In a lot of the tropical forests where they occur,
primates are the fairly dominant animals, both in terms of numbers of
individuals and in number of species. The highest count so far is 16
different species in one location. With this kind of species density,
inevitable that they'll interact. Two species might interact negatively
or they might interact positively;
Krau game reserve in Malaysia
Here there are two different species of gibbons, two kinds of leaf monkeys, two kinds of macaques, one loris. Some of these are very similar in diet and ecology but others are very different. Some species have up to 48% dietary overlap. Others however have only a 5.1% dietary overlap.
How much of these differences in diet have evolved in response to living together? Ecologists say there's no way two species could live together with the same niche- they'd have to diverge or else one would kill the other off. So they like to look at how this works in primate species and figure out how the ecology has affected their development and how they have changed to avoid direct competition.
A necessary prerequisite for competition is that some resource is in short supply. So like, they might both be using oxygen, but it's not a limiting resource for either one so they're not competing for it.
There are two types of resource competition;
For instance, between mangabeys and blue monkeys. Usually the larger primate will drive away the smaller one, but there are exceptions- gibbons are larger than macaques but macaques come in larger groups and usually drive away the gibbons.
This is harder to determine because you can't just sit back and watch who is chasing who, like with interference competition. In theory, all you have to do is to find out who is eating the same thing in the same place at the same time. The first thing you must look at is diet type. But even if two species are eating the same food, it doesn't necessarily mean that they're in competition. Similarly, if they're in the same location, it doesn't mean they're in competition- they might be different in their vertical displacement; some might be found in the canopy and some on the trunks etc. Also, even if they're in the same tree, they might still not be in competition, because of what kind of locomotion they use. For instance, gibbons and macaques might be in the same tree. The gibbons can get the fruit on the very ends of the branches but the macaques can't swing around and so they feed on food that's closer to the trunks.
People like to look at density of a species when it's around competition and when it's not- they say that if the population density is lower when the competitor is around and higher when the competitor is absent, then that's pretty good evidence that the two species are in competition.
Also you can look at range size. If food availability is decreased, then they'll have to look farther to find food. So, if when the potential competitor is there they have larger range sizes and when it's absent they have smaller range sizes, then it shows that the competitor really is a competitor and is taking away resources which they need.
and S America
But why would it be more advantageous to be in a group with a different species than a group all of your own species? Because the other species might eat different food so you get the benefits of living in large group but don't have the feeding competition costs.
In the case of # 2, it's also beneficial to have not only more individuals but also different kinds of individuals- for instance, if they feed at different heights, then one might keep watch for eagles and the other might keep watch for ground predators.
For #3, see the coursepack for a multi-species mobbing event.
Remember the associations between colobus monkeys and redtailed monkeys and looking at how often they're taken by hawks. When you look at how often they're taken compared to their distribution, both these species are the ones who are taken less often than you would expect. It has not been proven experimentally that this is why, but it is hypothesized that it's because of their interspecific interactions.
Another reason why monkeys might be together which can not be ruled out- it might just be chance. If there's a high population density of primates, it's odds-on that some are going to run into each other sometimes especially if they're after the same types of food. It's not too easy to calculate how much you'd expect them to be together based on random chance. People like Peter Waser have made up some mathematical models of how often you'd except one species to be with another if they were just bouncing around randomly. He figured out how much you'd expect them to be and how much they're observed to be together. Then he plotted observed:expected. Some were observed more than expected and some were less. More means they're probably seeking each other out, while less means they're probably avoiding each other.
Of course these
associations might not be mutual- one species might just be following the
other around. Need more study to figure out who is benefitting and who is
being taken advantage of.