In a related lecture, we spoke of kinship and dispersal, which lead to altruism. To avoid inbreeding, one sex usually disperses at maturity. Usually it's the males who disperse since benefits of dispersal are more to them than to females, since they get greater mating opportunities. Also, the costs are less; access to food is reduced which isn't as important to them as to females. When males don't leave, then females are forced to. This happens especially in the new world primates. So, because of different patterns of dispersal, you have variations in which sex is living with relatives. Whoever lives with relatives has the opportunity to treat relatives differently than non-relatives. Called kin-correlated behaviors, these include spatial proximity, agonistic helping, and grooming. So they treat relatives differently, but why? Can we explain it in terms of natural selection?

So how do Primates know kin from non-kin?

You can't treat your relatives differently if you don't know who they are.
There are two hypotheses:

Social learning

You grow up with your parents so you know who they are since they're always right there. Imprinting! Lots of evidence and research on this. Probably extended farther in primates since they have more extended relationships. You learn who your mother is, then pay attention to who she hangs out with and that teaches you who your other relatives are. Some problems: You can make mistakes or get fooled. Like cuckoos- trick other parents into taking care of their egg. Question: how do cuckoos avoid trying to mate with the kind of bird that raised them if they imprint on their mom?? Another problem is how to know who your dad is in non-monogamous societies. So people hypothesized another theory- this is not based o observations, just on people sitting around thinking.

Self-referential phenotype matching

What does this mean? All your physical characteristics as opposed to genotype which is all the genes which went into guiding your development. So if you see someone who looks like you then maybe they're related and maybe you should treat them better. This isn't just visual, but also smell or sound or something. Dawkins' armpit effect- imagining people sniffing another's armpit and then their own and deciding that they're related or not. Out text says this does exists but more recent publications are really getting to doubt it. One study where it was demonstrated on macaques and one in ground squirrels but hey have both been criticized as to methods and no on has been able to repeat their findings.

Natural Selection and the Expectation of Selfish Behavior

Remember the characteristics of natural selection:
1 phenotype variation that ...
2 affects reproductive success and...
3 is due at east partly to genetic differences

You expect individuals to act selfishly. It is easy to understand and explain through natural selection. This is the easiest way to ensure that you will have the highest reproductive success. By simple mathematics, genes for altruism will die out.

There are four ways of behavior to another- it might benefit or harm you, an it might benefit or harm the other guy.

Benefits recipientHarms recipient
Benefits donorcooperationselfish behavior
Harms donoraltruismspite

Selfish behavior, we expect to see. Cooperation we can understand. Spite is hard to explain in terms of natural selection, but that's ok because you almost never see it. However, in animals you see altruism but how can we explain it?

An example- grooming. It benefits the recipient, but what does it do for the donor? Another example is helping defend a group against a predator; sometimes male colobus will confront predator instead of running away with the rest of the group. In primates there are many examples of individuals taking care of infants which aren't their own. Another example is alarm calls.

A Counter-example of Apparently Altruistic Behavior: Alarm Calls

If the predator moves slowly, alarm calls don't cost much- just a few calories. But with quick predators, if you call out the alarm then that might catch attention of the predator who might come after you.

Group selection interpretation
This was the classic explanation; it was good for the group. As people sharpened up their logic, they debunked this theory. George Williams especially drove home the point that if you have a bunch of altruists it's really easy for a cheater to arise and take over.

Question; how to resolve the paradox?
How can altruistic behavior evolve in individuals within the framework of natural selection? The key insight in cracking this problem was realizing that it's not individuals who are passed down but genes. If your relatives are like you then they carry the same genes.

Two solutions were developed to answer this question: kin selection and reciprocal altruism, which is less well established.

Remember the basics of natural selection: If there's a trait in a population which spreads, then it's favored by natural selection. Now we will complicate things by adding inclusive fitness. We are going beyond thinking about individuals and are now thinking about the genes that they carry. Alternative forms of genes, called alleles, are in us. We can copy our genes not only by having offspring ourselves, but by helping our relatives to have offspring. You can calculate exactly what is the probability that another individual will have the same genes as you.

First, what is the probability that a copy of a particular gene in a parent is shared by one of its offspring?
(Thanks to William Hamilton)


The probability of sharing any particular gene can be computed as follows:
At each generational link between two individuals, there is a meiotic event, resulting in a 50% probability that a copy of a particular gene is passed on. For T generational links, this probability is:


(Note that our text uses L instead of T)

To compute r, you just sum this value for all possible pathways between 2 individuals. It can get a little more complex because you can be related to the same person in two different ways. Here are some examples;

Parent and offspring:
One link, parent to child. Thus, r=.5

Grandparent and grandchild:
Two links; child to parent, and parent to grandparent. (.5)^2 = r =.25

Half siblings:
Two links; up to the parent is one and back down to the other sibling is another. (.5)^2=.25, so your relatedness coefficient is .25

Full siblings:
These are a little tricky because there are two ways that their genes could be the same. Full siblings are related through both their mother and through their father, so each pathway is .25 and when you add them together, it's .5.

Two cousins:
Their parents are siblings, so there are two pathways with four links each that they're connected by. 2 * (.5)^4=.125

Genetically speaking, full siblings are just as related to you as your offspring. So an individual can benefit just as well by helping its siblings as by making offspring.

If given the choice to help a grand-offspring or a half-sibling (both have r=.25), which will an individual be better off choosing? They will usually choose the younger one because the benefits t the recipient will be greater since a young helpless infant will be more helped by a single act of helping than an old established animal. (Also, lifetime reproductive potential is greater for younger individuals than for older ones.)

Kin Selection

Hamilton noted that selection would favor behaviors or traits that not only increase one's own reproduction, but also that of others, insofar as those others are related and share genes with you. This form of selection was termed kin selection to contrast it with Darwinian natural selection.

To formalize the concept, Hamilton introduced the concept of inclusive fitness, a measure of one's own reproduction plus a summed component of the reproduction of others, devalued by the degree of relationship.

Inclusive fitness
individual fitness
fitness of others devalued by
the degree of relationship between donor and recipient

Conditions favoring altruism

Whether or not you will help someone depends on thee factors:
How related you are
How much it benefits them
How much it costs you

Hamilton's rule is for predicting whether or not altruistic behavior will be exhibited:

B/C > 1/r
ORrB - C > 0

B = benefit to recipient
C=cost to donor
r = coefficient of relatedness of donor to recipient

A Return to Alarm Calls

The theory would be that even though the alarm call has a cost, the benefit to kin must be more than the cost to the caller. So, if you can measure the costs and benefits, you should be able to predict whether or not an animals will call.There have been plenty of studies on this, especially done on ground squirrels. It turns out that the rate of alarm call depends on closeness of relatives and how many are nearby.

In ground squirrels, males disperse and don't live with relatives, but females stay so they're with more relatives. Based on their percentage of the population, females call much more than you'd expect and males much less.

Male gorillas are related to all the kids in the group, but females are only related to a few of the kids. So you'd expect males of do more alarm calls than females and they do.

Also, vervets were studied in captivity. They checked whether females did more alarm calls when offspring were present, and they did indeed call much more when offspring were present than when there were others present.

A cautionary note: To understand all this, you have to do a lot of math. You may be thinking, "Do monkeys really understand all these concepts and everything?" But you must understand that the animals don't have to be conscious beings for it to work, and they don't have to know calculus! Natural selection does all the math for them- if an individual's behavior works within the formulas, then its genes will be passed on, but if its behavior isn't within the formulas, then the overly- or underly-altruistic genes won't be around for long.

Note that there are different kinds of altruism:
Phenotypical altruism, which has a cost to the individual but is also genetically selfish because it benefits the genes that the individual carries. This is explainable by natural selection.
Genetic altruism, on the other hand, harms the individual and doesn't perpetuate any of the individual's genes, and so it can't be explained by natural selection.

This kin selection theory explains most acts of altruism that we see- they're generally benefitting relatives. This is in contrast to:

Reciprocal Altruism

(Developed by Robert Trivers in the 70's) This concept will be easy for us as humans to understand since we do it a lot. The idea is that the altruism is reciprocated at some other time- an individual does something nice and later gets it back. This has gotten a lot of theoretical discussion but there is little evidence.

Primate examples (From pg 287 in coursepack)
Our first example is baboons: Remember their consortships- when a female is receptive, a male will follow her around and keep others away as long as he can. Sometimes the individual in the consortship will be the highest male in the group and will be able of keep everyone away. But sometimes two males will gang up to drive him away and then one of them will mate with her. They often recruit an ally to help chase away the other guy. The helper incurs serious costs, but what benefits does he get? He doesn't get a female. You notice through further observations that in the future the helper will recruit the guy he helped to help him drive away a guy and get a female.

Our second example is vervets: Here, alliances are between females.When a fight between females breaks out, one participant will often scream for help and other individuals who hear her will come to her aid. Usually, it's relatives who come but sometimes it's non relatives. How likely is it that a non-relative comes to give help depends on how recently the recipient groomed the donor. Data from playback experiments shows that when kin hear a scream for help, they look around for the source for a long time when there was no prior grooming, while non-kin look for the source for a LOT longer when they had been recently groomed, and barely looked at all when they hadn't been recently groomed.

Conditions favoring reciprocal altruism
To illustrate this point, we played prisoner's dilemma. When you're playing a series with the same partner, the 'tit-for-tat' strategy works the best. When playing with a different partner every time, it's better to defect every time.

To learn the theory of prisoner's dilmma, check out this link: .HTML

So, how favored by natural selection a tit-for-tat strategy is depends on the likelihood of future interactions with the same individuals.

Conditions necessary for the evolution of reciprocal altruism