Evolution, 4th Edition

298 CHAPTER 12

Reciprocity

Robert Trivers suggested that cooperation can evolve when one individual pro-

vides a fitness benefit to another, as long as the second individual is likely to

return the favor later [74]. This kind of cooperation is called reciprocity. It can

evolve if there are repeated interactions between individuals, if individuals rec-

ognize and remember each other, and if the benefit received is great enough to

outweigh the cost of providing the benefit to others [47]. Reciprocal cooperation is

known in many species of fishes, birds, and mammals [73].

Mathematical models predict the conditions under which reciprocity is favored

by natural selection. While cooperative behaviors in most organisms are innate

and involve no active thought, those behaviors have close parallels to some kinds

of human interactions that have been well studied by economists. The famous evo-

lutionary biologist John Maynard Smith introduced game theory from economics

to the study of the evolution of social behaviors. Some situations that occur in

humans and other animals are described by the “prisoner’s dilemma” [4]. Here

each of two individuals will do best by acting selfishly, but if both individuals act

selfishly, they will do worse than if they both cooperate (BOX 12A). Game theory

models show that selfish behavior is favored if individuals interact only once, but

that repeated interactions can favor cooperative behavior. Thus, reciprocity can

be favored when the association between individuals is so long-lasting that the

benefits that each partner provides to the other feed back to the individual’s own

benefit [68].

Vampire bats (Desmodus rotundus) risk starvation if they do not get a blood meal

every night (FIGURE 12.4A). Unrelated individuals form long-term social bonds

and regurgitate blood to members of their group [83]. As a result, individuals with

friends are less likely to starve. Social bonding also pays off in primates (FIGURE

12.4B). Offspring of female yellow baboons (Papio cynocephalus) that have strong

bonds with other females survive better than offspring of females with weaker

bonds [69]. Cooperation was able to evolve in vampire bats and baboons because

individuals have long histories of repeated interactions.

Mathematical models also show that, under some conditions, cooperation is

enhanced if one of the partners in an interaction punishes selfish individuals:

Futuyma Kirkpatrick Evolution, 4e

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Evolution4e_12.04.ai Date 11-02-2016

(A) (B)

FIGURE 12.4 Mammal species that display reciprocity between individuals. (A) The

vampire bat Desmodus rotundus forms roosting groups in which members that have

fed successfully sometimes feed regurgitated blood to other members of the group.

(B) Among primates, such as these yellow baboons (Papio cynocephalus), social alli-

ances between individuals are reinforced by activities such as grooming.

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