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 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.


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.