Kinship and Dispersal
Today we'll see how primate social behavior works within groups. We are
now getting into group composition- what they're made up of and how
relations within groups work. Next time we will see how can we use
natural selection to explain why primate should treat their kin
differently.
It was only in the 50's that people got the clue that kinship groups
existed in primate societies. A group of Japanese researchers were
studying Japanese macaques and once they started tracking who was related
to who, they began to notice that there were groups of female relatives
who had a lot of interactions with each other within their kin groups and
not so much interactions outside of the groups. This is a female-bonded
society. You get several matrilines who hang out together. A
single group might include several matrilines, and interactions go on
within matrilines and not between matrilines. When the males reach sexual
maturity, they transfer to another group. So in a group, the males are
not socially interactive because they aren't related and they all came
from different groups.
Kin-Correlated Behaviors
These are behaviors that are directed mainly toward kin as opposed to
non-kin.
- Spatial proximity
- Among old world monkeys, related individuals living in multi-male
multi-female groups are often found near and in contact with each other.
Data about spatial proximity can be seen in macaques, yellow baboons, and
vervets. (All cercopithecines.)
- Grooming
- Grooming is distributed preferentially among related individuals. It
probably plays a hygienic role in terms of removing parasites and keeping
wounds clean, but it also has a social role; creating or strengthening
social relationships. This type of kin-correlated behavior is seen in
many types of macaques, patas monkeys, vervets, and savannah baboons.
- Rank
- In cercopithecines, rank is inherited from a female's mom. Males'
rank is affected a little too, but what rank they have as kids they lose
when they transfer to another group. Daughters however stay in the group
and they keep whatever rank they inherit from mom. Note that highest rank
is the old lady. Next highest is her youngest daughter. So every time a
daughter is born, she assumes a rank higher than any of her sisters. When
the daughters have kids, they rank just below their moms but above any of
their mom's older sisters.
- Rank acquisition and alliance formation
- Why does it work this way? Why do younger kids have more rank than
older? Rank acquisition depends on agonistic support from kin; because of
the agonistic support, the more siblings you have the more backup you
have in any fights. You can get a tiny little monkey dominating adults
simply because they know that this kid's mom will back her up. Note that
there is some alliance development outside of kinship but most alliances
are formed within kin lines. Maybe about 20% are formed outside of
family.
- Mate choice and inbreeding avoidance
- Three sets of proof that female mate choice is involve with
inbreeding avoidance; in Japanese macaques, females do not mate with
their nephews and cousins (Ones related through females, that is; no one
knows who is related through the males.) Male gorillas do not mate with
their daughters, and female chimps avoid mating with their brothers.
(Usually actually they're half-brothers)
Kinship and Male Behavior
What kind of kin-correlated behaviors do males show? In cercopithecines,
where it's the males who disperse, they often encounter hostility when
they get to the new group. The residents try to make the newcomers go
away and join a different group. Two brothers will often leave their
natal group at the same time and this waw they have a built-in ally.
Sometimes if they don't emigrate at the same time, they may still end up
joining the same group and so there'll be a bunch of male relatives there
to offer support to the new applicant.
Other kin behavior shown by males is in species where the females
disperse and the males form the social cohesion. These groups have
patrilines instad of matrilines. A group is made up of a bunch
of males related to each other with some disconnected female peripherals.
Cercopithecines are called female-bonded societies while species
like chimps, where the females disperse, are called male-bonded
societies. For instance, in Goodall's work there's the case of Figan
reaching alpha male because he had the support of his brother Faben. When
Faben disappeared, Figan lost power.
A bunch of scientists did some DNA checking on male-bonded societies and
they found that the males in a group are more genetically related than
the females.
How General are the Patterns?
The differences between old world and new world monkeys:
Most of the old world monkeys are male-dispersal monkeys. There are of
course some exceptions in the old world monkeys; chimps, gorillas, red
colobus, hamadryas baboons.
In new world monkeys, the females emigrate and the males have closer
social relationships. These are primates like tamarins, howlers, spider
monkeys, and marikis. New world exceptions are the cebus monkeys.
Explanations of Dispersal
First, consider the costs of dispersal
It is costly, and dangerous. This can be seen in the patterns of
mortality in a male-dispersing species. The females have high mortality
when very young and very old, but in the middle they tend to live. Males,
however, have three peaks- one at infancy, one right at the time of
dispersal, and one in old age. So, many males don't survive when they
attempt to transfer. This may be due to increased predation or
starvation. They don't know where the enemies and the good food spots
are.
You can also see this from data on times when monkeys are moving into new
areas; they tend to disappear more often than when they're circulating
into the same old areas.
Our conclusion? Leaving your home turf is dangerous! Be careful! Watch
out for lions!
So, Why do animals disperse?
Two main reasons;
- Incest avoidance
- When you begin mating close relatives, you invariably have lower
fertility, reduced numbers of offspring produced, and the offspring tend
to be less fertile. This really happened a lot in the early days in large
mammal zoo populations because they didn't have large enough
populations.
- Mate choice
- To avoid inbreeding, females, being the choosy sex, will often refuse
to mate with related males. The males might be ok with mating with any
female but if all the females are refusing them sex, then the males will
run out of mating opportunities and will be rewarded for looking farther
afield.
Why male dispersal?
Old world monkeys are typical mammals because mammals are usually
male-dispersing. To understand this, we must go back to parental
investment.
A benefits to males is greater than that to females (More mating
opportunities)
B costs to females is greater than that to males (Less efficient resource
acquisition)
Males disperse because it's a good way to get more mates which is what
they are driven by. Females, on the other hand, are more limited by
access to resources. When you move, you don't know the food sources as
well and you don't have allies to help in competition over food access,
so females don't gain anything by leaving their natal group.
So then, why female dispersal?
The Inbreeding Avoidance Hypothesis
- Female dispersal when;
- Average male tenure in group > age at first
reproduction for females
- Male dispersal when;
- Average male tenure in group < age at first
reproduction for females.
Males usually stay in a group as long as the costs of leaving outweigh
the benefits. So if the male tends to leave the group after a period
which is shorter than it takes a female to grow up, then a female can
stay because whoever fathered her has most likely moved on. If males tend
to stay in a group longer than it takes a female to mature, then the
female must leave because her dad is probably still in the group.