithecus; Polyspecific association; Cooperation; Mutualism. INTRODUCTION Mixed-species associations among primates are widespread and have been reported from tropical forests of Africa (GAUTIER & GAUTIER-HION, 1969; STRUHSAKER, 1978, 1981; GAUTIER-HION et al., 1983; GALAT & GALAT-LUONG, 1985; CORDS, 1987, 1990a, b; WHITESIDES, 1989; OATES & WHITESIDES, 1990; MCGRAW, 1994) and South America (TERBORGH, 1983, 1990; GARBER, 1988; HEYMANN, 1990; PERES, 1992a, b). A number of different types of associations have been described, ranging from chance encounters and temporary assemblies of two or more species to permanent bispecific groups. Within any community the composition of associations tends to be quite consistent. Some species combinations habitually form polyspeCific associations, while other combinations never do (TERBORGH, 1990). Polyspecific associations can be formed, either because primate groups meet each other by chance, or because groups actively seek the company of allospecifics. Chance encoun- ters, which can take place randomly either independently of resources throughout the range, or mainly at commonly used resources, do not ask for a functional explanation. The question of an ultimate cause can be asked when members of one species are apparent- ly directly attracted by groups of other species. Two such causes have been proposed: improved acquisition of resources, and better protection against predation. For a more extensive discussion, see BSHARY (1995), HOLENWEG et al. (1996), and WACHTER et al. (1997).