Interspecific Interactions

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, it's inevitable that they'll interact. Two species might interact negatively or they might interact positively;

Competition can be over food or over territory and so aggressive interactions are not uncommon.
Mutualistic interactions can be increasing each other's access to food or protecting each other from predators. Some species will form very stable mixed species groups.

Two primate communities: Krau reserve and Kibale forest

Kibale Forest in Uganda
Chimps are there, as well as two different species of colobus: black and white colobus and red colobus; also mangabeys. Three different cercopithecines; savannah baboons are there, two different species of bushbabies, one potto, and a partridge in a pear tree.

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.


Ecological competition does not equal behavioral competition!!
Two competing species might not be physically fighting over things- they may never even see each other. It may just be that one of them is depleting a resource that the other needs.

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;
Interference competition

One species aggressively excludes the other, like when two groups meet at a nice fruiting tree and one chases the other away.
Exploitation competition
One species obtains the resource first, like when there's a big fruiting tree with orangutans, macaques, and colobines all eating at once. There's no interaction or aggression between them, but they're still reducing the amount available to others.

When does competition take place?

There is a potential for interference competition when there is interspecifc aggression.
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.

There is a potential for exploitation competition when there is overlap in resource use.
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.

Does competition take place?

If two species are in mutual distribution then that probably means that they're in competition- for example, often when gibbons set up a territory they'll not only exclude members of their own species but also any other kind of gibbon.


Population density
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.

Ranging patterns
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.


You might think interactions between species are always negative, but they aren't. Primates can also benefit from being in the presence of other species. There are some species pairs which we see together in the wild quite a bit. Sometimes they have a lot of interaction and stay together for long periods of time. They play together, groom each other, and sometimes even eat with each other. You can read about this all in the CP article.

Examples: Africa and S America

(There are no Asian examples because there aren't a lot of examples of long term interspecific interactions in Asian primates.)

Potential benefits

Increases foraging success
By joining with another species, you might be able to gain access to otherwise inaccessible foods. Or you might just be more likely to find food in the first place. Both of these might be going on between mangabeys and redtails. Mangabeys have more powerful jaws and can open fruits that the redtails can't even get into. Mangabeys sometimes leave some behind, and the redtails get the leftovers. What do the mangabeys get out of it? Mangabeys have large ranges while the redtail monkeys have smaller ranges. So one mangabey group will range over an area occupied by several redtailed groups. When the mangabeys move into an area, they don't know it as well but if they follow the redtail monkeys then maybe the redtail monkeys will unwittingly lead them to the best food spots. So basically the mangabeys exploit any local knowledge that the redtails have.

Reduces risk of predation
Three ways;

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.