Chapter eight: The evolution of social behavior

Chapter Review

Altruism and Cooperation: A Selective Puzzle

Most of the anthropoid primates live in social groups that create many and diverse opportunities for interaction. Among dyadic (two-individual) interactions, a number of different outcomes are possible in which the actor and recipient can receive positive (+) or negative (-) outcomes (Table 8.1):

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Table 8.1 Range of Possible Outcomes for Four Types of Dyadic Interactions
Figure Credit: W.W. Norton.

So why would an actor engage in a potentially disadvantageous interaction—such as those denoted by spite and altruism—since Darwinian theory places individual fitness at a premium for evolutionary success? Surely, an individual engaging in altruistic acts is squandering time better spent mating or gathering resources. Plus, altruism can result in severe injury (or even death) without the promise of any tangible reward. How, then, did altruistic behavior evolve, given the apparent conflict with individual fitness?

Kin Selection

The problem of altruism as described above stumped evolutionary biologists until W. D. Hamilton, in 1964 (and J. B. S. Haldane, who foreshadowed the logic behind kin selection in the 1930s), addressed the question of altruism in quantitative terms. These findings introduced a new term—inclusive fitness—and describe the logic of genetics as maintained over evolutionary time.

If there is a genetic basis for altruistic tendencies, Mendelian genetics should produce siblings with a specific probability for sharing genes. Logically, siblings share approximately 50% of their total genetic makeup, a parent and offspring share about 50%, and first cousins share about 12.5%; the amount of genes shared by other relatives depends on the degree of relatedness. Hamilton took these assumptions about the degree of relatedness among kin into account when addressing altruistic social encounters, constructing a formula that we now call Hamilton's rule:

rb > c

where (in terms of fitness) c is the cost to the actor, b symbolizes the sum benefits to all individuals affected by the behavior, and r is the coefficient of relatedness between actor and recipient. Here, actors gain fitness by assisting relatives, provided that the cumulative benefit to the recipients is greater than is the cost to the actor. This logic, coupled with the fact that interaction between individuals is likely to be biased toward kin rather than nonkin (particularly those of an altruistic nature), provides a theory in which altruism and evolutionary success no longer seem contradictory.

Example

Imagine a situation in which the threat of predation can evoke one of two possible responses in a vigilant monkey (Fig. 8-1):

Give an alarm call: the caller incurs a cost (being eaten) at the benefit of the others.
Remain silent: some of the other members of the group are eaten while the actor escapes unharmed.

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Figure 8.1 Diagram of possible positive and negative effects of giving an alarm call (top) versus remaining silent (bottom).
Figure Credit: W.W. Norton.

If the beneficiaries of this act of ultimate altruism are close relatives of the actor, it actually benefits the inclusive fitness of the acting individual to give the call and save its kin. Because relatives have a higher tendency to share similar genetic material, the total genetic benefit for this behavioral strategy exceeds a purely selfish strategy. Natural selection, therefore provides for the enhancement of group fitness by individuals acting, really, in their own self-interest.

Reciprocal Altruism

Kin selection is not the only reason cooperative and seemingly altruistic behavior may have evolved. Reciprocal altruism—the continued, mutually beneficial interaction between individuals over time—is another useful behavioral strategy. For reciprocal altruism to work as a strategy, several conditions are required:

Frequent interaction.
The recognition of individuals.
Remembering past interactions with individuals.
Assisting only those who provided past assistance.

Given these conditions, the strategy of reciprocal altruistic behavior has proven to be both successful and evolutionarily stable.

Example

Researchers have observed reciprocal altruism in action in field studies with monkeys and apes, such as recording situations where monkeys endeavor to groom each other for roughly the same amount of time. More specific experiments to test the precepts of reciprocal altruism, though, were first devised by Robert Seyfarth and Dorothy Cheney who played recorded “recruitment calls” (invitations for grooming or support) to vervet monkeys in various situations. First, Vervet A’s call was played to Vervet B after A had groomed B. They then played the same call, but after a period when no grooming had occurred. Cheney and Seyfarth hypothesized that Vervet B should respond with more interest after having been groomed by Vervet A and with less interest after no grooming had occurred. In fact, their hypothesis was affirmed: Vervet B responded more strongly after having been groomed, suggesting that monkeys remember and maintain reciprocal altruistic relationships.

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