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of death for a child. What matters most for survival is the presence of the mother, and other relatives are not adequate replacements. Task cooperation occurs within the groups that work and eat together, but conflict is always present in ways that Strassmann carefully explains.

In an overview of vertebrate interactions, Dorothy Cheney demonstrates in Chapter 15 that animals ranging from chickadees to chimpanzees are aware of their own status, and that of their companions, and behave accordingly. Eavesdropping on how individuals interact with others can change behaviors. Relatednesses are often known and impact interactions. In vervet monkeys, for example, an individual who has been attacked may turn and subsequently attack a relative of her opponent. Dominance hierarchies also impact such interactions. But some animal interactions are more subtle. Ravens are more likely to cache food in hidden sites when competitors are present, for example. However, the calculations of gain, cost, and punishment necessary for reciprocal altruism (here called contingent altruism) seem largely lacking outside of humans. Instead, there is a great deal of tolerance in interactions and a lack of direct payback among close relatives and long-time partners. Yet it is in these relationships where cooperation overwhelmingly occurs. A common feature of cooperative acts is that they are not necessarily transitive. Some individuals consistently take on the risky jobs, be it male chimps patrolling their territorial edges or female lions leading the hunt. This is also true in organisms (such as wasps) with much simpler brains, where cooperation flows from workers to the queen.

Observations of humans and primates in natural situations can teach us much about behavior, but environmental complexity can make causation difficult to discern. An alternative is to examine choices made under highly regulated circumstances. To address social acts such as generosity, trust, fairness, and punishment, many purportedly relevant games have been applied to humans, one simple example being the Dictator Game that allows a subject to decide whether to share a quantifiable resource with an unseen other. [This game typically yields donations of 20–30% of the resource.] Although such games have weaknesses, they seem to indicate that humans are willing to donate but only at levels indicating they consistently value themselves most highly. These and other experiments further indicate that humans favor relatives, long-term partners, and group members over outsiders, and they will suffer costs to punish cheaters. As described by Joan Silk and Bailey House in Chapter 16, versions of social games involving food or tools that likewise have been used with primates produce complex results. Cooperation clearly occurs and tracks levels of sociality in the groups, but some results are controversial and remain open to alternative interpretations.

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