Monday, November 4 -- Interspecific Interactions

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Competition and Mutualism 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 ofspecies 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.

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.

Evidence

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.

Mutualism

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.



Wednesday, November 6 -- Social Relationships: Females

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Announcement: They've changed some of the course requirements/writing requirements. This'll depend on whether or not you're an ECB section. Go to discussion to find out what's going on.

Patterns of female social relationships: competition and cooperation

What is the most important factor that affects social relationships in primates?
Kinship!
What's most important in affecting kinship patterns; what determines who lives with relatives and who doesn't?
Dispersal!

Whether or not you live with kin depends on which sex disperses. The classic story with old world monkeys is that they're like most mammals in that it's the males who disperse and the females who remain in their natal group. You get a female bonded social organization. There are several matrilines in a group. (Matrilines are females who are all related through their maternal lines.) The males who are there are immigrants or are juveniles who haven't emigrated yet. The females form the social core of the group and they have a stable dominance hierarchy. (The male dominance hierarchies are usually really volatile.)

Let's compare gelada baboons and hamadryas baboons:

The gelada baboon is a typical old world monkey in that males disperse and females stay. In these groups, the male is supposed to be the central guy, but he doesn't have a lot of strong social bonds with the females, who all like each other and groom each other frequently. If the male disappears, the females will usually stay together.

In hamadryas baboons, females disperse and so they have weak bonds with each other. Most of the strong social bonds in the group are between the male and his females. If the male disappears, the whole group will usually disintegrate.

So while their social organization is pretty much the same, because the dispersal pattern is different, their kinship and social stuff are different.

A quick review on dispersal:

In some species males disperse and in some females disperse. Why? Because the benefits of dispersal are more beneficial to the males because it's mostly that you get increased access to mates. The costs of dispersal fall mainly on the females because it's mostly limiting access to food and this is more important to females. Since it's more beneficial to males, and more costly to females, its usually the males who disperse.

So in most cases it's advantageous for a male to move on. Sometimes they stay in a group only a short time and sometimes they stay a long time. When females reach sexual maturity, the thing that will affect whether or not inbreeding will happen is whether their dad is likely to still be in the group. In some species, males tend to move on before their offspring reach sexual maturity. In these species, there's no benefit for females to disperse, so they don't. In other cases, the males remain in the group long enough for their offspring to grow all the way up. In these groups, if the females stayed, they'd likely end up inbreeding- so they disperse.

Vervets and Gorillas- two example species

Both species have aggressive and cooperative interactions between females.

Vervets

Aggression
In vervets, the most common forms of aggression are supplants; one individual walks towards the other and makes it leave where it was sitting. These are pretty low-key. To get an idea of how often these kinds of things happen, here's some data from Dorothy Cheney:
She watched 75 individuals for 225 hours total. She saw about 13 aggressive interactions per hour. Unrelated individuals were primarily involved.

What are the contexts of aggression? 11% were over food or water; about 20% were over access to preferred social partners; for the rest there was no obvious resource being fought over- most were probably just one asserting dominance over the other.

Cooperation
One of the most conspicuous forms of cooperation is grooming, mostly between kin, within matrilines. Also, high-ranking females received more grooming than low-ranking females; You may remember seeing data before about how likely a female was to go to a non-relative when they called for help depended on how recently they'd been groomed by that monkey. So, if you're grooming to ensure future agonistic support, then you'd rather cultivate a high ranking individual who could help you out more.
You also find alliances, mostly between kin. When a fight breaks out and someone comes to aid one of the contestants and forms a coalition with them, the recipient of the aid is usually the higher ranking female of the two fighting- this data was based on 666 interactions where two females were having a conflict and one came and helped. 89% of the time, the recipient of aid was the higher ranking female. So, usually they like the support the winner, not the underdog.

Gorillas

Aggression
Recall that they have female dispersal- so the females are for the most part unrelated to each other. This is from data collected by David Watts. Most aggressions take the form of lunges, chases, and displays; a lot of nonphysical contacts. Also, they engage in shoving and hitting as well. Again, most aggression is low-level, not all out fights. In gorillas, most of it is just vocal threats- they have two vocalizations they use in agonistic encounters, called 'screams' and 'pig grunts'.
There were .26 displacements per hour (n=971 h) So only one displacement every 4 hours or so.
There were .9 harassments per hour (n=586 h) (harassment=low-level threats, not necessarily over a specific resource or anything.)
Again, unrelated individuals were the primary participants.

Another factor that turns out to be important in with gorillas is how long you've been in the group;

Frequency of harassmentResidentImmigrant
Give Harassment22550
Receive Harassment50225

So the residents are giving a lot of harassment but don't receive very much, while immigrants take a lot and don't dish it out too much. Older females just couldn't care less about the new ones coming in; they're just more food competition.

As far as contexts of aggression, there is some competition over feeding even though gorillas don't have a lot of food competition. Some intolerance of proximity, too; they don't like someone to come too close. Protection of infants, or course, and lastly unclear reasons which is termed harassments.

Another determinant of aggression is where you happen to be; feeding was 15%
most of other was when they were resting- if more than 2m from silverback, 74% but resting less than 2m from silverback was only 4% of harassment.Other was 7%.
So, the silverback male often intercedes to end aggressive encounters between females. It's not in his interest for them to fight since they'd use up energy fighting that they could use to raise his kids. So they're unlikely to be harassed when sitting near the male.

Cooperation
One conspicuous form is grooming but you don't see a lot in gorillas relative to other old world moneys because they're not related to each other for the most part. There are some close relative but they're the exception.
Another form is alliances- they're very structured by relatedness. David Watts saw 59 interventions by a third who formed coalition with one of two fighters. In 56 of those 59 they were going to help kin:
25 mom <-->daughter (r=.5)
7 full sisters (r=.5)
5 half sisters (r=.25)
1 otherwise related. (r=Very slight)

What proximate factors affect patterns of female competition and cooperation?


Evolutionary effects of female competition and cooperation

Benefits of high rank:

How this translates to evolutionary effects:

Reproduction
Robin Dunbar studied Gelada baboons and compared rank to matriline size. Found that a large matriline meant higher rank. Higher rank means less harassment, which means less energy wasted fighting. Thus, higher rank increases the number of offspring that an individual has.
Mortality
Richard Wrangham studied vervets. It was a severe drought. Vervets always sleep in trees at night, and some of the trees are closer to the rare water than others. Because it was such a drought, it became important to survival how close your tree was to the water, and he found that the higher-ranking individuals got the trees closer to the water. During this drought, more lower-ranking vervets died than higher-ranking; Three out of four higher ranking survived, while only one out of four lower-ranking survived.
Fertility
Fertility has been linked to higher rank in vervets, macaques, and mustached tamarins, to name a few. In all of these, high-ranking females gave birth more than low-ranking females.

Dominance is not heritable, but the abilities needed to gain it might be. So, social ability might be under sexual selection- watch for more on this in the lecture about cognition and social abilities.


Friday, November 8 -- Social Relationships: Males and Females

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Demographic Constraints on Male-Female Relations

Let's look at two different types of social system which can affect male-female relations:

Monogamous system, like in gibbons.
There is little opportunity to form relationship besides the one with the mate, so there's not much complexity in choosing who to mate with and who to hang out with. People have done preference experiments in the lab and it has been shown that monogamous animals even when given a choice, choose their usual partner to associate with.
Polygynous system, like in the gorilla.
The females are not related to each other and so have little to offer each other socially. The male protects the females from outside males. Long-term bond form between the dominant male and the female sin his group- if he disappears, the females all disperse.The females are submissive to the male, and take more of the responsibility for staying near the male and maintaining proximity. The subordinate males do tend to hang out with females more than other females, but these proximities are the responsibility of the subordinate males, not of the females he's hanging out with.

However, remember the differences between gelada and hamadryas baboons; They both live in one-male groups with several females, but because of dispersal differences, the relationships are different.

Gelada baboonsHamadryas baboons
Female-bonded; males disperseMale-bonded; females disperse
If the male disappears, the group will still stay together and probably find another male.If the male disappears, the group will disperse. He holds the group together by aggression and herding.
The females are power brokers and have a lot of influence in which males will join the group.There is a lot more competition and less cooperation among the females. (Since they're not related.)

Multi-male, multi-female system, like the common chimp.
The females are mostly solitary in the chimp, so they haven't been studied as much, since males and females don't have too much contact outside of estrous periods.
In bonobos, however, males and females spend a lot of time together. Females maintain strong relationships with their sons even after they're grown. Females are also sexually receptive throughout their lives, even when lactating or pregnant.
In Savannah baboons, male-female relationships have also been studied. They're a female bonded group, and most grooming and alliances occur within matrilines. There is extreme sexual dimorphism (2:1 ratio) and the males have much huger teeth than the females. Studied show that the higher ranking males do get most of the matings, but the lower-ranking males get more than their share as well.

Why? Female mate choice. Why would they prefer these low ranking males? We must look beyond their estrous period and into the rest of their lives; Baboons give birth every 5-8 years. After birth, they lactate for about two years, with no cycling. They only go through about 5 cycles before they conceive again, and their gestation period is about 6 months. So, they spend about 10% of their lives cycling, and that was usually the only portion that people studied when they wanted to know about mating behavior. Barb Smuts looked at the other 90% of their lives and how that affected the mating behavior.

Friendship in Baboons

She found that females have certain guys that they spend most of their time with. A female spends very little time with most of the males but much time with one male. This was a surprising find in a species we had always termed promiscuous. She termed these pairings "friendships," and defined friendship in terms of proximity and grooming. Females spend a lot of time in close proximity with their friend, and almost no time with other males. If grooming were randomly distributed throughout he groom, then you would expect that any pair would be grooming each other about 6% of the time, but friendship pairs mostly (or even exclusively!) groom together.

Most female shave one friend or maybe two. Besides proximity and grooming, they often travel together, feed together, and solicit grooming from each other. The female is relaxed around her friend. Usually, when a male approaches, the female goes through ritualized submission- she presents. However, when it's a friend who is approaching, the females don't do this.

So, who is maintaining the relationship? Robert Hinde can up with a system to quantify this. You watch any time a female or a male in your target pair approaches or moves away from the other. Then you subtract the percentage of the time that the female leaves from the percentage of the time that the female approaches. This gives you a continuum from -1 to 1. If your number is -1, then the male is responsible for maintaining the relationship, while if you get 1, the female is. At 0, they're both doing it. When they apply this to baboon friendships, they find that in most cases it's the responsibility of the females to maintain the friendship, but in some pairs, it's the male. If they're both doing some of the work, then what are they each getting out of it?

Benefits to Females

Benefits to Males

"Friendships in the past doubled the probability that a male would form a consortship with that female in the future."
Also, beside increasing mating probability, there's another benefit which may be important- the male often uses the baby of his friend to fend off aggression from other males; when someone threatens him, he grabs the kid because the other guy knows that if he hurt the baby he'd be mobbed by the mom and her matriline.

So, the males increase their mating chances, while females increase their survival and that of their infants- so this is a case of reciprocal altruism.


Discussion --

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We went over review questions, got back a paper, and handed in a paper.

Let me know your thoughts: phyl@umich.edu
Last modified: November, 1996