In Primate Behavioral Ecology, Strier outlines various polyspecific relationships and their contexts within the forest. Polyspecific associations occur when multiple species interact at the same time in the same place, and engaging in interactions that are beneficial for both parties involved. These behaviors must be repeated and meaningful actions and exist for a specific purpose. These interactions are less competitive and more interactive.
Red colobus monkeys and Diana monkeys, which are both old world monkeys that inhabit West Africa frequently associate with one another for various benefits. Red colobus monkeys are a species that establish hierarchies of dominance based upon various behaviors. They typically eat leaves, flowers and other fruits. An interesting aspect of red colobus monkeys that helps define their polyspecific association with Diana monkeys is their relationship with chimpanzees. Chimpanzees typically hunt and feed off of smaller mammals, such as these colobus monkeys. I imagine that this is what a Red colobus monkey looks like when it realizes a chimpanzee is near:
The other half of this polyspecific association is the Diana monkey. The Diana monkey is also an Old World monkey from West Africa. The Diana monkey is socially structured with a single male and several fertile females. These monkeys also have fairly obvious vocal and visual signals for things like predators and other social behavior. Similar to the Red colobus monkeys, Diana monkeys eat leaves, fruit, and some small insects. The Diana monkeys are really interesting-looking, so here is a picture:
By engaging in polyspecific associations, both species benefit in certain ways. In polyspecific associations there are typically two main ways in which species engage. One of the most common benefits some primates receive from these associations is increased foraging efficiency. Occasionally, two different species that have some dietary overlaps engage in mutually beneficial behaviors related to food gathering. If primates associate together but eat differing food resources there is very low competition, and finding appropriate food patches may be easier. Though there is relatively low levels of overlap in the diets of these two primates, it has been noted that some Diana monkeys will prey more on insects while associating with the colobus monkeys. Other than this, there is not, however, much data to prove that red colobus monkeys and Diana monkeys engage in polyspecific behaviors related to food foraging efficiency. According to Strier, some colobus monkeys will spread their groups more to match that of the Diana monkeys as well, signifying that there is some amount of repeated, meaningful behavioral shifts.
There does, however, seem to be a strong correlation between protection from predation and interactions between these two species. While associating together, both species benefit by avoiding their common predators. When two differing species interact for mutual benefit, each species brings to the table a certain set of skills that the other may not have. When the Diana and colobus monkeys group together there are simply more individuals to help keep a look out for predators. Differing behavior between the two monkeys also helps to watch for prey; they are spatially distributed differently. Diana monkeys typically inhabit trees at a lower vertical niche than the red colobus monkies. Both species, therefore, are further protected from either aerial or terrestrial predators than they would be on their own.
Various studies show that while there are high threats of chimpanzee attacks on Red colobus monkeys, the species will increase their association with Diana monkeys. Chimpanzees typically hunt for colobus during the rainy season, which is when associations between colobus and Diana monkeys are the highest, which proves the relationship is beneficial and meaningful.