Reciprocity in animal behavior deals with how animals interact with one another to further their own personal welfare beyond what they could do on their own.

In many species, behavior has been observed in which one animal will help another with the expectation of having the favor returned at some point in the future. One animal may help another so that if he ever finds himself in need of help later on, he can, in a sense, "collect his debt" from the animal he helped in the past situation.

In 1971, biologist Robert Trivers showed how natural selection could lead to seemingly altruistic exchanges between individuals, irrespective of their degree of genetic relatedness.

If in a population it is common for two or more individuals to exchange altruistic acts reciprocally and repeatedly in such a way that the summed benefits to each of them exceed the costs, in succeeding generations the genes underlying these behaviors will increase in frequency.

Imagine, for example, that at a time of food shortage one individual gives another one who has little enough food to keep him and his family from starving, but the gift does not endanger first individual's own survival. If this hypothetical situation is later reversed — the second helps the first and the benefit to first exceeds the cost to second — then, over time, for both individuals, the summed benefits of these exchanges exceed the costs.

Given the feeding behavior and food requirements of vampire bats, Wilkinson (1990) suggests that reciprocal altruism must be crucial to their survival.

These nocturnal bats feed off the blood of other animals during the nighttime; they go out in hopes of finding an animal upon which they make a small incision and collect blood from them.

If a bat is successful, he will return near dawn with a stomach full of more blood than he actually needs to survive.

On a given night about one third of the bats less than two years of age and 7% of older animals fail to feed. This poses a potential problem, however, because as some bats are weaker than others (specifically young ones), they may only be able to go three consecutive nights without a successful hunt before they die of starvation.

Thus, on any given night about 11% of the younger bats and 0.5% of the older bats are within a day of death by starvation.

In this environment, these bats have been observed to demonstrate seemingly altruistic behavior toward one another: a bat returning from a successful night of hunting (and therefore a gorged stomach) will actually regurgitate some of this blood to another bat that was unsuccessful that night, effectively transferring the nutrients needed to survive.

While this may, on the surface, seem like a selfless act unexplainable from an evolutionary perspective, this behavior is actually in a bat's self interest, because by helping a fellow bat one night, that same fellow bat will be more likely to help him in the future if he is in need.

This reciprocity clearly provides more to the sharing bats than they could provide for themselves. The benefit of sharing blood for a recipient is always greater than its cost to the donor. Bats that share food with partners among their roost mates have a significantly higher survival rate than those who do not.

Vampire bats are remarkably social. Long-term associations form where they regurgitate blood to one another on a regular basis.

Wilkinson saw individual adult females switched groups only once every two years (a previous study found females bats roost together as long as 12 years).

Some of these associations are between kin and some are not. Among adult females, Wilkinson observed 70% of the sharing was between a mother and her pup and 30% between non-relatives.

Vampire bats did not share blood randomly but preferentially with relatives or roost mates. He saw the roles among the adults reverse on a regular basis. Vampire bats are able to discriminate the needy from the well-fed, those likely to reciprocate from strangers, and those who have fed them in the past.

Wilkinson measured the costs and benefits and found that reciprocity increases the probability of survival for bats that share. He concluded that reciprocity and kin selection each promote sharing blood among roost mates.

  a cleaning station

Wrasses act as cleaner fish that pick parasites and dead tissue from the teeth and gills of the grouper in return for protection from predators. Groupers are frequent visitors to "cleaning stations," where the grouper will open its mouth in a non-threatening manner, attracting cleaner fish to enter its mouth to remove parasites.

As the big fish could be tempted to eat the small fish and the little fish could be tempted to eat live tissue instead of dead, Gintis (2000) describes this interaction as an infinitely-repeated Prisoners' Dilemma, or supergame (§6.2):

the component game
within the big fish-little fish supergame

Chimpanzees also engage in reciprocity.

Adult males hunt, often cooperating closely, and after a successful kill other members of the group come begging for their share, and they often do. De Waal (1996) concludes from his observations that the amount that the successful hunter himself possesses is much less important than who distributes meat to others. He shares meat in exchange for a greater allegiance to him later, which increases his power in the group.

Chimps value social status highly. A male chimp that holds the highest social status generally enjoys a large territory and sexual dominance among the female chimps, among other things.

Along with this superior position, however, comes the danger of being overthrown, not by a rival individual, but by a group of other male chimps in the community. These lower-status chimps may benefit greatly by forming a coalition that would be capable of combating, or at least threatening, the higher chimp in order to win back some of the luxuries he enjoys as the top male.

In order to form this coalition, the chimps must establish a system of communication in order to understand one another's intent and keep one another accountable for their part in the coalition. This is necessary in order for the group to be successful and acquire the "public good" within their coalition of overthrowing the top male chimp.

This communication seems to come in the form of reciprocal favors. To display their commitment to the coalition, the chimps offer food to each other, they protect members of the coalition who are threatened, and they provide for others in need. In return, they receive respect and support.

Also, by giving these gifts to one another, they effectively put the receiver of the gift in debt so that when a time comes when the chimp needs food from them, he can justify the favor by establishing the act as a payment of a debt. These forms of reciprocity appear to be essential to the strength and coordination of the chimpanzee coalitions.

The sharing of support and other social favors is quite prominent in chimapnzee colonies. Reciprocity has been observed in all of the following instances:

•  the forming of coalitions — A supports B, and vice versa;

•  nonintervention alliances — A remains neutral if B does the same;

•  sexual bargaining — A tolerates B mating with A, only after B has groomed A; and

•  reconciliation blackmail — A refuses to have contact with B unless B "greets" A.

Further, reciprocal acts such as these have been observed in both negative and positive ways.

A powerful male may, for example, punish females individually who a short time before joined forces against him. In this way, he repays a negative act with another negative act.

Apparently, chimps actively teach others a fundamental rule: good behavior is rewarded and bad behavior is punished.

Cultural norms may differ in different environment. We might see, for example, citizens from one state or region acting very differently in certain situations than citizens of another state in a different region.

One specific example that reflects these differences is found in human interactions when passing each other on a street.

Someone from a small town is generally more likely to smile, say hello, strike up conversation, or engage in some other form of social interaction with another person when meeting on a street.

A person from a big city, however, will be more likely to avoid such social interaction by either avoiding eye contact with the other person or signaling a desire to refrain from interaction through some other means of body language. This could include walking more briskly, walking further away from the person, or looking in another direction.

These reactions may be significantly different when compared to each other in this way, yet they are generally considered normal social behavior by the two different types of people based on the culture in which they have grown up.

How could this be so? Consider the difference between rural and urban life before easy transportation.

On a farm, several times each year some tasks were much more easily done with several people than one. The only other people available were neighboring farmers. Since they each would have this need, they each would have the opportunity to swap the favor of helping the other with expected reciprocity.

Continued interactions between few neighbors created a good environment for reciprocity to work. Also, these tasks were usually so sporadic that formal markets for these services, a possible alternative for gaining these services, would often be undeveloped. In addition, other people coming to the farm rarely provided any concern about security.

Over time these factors led to a cultural norm of being nice and open to strangers, to be willing to provide small favors and to make one's intentions transparent. People that you would meet would be more likely to be able to help you in the future than to hurt you. Being nice and open encouraged reciprocity to form, yielding gains for this behavior, and relatively few people failed to reciprocate or caused harm due to this openness.

Further, after generations with this farm life, people with the history of these cultural norms in their families remain nice and open, at least for awhile, even if they move to the city.

By contrast, if the urban dweller needed certain tasks done, markets for them were fairly well developed. Paying to have them done from a number of possible, relatively anonymous sources met this need. Reciprocity was replaced by markets.

An incentive to be nice and open did not develop in these urban environments.

Interactions with someone other than family, colleagues at work, or friends were infrequent. If one did provide a favor to such a person, having it returned would be rare.

At the same time, security concerns were more frequent, and even with more locked doors and gates, the probability of an approaching stranger wanting to hurt you rather than help you is greater than on the farm. Openness could hurt you.

So, being nice and open had fewer benefits and higher costs. Being wary, or "street smart," becomes more likely.