Monday, November 4 -- Interspecific
Interactions
[Topic List] |
[Next Lecture] | [Previous Lecture] | [Discussion]
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;
- negative interactions are
called competition
- positive interactions are
called mutualism (but it's not always two-way)
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
- In Africa, red colobus monkeys form
longterm associations with redtailed monkeys. This is like in CP
article.
- From South America, cebus (capuchin) monkeys form
longterm associations with squirrel monkeys.
(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;
- It decreases the chances of an
individual being taken by a predator. (Providing, or course, that the
predators take only a few animals per attack and that the rate of attack
is independent of group size)
- It increases the probability that
predators are detected: more eyes.
- It increases the probability
of deterring predators: mobbing.
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
[Topic
List] | [Next Lecture] | [Previous Lecture] | [Discussion]
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 harassment | Resident | Immigrant |
Give Harassment | 225 | 50 |
Receive
Harassment | 50 | 225 |
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?
- Kinship: Kin engage in serious aggression less
often than non-kin. Also, a substantial portion of cooperative acts are
performed between kin.
- Rank: High ranking females
are more attractive social partners.
- Reproductive
state: Lactating females are attractive social partners for both
immature and adult females- they receive a lot more grooming than when
they're not lactating. This is probably mostly because the others are
trying to get close to the newborn so they're being nice to the mom.
Also, sexually receptive females receive more grooming and give
less, and are also more likely to receive agonistic support from males
against other females.
- Immigration status:
Resident females harass and attack immigrant females in both gorillas and
chimps, for example.
Evolutionary effects of female
competition and cooperation
Benefits of high rank:
- Better access to food
- Better feeding efficiency since
they get interrupted less
- Receive more grooming
- Get
harassed less
- More likely to receive agonistic support
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
[Topic List] | [Next Lecture] | [Previous Lecture] | [Discussion]
Demographic
Constraints on Male-Female Relations
- One is the ratio of
males to females- the social organization of their species will affect
the types of relationships that males and female scan have.
- Another is the biology of the species- in some, the males are
stronger, larger, and have better weapons.
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
baboons | Hamadryas baboons |
Female-bonded; males disperse | Male-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
- The friends protect
females from aggression from other males- a female gets attacked about
once a week, and receives a serious wound about once a year. A male will
protect his friend from other males. In instances where a male went tot
he defense of a female, about 90% are friends of the female, and only
about 10% were other males.
- The friends also defend their
friend's babies.
- Also beneficial for the female, her friend often
develops good relations with her infant. He will carry it around, groom
it, pick it up, and allow it access to good feeding sites. What's good
for the baby is good for mommy.
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 --
[Topic List] | [Next Lecture]
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