What is the most important factor that
affects social relationships in primates?
What's most important in affecting kinship patterns;
what determines who lives with relatives and who doesn't?
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
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
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
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
Both species have aggressive and cooperative interactions
- 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
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.
- 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.
- 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.
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 |
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.
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
- 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.
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
7 full sisters (r=.5)
5 half sisters (r=.25)
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.
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
- More likely to receive agonistic support
How this translates to evolutionary effects:
- 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.
- 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
- 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.
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.